# Phase coherency: The Holy Grail of Car Audio



## Speakermakers (Nov 9, 2013)

*Phase coherency: The Holy Grail of Car Audio*

Is your system phase coherent? 

have you ever wondered why some sound systems sound way better than others? Even though you did everything right. Or you did almost exactly everything the same as a previous installation but the outcome was very different? More often than not it is because the audio system has one or several points of phase misalignment.


*Four points of phase trouble*


There are four common points of phase issues that I think most enthusiasts and installers are largely unaware of that have a profound effect on the performance of every car audio system.


*Phase shift due to analogue crossover points
Phase shift due to DSP latency in processors & head units
Phase shift at speaker/enclosure resonant frequencies
Mid bass speakers that are time correct and consequently not phase correct*


Each of these topics are complex by nature but I will attempt to address them as directly and simplistically as possible.


*Phase shift at analogue crossover points*

A more accurate definition of this problem would be: phase shift due to electrical resonance. Analogue devices that modify frequency response by means of electrical resonance can potentially cause phase shift and this includes equalizers as well as crossovers. Put in simpler terms; every time you apply a crossover (active or passive), or make an adjustment with an analogue equalizer, you are potentially introducing phase shift at that frequency. Not all phase shift is necessarily going to have a devastating effect on sound reproduction. Phase distortion that happens gradually and smoothly over a wide frequency range is much less or even unperceivable. The problem arises when a crossover filter is applied to a channel and no compensation is made to the adjacent channel reproducing the next lower or higher frequency range. Those two channels are now misaligned due to the phase distortion inserted by the crossover filter. This also happens when making equalization adjustments close to a speakers crossover point.


*Phase shift due to DSP latency*

Digital sound processors typically allow you to make a wide range of adjustments to crossover points and equalization without inserting any phase distortion, so modify to your hearts content. 

But here is the problem: Digital sound processors have digital latency. In other words it takes time for the information that enters a processor to be processed and come out the other side. The more processing (equalization, crossover) you do the longer it takes for the information to be processed. The amount of delay added due to latency is a substantial amount that can through a big wrench in your plans. What often happens is that you get your basic crossover points set up and then proceed to tweak the equalization time alignment and crossover points. Every time you make an adjustment you are inserting an extra unknown amount of delay into that channel. That extra delay amounts to real world phase problems in relation to all of the other channels in the system. 

The good news is that you can simply add additional delay to all of the other channels in order to compensate. The bad news is that this can become complex quickly.


*Points to remember*

Head units with DSP (most all of them) are equally affected
readjust your time delay if changing crossover slopes and after any major equalization. Don’t assume that a better or worst sound is due to the slope change alone; phase shift relative to the rest of the channels is likely a contributing factor in any change that you might hear.
It's advisable to start with physical measurements for your delay settings but due to your processors latencies know that those settings will change 

*Phase shift due to speaker/enclosure resonant frequencies * 

speakers have a shift in their phase response near the resonant frequency and the enclosure that the speaker is in alters the characteristics of that phase response. This particular topic can get quite complex and lengthy so I am only going to address the basics. 

Don’t assume that your subwoofer is the farthest speaker form the listening point and therefore not add any delay to it. Due to the likelihood of phase shift in the low pass/band pass crossover point and the nature of how the subwoofer enclosure interacts with the vehicle; it is advisable to add an equal and extra amount of delay to all channels including the subwoofer. This way you have the option of both adding more delay or subtracting delay from the subwoofer. 

*Mid Bass Phase Shift*

Be aware that mid bass speakers are typically played at or near their resonant frequencies. Therefore phase at the lower limit of the mid bass speaker is potentially not the same as it is at the upper frequency limit of that same speaker. You will want your subwoofer to be phase aligned with your mid bass speakers if you expect to get optimal mid bass response. 

Mid bass speakers should be phase correct with the subwoofer and not necessarily time correct with the midrange drivers (most of the time). 

This is a general rule and does not apply to all configurations but the science behind it does apply to all in vehicle sound systems. This can be a confusing idea for some people to wrap their head around. I know it was for me when I first discovered it. So think about it like this: for powerful low frequency response including mid bass, all speakers producing that passband must be sonorously creating positive and negative pressure waves. This is somewhat unique to car audio because all speakers are within a common pressure chamber (the car). Human perception of sound is not affected much if any in this low frequency range. Time alignment for the purposes of imaging and staging do not become relevant until a much higher frequency range. 

*Note:* don’t mistake the relevant range for time alignment for the range at which humans begin to perceive location due to frequency. This is a different phenomenon entirely. 

If you time align all of your mid bass speakers individually in relation to the listening position, the speakers will all be out of  phase with respect to the subwoofer. 

*Auto Tune*

I have yet to see an auto tune device that sets delay with a discrete algorithm for the mid bass channels. This leads me to believe that there is no compensation engineered into these devices and that they all set delay based on time arrival alone. I don’t know that there is not an auto tune that has this compensation on the market but it does explain a thing or two. Horribly thin sounding systems that image well. Lots of them out there.


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## RedRaider (Mar 8, 2013)

sub'd


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## bkjay (Jul 7, 2009)

Sticky!! good stuff Sir.


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## nickt (Sep 22, 2013)

Is there a way to detect the problem and to solve it?


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## diy.phil (May 23, 2011)

good stuff!
everybody please +rep the OP Speakermakers

are you in Lincoln near rocklin/Roseville?


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## Hoptologist (Sep 14, 2012)

Oh man, you're right by Knee Deep Brewery! Some of the best, hoppiest beers on the planet. Have you visited the new tap room yet? Next brewery trip I take up north I'm definitely stopping by there, and I'd like to check out your shop as well.


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## Finesa (Aug 12, 2013)

This is one of the reasons why buying a component set from the store is not the way to go unless it is designed specifically for your car and the manufacturer has actually tried to time-allign. In home speakers, about 75% have poor phase tracking, I have measured enough. Car audio - that very much depends on loudspeakers position in the vehicle and sitting position ( sweet spot ). If your installer puts the tweeter 0.5cm behind the needed location, phase tracking would be bad, probably over +/-20 degrees. Does a phase coherent speaker sound better - yes it does. This is often the most ignored feature in car audio design and all you will have is poor imaging and often dips in response due to close opposite phase behaviour.
Achieving time alignment requires the use of measuring equipment and you should start from the tweeter to obtain the initial signal and lock it for the rest of the speakers. Phase tracking can be monitored and should be close, no more than 5 degrees in the crossover region. Agood indication is the presence of a deep null at the crossover frequency, for car audio I would be happy with a null of -25db considering all the reflections and obsolete information below 1.5Khz ( usually ).


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## robtr8 (Dec 6, 2011)

Is there an APP for that?


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## turbo5upra (Oct 3, 2008)

sub'ing


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## quietfly (Mar 23, 2011)

Sub'd


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## rton20s (Feb 14, 2011)

Hoptologist said:


> Oh man, you're right by Knee Deep Brewery! Some of the best, hoppiest beers on the planet. Have you visited the new tap room yet? Next brewery trip I take up north I'm definitely stopping by there, and I'd like to check out your shop as well.


Do you talk car audio stuff in your beer forums?  

Sub'd as well.


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## cajunner (Apr 13, 2007)

how about this:

if midbass phase is different for every mounting location, why is it supposed that tuning for the driver is optimal?

tuning all drivers to create the pressure peak and null in the acoustic center of the vehicle, and locating the subwoofer's output in that location, should result in the most congruence, or synergy of wave propagation and negate many issues with subwoofer phase interfering with many different propagation points around the vehicle.

true, or poppycock?


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## Speakermakers (Nov 9, 2013)

cajunner, can you rephrase that? I am not quite sure what your question is and want to get my answer right.

Beer is good in all places. I live in Lincoln but the shop I work at (Car Audio Innovations) is in Roseville. Going to have to check that brewery out!

There are software based time arrival tools that work in the electrical, digital, as well as acoustical domain. They typically use a single channel of your computers sound card and loop the second channel of the sound card back to the mic input so that frequency response and latency of the sound card it's self can be automatically accounted for. I have briefly looked in this as a solution but have yet to find it to be a necessity (would be nice to have). At this point a more practical solution for myself is a simple three step process.


Be aware of each point of delay
Add an additional & equal amount of delay to each channel
Re-adjust delay throughout the entire tuning process

Fact of the matter is that no matter what your process is, you will have to jump back and forth between delay adjustments and equalization if you are going to get it right.

For vehicles that come in my shop that do not pay for a real tuning (and won't because they are just not into that sort of thing) I have some short cuts that get me real close in just a few minutes. This happens when I sell a deck that has TA built in but the customer is more concerned about the Bluetooth or Nav.

There are even more variables than the four that I have pointed out (those are just the four big ones). Other variables would be:

multiple speakers on the same channel but in different locations (dash & door, or mid & tweet with a passive)
The acoustic center of a speaker is not necessarily at the dust cap and can move back and forth with frequency
Rogue yet prominent reflections can change the optimal delay for a given speaker location

Last but not least. I am new to this forum and as a rule I generally do not participate on forums. So I might be a bit ignorant. What the ef is Sub'd?


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## REGULARCAB (Sep 26, 2013)

Speakermakers said:


> Last but not least. I am new to this forum and as a rule I generally do not participate on forums. So I might be a bit ignorant. What the ef is Sub'd?


Sub'd is subscribed. If you post in a thread you will get email updates anytime anyone posts something.


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## Speakermakers (Nov 9, 2013)

Thanks! My OCD dictated that I either figured that one out or stick a needle in my eye. Good save.


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## Finesa (Aug 12, 2013)

Speakermakers said:


> cajunner, can you rephrase that? I am not quite sure what your question is and want to get my answer right.
> 
> [*]The acoustic center of a speaker is not necessarily at the dust cap and can move back and forth with frequency


I think he is talking about time allignment of subs to front components, correct me if wrong. My two pennies on this: It really depends on crossover frequency ( sub to woofers ), if below 80Hz, I would generally say that you won`t feel phase due to psychoacoustical reasons. Bass is omnidirectional under such frequencies and if you are noticing the sub being behind you, then your crossover is too high.

Acoustic center is different for different speakers. Assuming it is at a specific location is a mistake, especially if trying to measure it with a ruller or tape to offset tweeter, for example ( common ). With many woofers it is around the voice coil, tweeters - somewhere behind the voice coil but time domain should be measured to obtain proper allignment.

Nice thread by the way, I never encountered an installer until now that would talk about phase.


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## Kevin K (Feb 11, 2013)

Per the sub, phasing plays a big part. I'm using 70 at 24 and did the spl measurement with mid for lowest point and flipped phase and very pleased with the results. You could hear as well as see the meter when phase was just right. Very neat.


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## cajunner (Apr 13, 2007)

it's a compliment to your thread if people subscribe to your thread, sort of an acknowledgement that you've created interest beyond the normal routine.

what I'm wondering, is whether or not the acoustic center, or the place where mapping, particularly bass contour mapping, is highest, can be manipulated by time and phase changes in the electronic/digital domain.


my "perfect" response vehicle would be one where if I have midbass array of front and rears, timed to create a positive gradient in the region where digital phase can be evenly applied, you would be able to create a perfect overlap of the sub's phase and in all the harmonics of the sub you would not produce a single artifact of non-linear modes that make the sub's output apparent, or distinct from the mid bass array's output.

I believe this is the principle used in the magic bus, where the general middle of the air space, or "acoustic center" is where the subs are aligned, and although many decay artifacts and harmonics are absorbed through diffusive devices or resonators, the idea is that the subs do not betray the initial phase set by the midbass.

now, it's easier with a vehicle where there are only two midbass producing "waves in the pond" but the general bane is that subwoofers turned up loud, make harmonic distortion that ruins the stage.

if you could map the interior you'd see areas of midbass drop-outs and peaks, and when you combine extra drivers, vis a vis lycan's theory of 'cone of confusion' we are still within Haas' dimensions, so I feel like when people 'get it right' by flipping polarity, it's just that they move the contours of the bass mapping so that there's more output, ie, a peak area, in the front left, seating position.

I think that DSP has come far enough that control of the vehicle's resonant modes from first bounce at the floor/wheelwells, second bounce of console/tranny hump, third bounce across the vehicle, etc. can now be accounted for, in the same way that they are now able to map horn geometry. It used to be where Fast Fourier Transform analysis, and FEA let you see the "air" above a driver, and that was used to determine break-up modes in substrates, but I think the time has come to be able to go further.

my idea of having the sub's output as equidistant from all other boundaries, is because of the harmonic distortion at high drive levels, but I've never heard a true low distortion, or servo controlled subwoofer in a vehicle so I may not be on the right track here. It has been an observation of mine, maybe others not so much. Absolute phase, after reflection control and all the various ripples from IIR filters and crossover manipulation, is hardly necessary to have great sound, but if one can approach the maximum in the mid bass, it would certainly be the bees knees of car audio. That impact you get in only certain cars, where the tactile sensation on the skin is felt from a few feet, the same as when you put your hand directly above a single driver, is a pretty rare bird.

message boards are great for generating ideas and sharing knowledge, and sometimes just to muddy the water if possible.


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## cajunner (Apr 13, 2007)

and I probably should point out that this idea flies in the face of conventional theory of boundary loading and placing the sub at the farthest corner of the vehicle, to produce "phase" that doesn't cancel in the driver's seat.

it's not an SPL derivative, it's designed to make the sonic signature of an acoustic recreation, or stage and not a hair trick maneuver.

after seeing what others are doing through sound propagation/transmission through solid materials, (the car body, seat frame, etc.) and timing those arrivals to either shorten the perceptual pathlength of long distanced drivers, or create tactile confusion so localization doesn't occur, it reminds me of the manipulation of phase, and acoustically, the manipulation of the acoustic space or scene. The one where lycan supposed that a cone of confusion could trick the mind into believing that acoustic sense, and scene.

I hardly listen to music so perfectly dedicated to the recording and preservation of the acoustic scene, that it is only rarely I feel it worth pondering. If I ever get into these jazz/folk recordings and that ethereal female voice trance some latent audiophiles appear to be delivered by, and not from then maybe it would be worth more time...


no, I give it the twist and slam, classic rock recordings loud enough for the neighborhood to know what southern fried is all about, but it's fun thinking of ways to confound the senses and create the audio magic.


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## Speakermakers (Nov 9, 2013)

Well I don't have time to fully answer the questions because I am on my way to work but I will make a more complete response soon. For now I just want to point out that the frequency range at which you perceive the location of a sound due to time arrival has nothing to due with your ability to locate or not locate a sound due to its frequency. These are two different things. 

Also I would like to point out that changing delay will have no effect on a vehicle's resonant modes or reflections. The only way you can change this is to relocate the speakers in the vehicle.

Merry Christmas everyone!!!


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## fcarpio (Apr 29, 2008)

Wouldn't this problem be corrected during tuning? An RTA (impulse response and sweep) will tell you whether you are in phase or not, and it will do so acurately.

Please correct me if I am wrong.


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## cajunner (Apr 13, 2007)

but that's sort of my proposition.

the resonant signature of the vehicle is most affected in the region of the transition between waves compounding and that of the pressure differential, and it's marked on either side by the Schroeder frequency of the resonant space.

this region is in the same region as kickdrum fundamental notes, which you'd like to hear in the center of the stage, because it's drums that are mono...

anyways, getting away from frequency dependent source material, what I'm saying is it's possible to tune the mid bass and the subwoofer to be in phase, easier if the sub is in the middle of the vehicle, or perhaps just equidistant between the two most prominent nodes as reflections or compression artifacts, from the windshield/dash or footwell space, may be enough to suppress the localization of the sub... but then, most sub content is mixed mono anyway, so the center of the vehicle seems to be the most natural place to "hear" it coming from anyway...


it's that darn transition that gets hammered, you've got wavelengths that can be measured in feet, and getting those to "sync" is where your mid bass meat is, while the inch-sized wavelengths, aren't as important.

And that does sort of coincide with the localization/Head related transfer function labels of which frequencies, matter and how.

If you get the mid bass right, you've won. That's pretty much it to me, and to do that you need a lot of DSP in an off-set listening position, maybe I didn't do well enough explaining..


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## Speakermakers (Nov 9, 2013)

Another quick reply while on my coffee break.

Subwoofer in the center of the vehicle is a situation that can look good on paper but does not work out in real life. I have tried this and the end result is that you experience massive cancelation across a wide band of frequencies. The best placement for a sub is at an extreme end of a vehicle. Fortunately this works out well because your head is located above an in front of center. The result is that you experience a nice rise in gain due to the reflective boundaries and the outcome is very predictable.

I have built many large center console enclosures for double cab pickup trucks and I have learned the hard way that the sound must be directed to either the front or rear of the enclosure. If the sound is allowed to radiate equally in all directions there is massive cancelation. You end up with poor, uneven bass response that sounds wildly better when the windows are rolled down.

Never forget that you are inside of a big reflective enclosure that is many times smaller than the sound waves that you are dealing with.


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## Speakermakers (Nov 9, 2013)

Also RTA results are not nearly definitive enough to adequately identify phase problems. You must use that information as one of many tools.


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## PsyCLown (May 17, 2013)

Damn, my head hurts a bit.

The few things I understood were quite interesting - I have a loooot to learn still. 

Shall be checking in on this every now and then, learn as much as I can and then try apply as much of it as possible later on.


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## cajunner (Apr 13, 2007)

well, I disagree with the DSP latency issue, and think you're possibly confusing it with IIR and FIR filter equalization...

I don't believe the amount you adjust one channel in time alignment, causes a shift in latency through the processor affecting the other channels. Electricity is moving faster than that. Now, in the acoustic space the other channels are affected by amplitude and time domain changes but it's not the fault of the processor's lag, as you "load up" the DSP with changes.

I'd have to see some sort of corroboration from the geeks, sorry.

I liked your statement about how each mid bass speaker has a different phase in relation to the subwoofer, but it's also possible that in frequencies that are 5 to 10 feet long, a couple of inches either way isn't going to be so noticeable. And you could have 2 pairs of speakers equidistant to the listening position, and the subwoofer actually located by phase, at an equivalent distance to 1 pair, so out of 5 speakers playing the same frequencies due to overlap, 3 of them could be phase coherent without any DSP at all...


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## Hanatsu (Nov 9, 2010)

fcarpio said:


> Wouldn't this problem be corrected during tuning? An RTA (impulse response and sweep) will tell you whether you are in phase or not, and it will do so acurately.
> 
> Please correct me if I am wrong.


I agree. The acoustic phase data can be extracted from the IR. Phase data cannot be extracted from RTA/Pink noise though. Use SineSweep or MLS.

"Phase distortion" as in deviations in phase vs frequency is linear distortion and is more commonly known as group delay. The audibility threshold varies vs frequency. We are fairly insensitive to group delay in the lows fyi, it's NOT the primary source for sloppy midbass unless the GD is over the roof (beyond 1-1,5 cycle of given frequency).

Focus more on the acoustic data and less of what the electronics might do... imo. Many issues can be fixed with a DSP, other issues as in phase shifts around Fsc and due to port gain (vented boxes) are "install" related and cannot be fixed without physical modifications. A slow increase in GD in the lows are very common and can often be disregarded as an issue in most situations. 

Sent from my Samsung Galaxy 3 via Tapatalk.


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## cajunner (Apr 13, 2007)

Hanatsu said:


> I agree. The acoustic phase data can be extracted from the IR. Phase data cannot be extracted from RTA/Pink noise though. Use SineSweep or MLS.
> 
> "Phase distortion" as in deviations in phase vs frequency is linear distortion and is more commonly known as group delay. The audibility threshold varies vs frequency. We are fairly insensitive to group delay in the lows fyi, it's NOT the primary source for sloppy midbass unless the GD is over the roof (beyond 1-1,5 cycle of given frequency).
> 
> ...


nice post.

I think the issue of phase coherency is more subtle than say, tonality or stage width, but it affects every aspect of an install's ability to deliver the goods.

group delay is another of these "sometimes it counts, sometimes it don't" measurements, and being able to factor in all the data and conclude a threshold beyond which, you have to literally "fix" something to get it into a more acceptable range, isn't always apparent.

the same goes for the impulse response, I think.

if you have the right methods of achieving this "coherence" through phase, of all drivers in a system "IN THE HOME ENVIRONMENT" you're on a scale of Thiel, Dunlavy and others who did it passively first, but still in good company.


You're gonna do it in a car, you're dealing with reflections as much as that "good, clean usable signal" and I won't say it's hopeless or it doesn't matter, but it's not on the level of getting a good l/r equalized match, or putting the kibosh on vibrating parts of the car.

I'm hoping the DSP coders out there understand this challenge and bring to market the product that fixes the phase ailments, but I haven't seen it all come together just yet.


2 Raspberry Pi/1 Sharc X 1 Lake... :blush:


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## Hanatsu (Nov 9, 2010)

To correlate what we measure with what we hear is a topic that I've been interested in for some time now. After repeatably tested different subwoofer configurations and EQed them to the same FR is now find group delay irrelevant in most cases unless it's really high. I'm convinced what we hear is dominated by the linear domain distortion issues. Group delay falls under this category but I think it's getting less important as frequency goes down, roughly below the schoeder frequency (in a car) group delay can be quite high as indicated by both mine and other tests I've read (link below). Other "linear aspects", most importantly how good the sound power response is - is far more important imo. Home audio and car audio is as similar as it is completely different worlds, some analogues originating from the home-audio-audiophile-planet simply cannot be transferred into car audio. Phase coherency is such a wide concept to begin with, people are worried about phase shift from tall order IIR filters and talk about FIR linear filters to be the holy grail of audio when there 1000 other issues that degrade the audio FAR FAR FAR more than any phase shift of a crossover could ever do - this is related to the environment, NOT the electronics. As sooner people realize that it's the environment that is the main bottleneck in their car system we can start attacking the real issues - the acoustic ones. 



> The audibility of phase distortion in audio signals was also highly dependent upon individual ability, although for statistical analysis individual data was not considered. For example, while most test subjects were very good at recognizing what was in general perceivable as phase distortion such as the impulse and the 70 Hz sawtooth wave, a few others had greater difficulty. Specifically, a few subjects seemed to hear clearly the presence of phase distortion in the jazz-vocal test signal for the headphone listening test, while a few test subjects seemed to perceive phase distortion better than others during the loudspeaker listening test.
> 
> Table 5.1 indicates that even for the headphone listening test, phase distortion audibility was of very subtle nature. This is surprising, since there exists gross phase distortion present in the all-pass filtered test signals. In this test, human ears seem to be tolerant of even large phase distortions in audio signals.


From audioholics:



> 1. Given the data provided by the above cited references [in the original article] we can conclude that phase distortion is indeed audible, though generally speaking, only very subtly so and only under certain specific test conditions and perception circumstances.
> 
> 2. The degree of subtlety depends upon the nature of the test signal, the dB SPL level at which the signal is perceived, the acoustic environment in which the signal was recorded and/or played back as well as the Q (Quality factor; a unitless factor of merit) & fo (crossover frequency, Hz) of any filter networks in the signal stream. Certain combinations of conditions can render it utterly inaudible.
> 
> ...


Chapter 5

Audibility of Phase Distortion

Human Hearing - How We Hear and Perceive Audio Quality Part 4 | Audioholics

Discussion of Group Delay in Loudspeakers

Phase, Time and Distortion in Loudspeakers

Can't find that subwoofer GD test I wanted to show though :<


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## Hanatsu (Nov 9, 2010)

Speakermakers said:


> Also RTA results are not nearly definitive enough to adequately identify phase problems. You must use that information as one of many tools.


I have a problem with this. First off, an RTA is a real time analyzer. It's often used together with noise and this method cannot display anything but FR (or HD/IMD if sine tones are used).

Sine Sweep and MLS measurement methods CAN accurately measure the time domain without any issues. Measuring the system with a microphone is the best way to find out issues in any system. Depending HOW you measure the results might be flawed, yes. At higher frequencies the phase data is simply not of any use, move the mic a fraction of an inch and you'll get different results due to the small wavelengths.

I have to disagree that measurements software like ARTA/REW cannot be used to identify phase problems though.


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## cajunner (Apr 13, 2007)

and the wind goes out of the sails..


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## SPLEclipse (Aug 17, 2012)

AS to the latency in DSP issue: I think this comes into play when you're using multiple components and signal routing. If you have two signals from a headunit, one going through DSP and the other straight to an amp, you'll get latency issues between the two signals. 



Hanatsu said:


> To correlate what we measure with what we hear is a topic that I've been interested in for some time now. After repeatably tested different subwoofer configurations and EQed them to the same FR is now find group delay irrelevant in most cases unless it's really high.


Did you specifically mention only GD as it relates to subwoofer tuning just because the threshold of distortion audibility is different at lower frequencies? I agree that GD is irrelevant if it's a gradually decreasing slope as you go up in frequency, but I've noticed in areas of high group delay where EQ is required to even out frequency response, distortion (in my experience usually the second harmonic) can increase rapidly. This really only applies to a peak in group delay that would signify a rapid change of phase around the peak of the spike.

I don't think the original intent of the OP was to get into linear phase design, but I'm kinda glad it did, lol.


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## Hoptologist (Sep 14, 2012)

rton20s said:


> Do you talk car audio stuff in your beer forums?
> 
> Sub'd as well.


Haha, either I haven't found a good beer forum yet or I simply prefer talking about beer with car audio people 



Speakermakers said:


> Beer is good in all places. I live in Lincoln but the shop I work at (Car Audio Innovations) is in Roseville. Going to have to check that brewery out!


Right on, thanks again for starting this discussion, lots of good posts which are mostly above my pay grade but which I enjoy to ponder over.



Oh yeah, and Merry Christmas! I better get to bed now, otherwise Santa might not come


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## Hanatsu (Nov 9, 2010)

SPLEclipse said:


> Did you specifically mention only GD as it relates to subwoofer tuning just because the threshold of distortion audibility is different at lower frequencies? I agree that GD is irrelevant if it's a gradually decreasing slope as you go up in frequency, but I've noticed in areas of high group delay where EQ is required to even out frequency response, distortion (in my experience usually the second harmonic) can increase rapidly. This really only applies to a peak in group delay that would signify a rapid change of phase around the peak of the spike.


The GD audibility threshold is different for different frequencies, different for what material is used, how it's presented and actually (according to the study I linked before) different between people. It's the least audible in the lowest octaves and most audible in the midrange.

Frequency response dips (nulls) and group delay peaks are related as group delay is a FR/phase derivation. If you try to EQ those (boost), the speaker will be overdriven at that given frequency causing an increase in HD. Since you mention HD2, it's more likely it's the speaker itself other than amp clipping which normally is mainly odd order HD. To view GD you must extract minimum phase data and view excess group delay and smooth it at least 1/12. Make several measurements and see if the GD peak moves around as you move the mic. If it does you probably found a null due to the "room". You can expect lots of those peaks in the modal region. If you find excess GD peaks in the smack of a crossover point; chances are that a phase inversion of one driver is required. 

Lots of good data can be seen in the EGD plot, you can even T/A with it.

Not sure if I interpreted your question right though... ^.^

Sent from my Samsung Galaxy 3 via Tapatalk.


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## cajunner (Apr 13, 2007)

and group delay can be a monster, if you're considering putting in reverberation and structural modes from the car body into the mix as a close cousin in determining what is distortion from the enclosure, versus what's coming from the acoustic space.

I'm not that well-versed on how the natural impedance spike at resonance, contributes to or subtracts from the group delay involved in an enclosure, such as when you've gone 6th order and there are large delays from the acoustic transfer of energy to a resonant tuning, but it's a good topic that does overlap with phase coherency details.

If you look at most amplifier outputs, they too have phase angle measurements, which means that as you go through a power cycle what comes out of the amplifier itself, can change phase.

so how does this relate to absolute phase, where you end up with the impulse response at the listening position being time correct through every driver's portion of the signal?

I don't know, really. 

but treating the car interior like an anechoic space, and then substituting driver physical measurement, or even the acoustic center measurement through impulse response analysis, is what seems to be the appropriate methodology for getting phase to conform, but is there really a huge difference with these minute changes, or is it within the mind's nimble process to exclude most "gaps" in the coherent modeling, and "fill in" whatever is necessary?

I think that this goes towards the psychoacoustics of it, and in these frequencies where localization is first detectable, say a fundamental of 80 hz to be within the established norms, does every speaker have to coincide, or is it just the engineer, the OCD part of finding audio nuance, making us believe that this is necessary?

I have had really enjoyable sound systems where none of the observed methodologies were followed, where huge delays and unfiltered, raw audio signal went straight into the air, unfixed by DSP machines and it was still pretty darn good...


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## Hanatsu (Nov 9, 2010)

Absolute phase is inaudible. 

The cumulitive effects of several forms of distortions might be an issue, I haven't seen any studies adressing this topic. BP enclosures can have severe phase distortion, especially dual vented. The higher port tuning you use the more audible it gets (GD).

Look at the waterfall plot displayed over 3000ms in the modal region in a typical car... some frequencies linger for 1500-2000ms in the lower octaves. I believe THAT's one big issue in car audio, the dark side of cabin gain :


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## SPLEclipse (Aug 17, 2012)

Hanatsu said:


> The GD audibility threshold is different for different frequencies, different for what material is used, how it's presented and actually (according to the study I linked before) different between people. It's the least audible in the lowest octaves and most audible in the midrange.
> 
> Frequency response dips (nulls) and group delay peaks are related as group delay is a FR/phase derivation. If you try to EQ those (boost), the speaker will be overdriven at that given frequency causing an increase in HD. Since you mention HD2, it's more likely it's the speaker itself other than amp clipping which normally is mainly odd order HD. To view GD you must extract minimum phase data and view excess group delay and smooth it at least 1/12. Make several measurements and see if the GD peak moves around as you move the mic. If it does you probably found a null due to the "room". You can expect lots of those peaks in the modal region. If you find excess GD peaks in the smack of a crossover point; chances are that a phase inversion of one driver is required.
> 
> ...


Yup, that's what I was asking. I use GD for the very reasons you mention (it's more intuitive for me to look at that at a glance than anything else) and by your first post on the subject it seemed like you were discounting it as a method for tuning.

cajunner: I've always understood absolute phase to be, in essence, not a "real" thing as it has no frame of reference.


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## cajunner (Apr 13, 2007)

I have to include this:

equalizers change phase, and the agreed upon principle of FIR filters that maintain phase relationships through changes in amplitude, are supposed to be superior to that of IIR filters that do not.

digital equalizers are almost all IIR filter types, except for the professional units, the Dolby/Lake or the Rane, or whatever.

when you get to the really expensive stuff and pro quality, they don't use the cheaper solution.

does this mean that there is an audible difference between equalizers at the consumer level, IN A freak'n CAR?


probably, lol....

and this all related to phase, means that most of us are still one level down in the consumer equipment, no matter how expensive it is.

I feel like if phase was that important through a DSP that it made such a noticeable difference, we'd already have the state of the art in the consumer products by now. I think the real reason why it's been mostly the domain of the professional market is not especially because of cost associations, but because you just can't tell the better equalizer from the worse one, in a car's reflective interior, and reverberant space.

that's why I say it's possibly not worth pursuit, this extreme position of phase integrity through the measurement axes of drivers placed willy nilly about a car's interior.

better perhaps, to spend time on making reflection control a priority, or increasing the state of the art in wave entrapment, or resonant control circuits in the active realm.

I'd like to see more servo than FIR filters, hey... maybe we could combine those, haha.... FIR servo, egads...


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## sqnut (Dec 24, 2009)

All the science on phase is useless if it doesn't answer two simple questions. Can you hear it? and secondly, can you correct for it? If you can't hear it or if you can't correct for it, the discussion is pointless. You can only correct in the time or response domain. Anything that doesn't get corrected by this or if it isn't audible, is not worth thinking about in my book.

It's very easy to pick speakers that are out of phase. Speakers in phase give the sound its sense of unity. The sense that the sound is one entity. If you hear the sound as the sum of it's parts or if you can sort of locate the speakers, they're out of phase for sure. Once you set arrival times via impulse response, TA is something you will tweak in small increments from time to time to get back that sense of unity. Beyond this fretting about phase is irrelevant.

The thing to remember about phase is that you hear phase changes in the response domain. So if you add/reduce delay on a driver by that 0.02m/s, you will hear a corresponding nominal change in response, that's it. Conversely, a change in the response from your eq or a change of slopes / xover points etc will affect the phase. The two are linked in that sense. When you change one you affect the other. But how much of it can you hear?

Imo in a car, response is a MUCH bigger holy grail than phase. You will spend much more time dialing in response than TA. In a car phase can't be a holy grail because:

1. Theoretically it changes every time I tweak the response, every time my head position changes etc. But I will have to make some radical eq settings in the phase sensitive zone to hear phase/response issues. Other than that if you have the speakers in phase to begin with, normal response tweaking isn't going to give you the sensation of drivers falling out of phase. A tweak now and again will be more than enough.

2. I can't set arrival times at 1/3 octave.

A lot of home audio buffs will talk about speakers that are phase coherent. What they are really hearing are components in L and R speakers that are better response matched. With any pair of speakers if you measure the same driver type in L&R speaker, you will measure a difference. Let's say the average difference is 10%, now if you can get this difference down to say 5%, BAM! better phase coherence.

P.S. Merry Christmas everyone!!


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## cajunner (Apr 13, 2007)

without a background, or study of acoustic theory I think we have hit (or maybe it's just me) a threshold where the math has to enter the equation to really understand what's going on.

I remember reading about lycan's exposition on phase changes through crossovers, like how an inductor or capacitor changes phase that varies by frequency and it travels through the scale from 0 hz to infinity, but a 6 db/oct crossover component, puts like, 90 degrees of rotation into a circuit. You might only have 45 degrees of rotation in the frequency region where the circuit affects a speakers output. Or maybe even less?

And then there's the Linkwitz solution, also in an analog form. An all-pass filter moves the time delay, does it also move group delay, is this related to phase? How does this work, does the changing speaker phase (inductor?) of the voice coil, also coincide with the opposing circuit, creating absolute phase, and not even in the digital?

And what about crossover designs that have odd orders of slope, like the 18 db/oct variant, when matched with the 6 db/oct of the other driver? Is acoustic phase represented here, or is it just acoustic crossover characteristics we define for?

there's still a lot I don't quite grasp yet, but short of a college course or two, I may not get it on the forums.

still, it's easier to think through, basically each time I try to process it.


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## 14642 (May 19, 2008)

I have never in my life read a thread quite as incomprehensible as this one, but that usually happens when one begins talking about phase.


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## cajunner (Apr 13, 2007)

Andy Wehmeyer said:


> I have never in my life read a thread quite as incomprehensible as this one, but that usually happens when one begins talking about phase.


why is it so hard to understand?

the little signal line goes up, and it goes down. Up, and down. It's not complicated, I mean it's not like it's economics.


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## aj1735 (Feb 27, 2011)

Sub'd. I have a lot to learn yet.


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## fcarpio (Apr 29, 2008)

cajunner said:


> without a background, or study of acoustic theory I think we have hit (or maybe it's just me) a threshold where the math has to enter the equation to really understand what's going on.


To me this is a hobby and I would hate to make it a science project.


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## quality_sound (Dec 25, 2005)

Except it IS a science project. 

Sent from my Moto X using Tapatalk


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## 14642 (May 19, 2008)

It's incomprehensible because it has turned into an exercise in digging up the longest words and the most preposterously constructed sentences to describe a perceived evil that no one on this thread has a clue about. 

It's all a bunch of theoretical mumbo jumbo that isn't even based on a measurement that's relatively easy to make--AN IMPULSE RESPONSE MEASUREMENT AND AN ATTENDANT FFT WITH A PHASE DISPLAY.

This whole thing is tantamount to a discussion of the flavor of an apple conducted by a bunch of people who have never, in fact, tasted one, but have read every backwoods apple-tasting forum post by everyone who's ever had anything to say about apple tasting despite having never tasted one. 

Move along, boys, nothing to see here.


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## rton20s (Feb 14, 2011)

Andy Wehmeyer said:


> Move along, boys, nothing to see here.




(Min. letter requirement fulfilled.)


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## cajunner (Apr 13, 2007)

see, it's not that I write preposterously worded/constructed sentences, on purpose. It's just the way it comes natural, so if it's difficult to read, if it's that my questions aren't sensible enough, if I turn normally even-keeled citizenry into irascible, perturbed, and incommunicado..

please excuse the intrusion, as it's not pointed in anyone's particular direction.


Phase, polar plots, and the best, power response or sound power.

obsequious by nature, impure thoughts.


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## bkjay (Jul 7, 2009)

bkjay said:


> Sticky!! good stuff Sir.


Unsticky !:laugh:


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## Hanatsu (Nov 9, 2010)

Andy Wehmeyer said:


> It's all a bunch of theoretical mumbo jumbo that isn't even based on a measurement that's relatively easy to make--AN IMPULSE RESPONSE MEASUREMENT AND AN ATTENDANT FFT WITH A PHASE DISPLAY.


...which I pointed out in post #27


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## 14642 (May 19, 2008)

Yes, you did. One glance at the measured phase of a car audio system should convince one of two things: 

1. If phase coherence was important for intelligibility or anything else, for that matter, there would be no point in even attempting to listen in a car. It's a mess, yet it can be made to sound fine, even though the phase is still a mess.

2. That there are many other things to tackle first in improving the sound of your system. 

I consider any discussion of phase coherence in a car to be about as useful as one of those LSD induced metaphysical discussions. It only seems like enlightenment when one is tripping. Once the trip is over, you realize that it was all ******** and being incoherent is NOT a window into another dimension except for the dimension of being F&*ed up and that dimension isn't all that useful. Sometimes fun, but not to be taken seriously.


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## cajunner (Apr 13, 2007)

but I don't want any FOP! I'm a Dapper Dan man...


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## REGULARCAB (Sep 26, 2013)

Andy Wehmeyer said:


> This whole thing is tantamount to a discussion of the flavor of an apple conducted by a bunch of people who have never, in fact, tasted one, but have read every backwoods apple-tasting forum post by everyone who's ever had anything to say about apple tasting despite having never tasted one.


:laugh: this is going in my signature. Not that i agree or disagree or have any idea what was said in this thread but just because its damn funny.


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## sqnut (Dec 24, 2009)

Well I did call BS on the scientific mumbo jumbo and hair splitting on phase. Threads like this also highlight how far removed some folks are from good sound in a car. If you seriously think phase is the holy grail or that phase issues in a car will hold you back from great sound, then you need to learn how to tune.


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## Hanatsu (Nov 9, 2010)

I love "phase" threads... xD

Sent from my Samsung Galaxy 3 via Tapatalk.


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## sqnut (Dec 24, 2009)

Hanatsu said:


> I love "phase" threads... xD
> 
> Sent from my Samsung Galaxy 3 via Tapatalk.


Wonder how far the thread would have gone if Andy had not intervened.


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## cajunner (Apr 13, 2007)

to be fair, Andy has been the expositor of phase in his explanations of certain circuits in the MS-8.

the bass recombiner circuit, the way a digital logic matrix/logic matrix digital/matrix digital logic, or whatever, takes out the bass and that neat circuit that makes the levels follow changes in crossover points, there was some mention of phase in there that may have not been easily digested.

phase permeates a lot of audio, and to understand what the cumulative effect is, besides the obvious of doing an impulse measurement to see what the total derived at, is...

well, I and I suppose many others would like to know each step of the process, as eliminating these muddying additions to the "clean usable signal" is not just a fantasy pursuit of the holy grail variety, it's been observed that with each new "signal adjuster" comes a deterioration of the ess que. Knowing what is necessary (time alignment, L/R equalization, resonant control, etc.) and what is better left out of the chain, is really only possible if you understand what you've put into the signal.

It's hard to hear small changes in phase, I think there are several studies affirming this psychoacoustic phenomenon. In nature, everything is analog, there's no phase rotation within an electrically manipulated reproduction system, and sound is, for lack of a better descriptive term, "natural."

Getting as close to that "natural" is the common goal, and audiophiles seek methods to retrieve that purity with all sorts of gadgets, phase manipulation tools, being high on the list.

they even sell wire, with "coherent winding structure" for bass, mid and treble signals.

that's phase too, it's everywhere... so dismissing it's intrusion in the car environment is going to be difficult, even if controlling phase is a home audio cornerstone of audiophile precepts.


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## cajunner (Apr 13, 2007)

and in lieu of going back in off-topic, I thought I raised some decent questions.

like, why is it the most expensive equalizers and room correction equipment, have FIR filters?

my understanding of their use is that they can be made not to or won't mess with phase, and the professional market pays for that, the Lake processor is 3 grand or whatever, if phase doesn't matter why wouldn't an Audio Control piece be just as good?

why do knowledgeable people here, prefer and would own a Lake processor over say, something that Kicker puts out, if not because of the circuits used, and how they affect the sonics?

At some point, phase entering the car audio discussion is inevitable. Making out like it's just a paper tiger or untenable as the environment doesn't allow changes in phase to be audible...

how about this:

the Haas effect, says that outside of what, 15 to 30 milliseconds, you can't distinguish between two sound source locations, or wait.. I'd like to get this right.

the time it takes for one full note to move your eardrum, corresponds to a frequency, because hertz, is cycles per second.

and at an area of the frequency spectrum, the wavelengths are longer than what the Haas phenomenon says, phase is unimportant. Then, going down the scale, the wavelengths are so long, in the car interior, phase isn't important.

okay, I lost you there. Don't feel bad, I wasn't on course anymore anyway.


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## cajunner (Apr 13, 2007)

for funsies, isn't baffle diffraction, a distortion of phase?

when you radius the corner edges of a home audio enclosure/cabinet, or you make a transition panel for a horn terminus, to deal with diffraction...

and what about cardioid enclosure design, where you slow down the sound waves in a diffuse media until what comes out the back, is in phase with the front leaving a cancellation so complete that the polar plot of the speaker is all in front?

if phase is not important, why is there a golden ratio at all, for loudspeaker enclosures...

isn't that relevant? aren't those things attempts to control phase from a physical standpoint?


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## 14642 (May 19, 2008)

cajunner said:


> for funsies, isn't baffle diffraction, a distortion of phase?
> 
> when you radius the corner edges of a home audio enclosure/cabinet, or you make a transition panel for a horn terminus, to deal with diffraction...
> 
> ...


Yes, many of these are real. They are, however, small parts of an audio system. I object to the focus on it and the lack of rigor in really understanding it. It's a buzzword used to obfuscate the real issues and sell a bunch of ******** that, even if substantiated, can't be understood by most. It's just the new snake oil. 

The real problem is that it isn't the real reason that systems don't sound good. Systems don't sound good because speakers are in the wrong places, in the wrong enclosures and too may people have no idea how to use even a basic EQ. 

FIR filters are more expensive because they are more computationally intense if resolution at low frequencies is important. They also make setting the EQ simpler for an algorithm. Measure, invert, done. They also can EQ phase, but the EQ is only valid for a single point in space, especially at high frequencies.


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## cajunner (Apr 13, 2007)

Andy Wehmeyer said:


> Yes, many of these are real. They are, however, small parts of an audio system. I object to the focus on it and the lack of rigor in really understanding it. It's a buzzword used to obfuscate the real issues and sell a bunch of ******** that, even if substantiated, can't be understood by most. It's just the new snake oil.
> 
> The real problem is that it isn't the real reason that systems don't sound good. Systems don't sound good because speakers are in the wrong places, in the wrong enclosures and too may people have no idea how to use even a basic EQ.
> 
> FIR filters are more expensive because they are more computationally intense if resolution at low frequencies is important. They also make setting the EQ simpler for an algorithm. Measure, invert, done. They also can EQ phase, but the EQ is only valid for a single point in space, especially at high frequencies.




it's just one thread, I think there's a little leeway for some concepts to overlap, and some undue importance or attention paid to the phase pet industry or phase gods.

I know you're steeped in separating the boolsheet, from the snake oil to offering useful product as a part of your job, but I don't have any particular level of professionalism to live up to, I'm sort of a free-floater when it comes to radical ideas and making audio a fascination, instead of a truth serum exercise.

I always enjoy when the concepts presented become real, lycan did that a lot with his useful models of electrical representation of acoustics, I never did more than look at his equations and problems solved through math, I'm not at that level of involvement.

so FIR is more expensive to implement. My understanding is that their use over IIR is they are actually better sounding circuits, because they don't wonky with phase.

Phase to me, isn't nearly as important as all the other stuff, but it's the last thing that people have, now that they're at the end of their bill of materials, or wishes on their personal audio quests, to focus on. A fool's errand, to some and the holy grail to others. It may be like people who compete in swimming and shave their eyebrows. Or it may be that phase itself, like natural phenomena in the physical world, is sought out for it's authenticity to the source.

either way, I hope you don't get too upset with those of us who are trying to decipher what sticks to the wall, and what slides down for ourselves. Your fact-finding posts may be what helps keep the vessel moving, even if we're just sailing on a sea of what-if's.


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## 14642 (May 19, 2008)

^^ I just get tired of cleaning the **** off the wall.


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## cajunner (Apr 13, 2007)

Andy Wehmeyer said:


> ^^ I just get tired of cleaning the **** off the wall.


although largely unsung, your janitorial duty is a voluntary endeavor...





that is greatly appreciated.


it's strange that you were tasked with fielding questions from the SMD pocket, over a distortion circuit?

remember how lycan's virtual driver concept, couldn't be managed by DSP modeling? something about how there were major reflection issues, creating minor reflection issues, all of it, pertaining to phase?

do you still feel the same way? Has the state of the DSP art, gotten any further along in developing a room correction algorithm that gets better results than what was done with the Audyssey and MS-8 circuit designs, and that approached your VST plug-in/MS-2 software, is there anything Audiofrog being developed, or in the pipeline, then?


sorry if I approach you as our resident mini-celebrity, guru but you did take a towel to the wall...


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## jtaudioacc (Apr 6, 2010)

where'd the OP go?


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## 14642 (May 19, 2008)

OK. I owe everyone here an apology for the freak out. I sent the OP a message and tomorrow when I have a few minutes, I'll try to address his questions and suggestions directly. 

I'll be sure I've remembered to take my medicine before I read Cajunner's posts.


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## cajunner (Apr 13, 2007)

mmm..... medicine.


(Homer Simpson?)


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## bkjay (Jul 7, 2009)

I want to say thank you Andy. I don't know much,but people like me need to know the stuff that matters, and not of bunch crap that you only see in a lab. Please keep giving us the stuff that really matters.


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## SPLEclipse (Aug 17, 2012)

I don't think it's right at all to shrug off phase and decry it as "unimportant", but that depends on how you approach the intent of the discussion. If someone were to pose a question such as "I want my system to sound good, how do I tune to get my phase perfect?" that would seem obviously silly. Having a basic understanding of phase, however, helps people intuitively understand things like why reversing a couple wires on your subs can make you have more amplitude in the midbass frequencies, or why when you increase the delay on your speaker by "x" amount the center of the soundstage pops into focus, or recognizing a combing pattern when tuning and being able to narrow it down to a certain reflective surface.

I don't think the OP was ever intending to promote some idea of tuning _for _phase, but tuning _because_ of what phase interaction can result in.


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## Speakermakers (Nov 9, 2013)

Well said!


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## Speakermakers (Nov 9, 2013)

OK, the Original Poster is back from Christmas vacation.

Wow! you guys really know how to fill a thread up with a bunch of.......umm? ..... Stuff?

Let me put a finer point on what this post was about.

1. You can easily identify and account for four common and audible phase inconsistencies in a typical car audio system. Please reread the original post and try to forget some of the off topic word bending that came afterwards.

2. Phase shifts due to DSP latency occurs as a result of processing. Not how long it takes electricity to follow a path. DSP latency is real, and amounts up to large amounts of sudden phase shift between channels. That's a fact. 

3. Group delay is a great way of displaying how phase changes with frequency for a given signal path and I have no problem with that. It's true that smooth variances in group delay have little to do with how humans perceive sound. That's why I did not refer to group delay as the problem. The problem occurs in mid bass channels when a mid bass speaker has a large shift in phase due to the crossover frequency being to close to the drivers resonant frequency. The problem does not reside on that channel. The problem is when you mate that channel with a subwoofer that has a very different phase angle.

4. If phase is not an issue, and can not be heard; stop wiring any of your speakers in phase. Just don't pay any attention to polarity. Let's see how that works out for you.

Now I would like to add a few things:

I am not talking about splitting hairs with respect to phase coherency. I don't know where that came from. I am not purposing that anyone can achieve perfect phase response across the entire frequency spectrum within a car. Didn't say that.

The holy grail of car audio is achieving a friendly frequency response as well as a friendly phase response from multiple speakers from within the confines of the reflective environment that a car is. Right Andy?

My experience in this matter does not come from a few systems that I have built, or a few articles that I have read online. Nor does it come from a typical experience as a professional that has built dozens or hundreds of fully active systems with top of the line equipment. I tend not to speak out about things that are in theory. I rank in the thousands with very notable builds in the mix. 

I love and embrace theory, debate, projects, and experiments; but make no mistake. I do not speak of speculation. That's a waist of my time. I don't have time to waist.


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## ErinH (Feb 14, 2007)

I think Andy's point of contention really comes from the angle of keeping people (especially newcomers) from missing the forest for the trees. of course phase matters, but the level that has been discussed here is taking things to the point where it seems more a competition to show off the buzzwords read rather than apply practical use of the phase elements that matter IN OUR APPLICATION. 

Have you guys really ever looked at the phase response of a single speaker in a car? Take it a step further; have you measured the same speaker 'anechoically' (quasi-anechoically) as well? That's a legitimate question. I'm not asking to be an a-hole or to get people talking off topic for the sake of looking cool. If you have, then you understand where Andy is coming from.


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## Speakermakers (Nov 9, 2013)

I agree. I avoid buzz words and intentionally maintain a continuity to the words that I do use. It's not always easy to choose smaller words but the fact of the matter is that a more basic and consistent vocabulary can be digested faster by people of all education levels. I assume that Andy was responding to other posters technobabble rants (i could be wrong). 

Properly phrasing your posting is important. In order to have the greatest effect on the masses. It's right up there with writing a provocative thread title like "Phase Coherency: The Holy Grail of Car Audio".


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## Speakermakers (Nov 9, 2013)

Besides last I checked this was the place to post ADVANCED topics. I have been gone for a couple of days so i missed out on the big word contest.


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## ErinH (Feb 14, 2007)

Speakermakers said:


> I assume that Andy was responding to other posters technobabble rants (i could be wrong).


Actually, that's what I was referencing. In the forums, there is a strong tendency for people to start speaking beyond others, sometimes intentionally, just to give some brovato to their post. From looking over this thread, that seems to be where this thread went off track, IMHO. 

It wasn't the sentence structure or the words themselves; moreso, the application of them. I'm all for pushing the science (seriously, look at all the crap I've posted over the years and all the testing I've done) but even I have to know when enough is enough. When am I posting just to post, and when am I posting something that's beneficial to the community. I liken it to people and those ridiculous round balls to lessen diffraction from their tweeters, when the pillar or dash is sitting right next to the drive unit itself. On top of that, research has shown that it's a moot point. But there's a very long thread going on about it still. In that case, I've read numerous people sacrifice an otherwise useful install just to get rounded edges on their 'baffle'. It's those kind of cases where I think we as a community lose site of the essentials of audio and strive to hit the outer space stuff just because it's new or different for us. That's fine. The problem is, people who have no understanding of even controlled directivity leap straight in to buzz worthy topics (or provocative, to use your own word) and a few months later are still asking someone why their system doesn't sound right and are switching gear constantly to achieve the level of sound they can't explain because they have no reference. There's something fundamentally wrong there. 

If that's not where Andy is coming from, then my bad. But, I will say, after reading this thread, that's where I'm coming from. 

Anyway, I myself am about to leave for vacation. So, I'm stepping back out because I don't want to be by my phone all weekend trying to reply with my stupid thumbs.


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## Finesa (Aug 12, 2013)

Andy Wehmeyer said:


> Yes, you did. One glance at the measured phase of a car audio system should convince one of two things:
> 
> 1. If phase coherence was important for intelligibility or anything else, for that matter, there would be no point in even attempting to listen in a car. It's a mess, yet it can be made to sound fine, even though the phase is still a mess.
> 
> 2. That there are many other things to tackle first in improving the sound of your system.


Should you want to experience music in its finest, you should stay away of cars, so your first point I agree with. 
Can is sound fine if phase is a mess - depends on how close the phase tracking is. The higher the frequency, the higher the sensitivity to phase, our ears are phase sensitive, especially in the mid-band. I can hear it quite well and we have done experments with a home speaker where one crossover was constructed with poor phase tracking and one with +/- 5 degrees starting from two octaves below to one octave and a half above crossover frequency. The phase-coherent speaker sounded better to all that participated in the listening test and they did not know which filter was used first.

Two is well said too.

Car environment perhaps makes some of you speak about phase irrelevancy due to the many reflections. While this is generally considerable, the cars I have measured have indicated Gating to be below 1500Hz where a home system in a small 3x5sq.m room can get as good as 400Hz. This is still not that bad and considering that crossover point is usually in the 1.8-2.6Khz range for a 6.5/5.25" midwoofer, design of a relatively phase coherent system* is possible in a car. Will do one more in January ( have to finish up the fiberglass enclosures ) and can post measurements here, if interested. One thing to note is that you will be able to usually achieve this only around the crossover region. At higher frequencies vertical lobbing will produce nulls at some spots and should you move your head forward or backwards, phase tracking will be gone, completely. Hard cone drivers would also exhibit their own null at some point ( usually around 15Khz for hard dome tweeters without a diffuser ) due to the driver radiating equally from all parts of the diaphragm which is not flat, thus causing cancellations at some frequencies that cannot be avoided.

*Relatively as in many cars drivers cannot be moved around and are bound to a fixed location where phase tracking is pursued through the crossover, thus not using the advantages and disadvantages of physical time allignment and with limited phase tracking band. Also, phase is position dependent and considering that you often move in the car, as well as vertical lobbing patterns and woofer being able to emit sound by its whole surface ( until vc decoupling is present ) which is concave...

"If you look at most amplifier outputs, they too have phase angle measurements, which means that as you go through a power cycle what comes out of the amplifier itself, can change phase."

The signal that goes through the amplifier would have a phase shift and also due to the crossover of the loudspeaker`s interaction with both the input impedance of the loudspeakers and the output impedance of the amplifier. Phase in the amp is not important as phase in multi-way loudspeaker systems as the delay on the signal in the amp would be for the whole signal, where for a two way loudspeaker system time domain for the tweeter and woofer would be different due to misplacement of acoustic centers of both drivers and phase shift induced by the crossover to each driver, thus causing the sound from one driver to arrive sooner or later than the other one. I hope you make a difference between acoustic phase and signal polarity.


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## Woosey (Feb 2, 2011)

sub'd


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## cajunner (Apr 13, 2007)

wow, there's a lot of blowback for "someone's" contribution to the thread, but I believe if you try hard enough, you'll see where my own posts say the same thing Andy says.

More things to worry about first, than phase.

But, why not talk about an advanced topic, it's still just one thread and if you're a noob and go astray in here because of someone's intense scrutinizing and contemplation of phase, then you've missed the other 35 gazillion other places where all those other things to get right, have been addressed.

Let's not assume the wordy guy is purposely trying to confuse people, or make himself look good because I assure you, he's not.


there, I'm third person, pity me now..


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## cajunner (Apr 13, 2007)

Speakermakers said:


> 2. Phase shifts due to DSP latency occurs as a result of processing. Not how long it takes electricity to follow a path. DSP latency is real, and amounts up to large amounts of sudden phase shift between channels. That's a fact.
> 
> I love and embrace theory, debate, projects, and experiments; but make no mistake. I do not speak of speculation.



this is my "speculation" because I don't have a full understanding yet, of phase. 

I'm going by things I've read, so correct me with more accuracy if you can.

the time a processor needs to "do it's thing" is about 8 milliseconds. This is most processors. There is a lag, or latency that the DSP takes, and it doesn't matter how many changes in the processor you make, no amount of extra filter changes or custom parametric q changes or whatever, is going to make the DSP have more than it's inherent latency, that 8 milliseconds is the same if you run everything flat, or you make the equalization curve look like a mountain range.

If changes in phase would cause DSP latency, wouldn't you see more lag with more computational complexity in the settings?

also, if you listen to music, the static changes you made would be harder for the DSP to "process" and the time required would increase, based on the complexity of the music. A single sine wave tone should be easier, and less computationally dense, so if phase changes mattered in creating latency, you'd see the processor "choke up" or chop the signal up, or something.



Finesa said:


> "If you look at most amplifier outputs, they too have phase angle measurements, which means that as you go through a power cycle what comes out of the amplifier itself, can change phase."
> 
> The signal that goes through the amplifier would have a phase shift and also due to the crossover of the loudspeaker`s interaction with both the input impedance of the loudspeakers and the output impedance of the amplifier. Phase in the amp is not important as phase in multi-way loudspeaker systems as the delay on the signal in the amp would be for the whole signal, where for a two way loudspeaker system time domain for the tweeter and woofer would be different due to misplacement of acoustic centers of both drivers and phase shift induced by the crossover to each driver, thus causing the sound from one driver to arrive sooner or later than the other one. I hope you make a difference between acoustic phase and signal polarity.


this is a good point, but phase angle changes in an amplifier are based on power output and frequency region, so how can it be global? Some amps display more favorable characteristics than others. I know it's not an audible shift, happening in the electrical domain but my example was to add traction to the discussion that phase is part of every audio process, and understanding various permutations of phase as it applies to various audio principles, is not 'techno-babble' but something worthy of discussing.

I wasn't trying to say you should weight the phase angle of amplifiers on the same plane as other "phase phenomena" but just making the argument that when it comes to phase discussions, it's not just some mythical goal for golden ears, but a valid and physics-related topic that has many vectors.


Phase coherency, is a subsect of that topic. If we allow the discussion to narrow to that particular vein, then it does become the goal of golden ears for most people's purposes. Manipulating phase in a car for better sound, is not really the "phase coherency" thing that home audio speaker designers mull over.


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## 14642 (May 19, 2008)

Speakermakers said:


> *Phase coherency: The Holy Grail of Car Audio*
> 
> Is your system phase coherent?
> 
> ...


OK. Let's try to separate the electrical signal from the acoustic signal. 

First, anytime you combine two audio signals (or any AC signal), the sum depends on the magnitude and the phase. Crossovers, so long as they are properly designed, don't screw up phase. For a high pass and a low pass the phase of one leads and the phase of the other lags. Designed properly, the sum of the magnitude and the sum of the phase are both "0". Crossovers don't screw up phase, unless they are improperly designed. 

With that said, if you're applying crossovers to speakers, then the ONLY thing that matters is the ACOUSTIC response--phase and magnitude. It doesn't matter whether your electrical filters are 12 or 24 or 36 or linkwitz Riley or butterworth. All that matters is how those filters combine with the EQ and the speaker's response to make the final acoustic response. 

Latency through the DSP is an issue only if you use several separate DSPs or FIR filters of different lengths, or some other process that takes a bunch of time. There aren't any car audio processors that either do any of this or are designed so this could screw you up. In my last system, I had a center channel extraction algorithm that took 40mS. I had to delay all the channels by 40mS in order for them all to be aligned. If this was built into an aftermarket processor, then the other channels would be delayed automatically anytime you used this part of the algorithm. If you used FIR filters in your processor and they were different lengths, then that would require compensation. There are n car audio processors that do this and if there were, they would also be designed to fix this. IF you're using a DSP, it's a good idea to run all the signals through it. Latency through the DSP is NOT a standard value for all parts and it depends a great deal on what KIND of DSP you're using. Electrons travel at the speed of light, which is much faster than the speed of sound. 

Phase shift at resonance? Not so much. The phase of the impedance changes, of course, but that doesn't really affect the acoustic output, which is what we care about. The phase of the acoustic output of the speaker is a function of the frequency response. When you build crossovers BETWEEN drivers, this phase is accounted for in the design of the slopes and the sum of the output of the two speakers being crossed over. At the top and at the bottom of the response, that phase shift is really just absolute phase, and that doesn't matter so much.

Phase from midbass drivers that are time aligned but not phase aligned? Hmmm...this is just designing a proper crossover between the subs and the midbass. Same as the first point above. One leads and one lags. The sum should be flat. 

Now, once you put all of this stuff in a car, it's basically out the window, because it isn't a minimum phase system. IN a minimum phase system, INVERTING the transfer function as a correction filter fixes both time and phase. In a non-minimum phase system, inverting it doesn't fix time and phase. A single reflection means that your system is NOT minimum phase. Some regions of your response may act like minimum phase and others won't. REW includes a view they call "excess phase" and that view will help you understand the difference. Read the explanation. 

From a practical standpoint, you can NEVER have a system in your car that's completely phase coherent because of all the reflections. It ain't happening and fortunately, it isn't necessary. It's sufficient to pay close attention to the phase at crossover points in order to have a smooth response without holes and to pay close attention to the phase between left and right channels in order to produce a stable image, but that's IT. You CANNOT eq the phase or the frequency response of reflections differently than the sound from the speaker. It is helpful for the left and right speakers to be in phase at all frequencies, but the ABSOLUTE phase doesn't matter. Phase that isn't zero is fine, so long as left and right aren't 180 degrees out of phase. A few degrees doesn't cause any big problems.


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## jtaudioacc (Apr 6, 2010)

^^^great breakdown, Andy.


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## Speakermakers (Nov 9, 2013)

And the battle rages on.

Manufactures and their engineers refuse to accept that they can not solve all problems on paper and on the bench while spending minimal time in a car.

This might have something to do with why you can't get two tunings from an MS8 that sound the same. Auto tune it 12 times and get 12 different outcomes (all with nearly identical frequency response). 

With more than 20 years, and conservatively 10,000 cars under my belt; I would venture to say that I am not speaking of theory alone. 

I have two different types of naysayers here: 

1. The type that wants to get into a word length battle.

2. The type that wants to police the internet and make sure that consumers are only feed the food they deem proper.

I don't have a problem with debate on finer points, but don't claim that I am handing out snake oil. Andy admits to adding additional delay to account for processing latency himself yet shouts that the rest of us should disregard the voodoo witchcraft of attempting to keep track of phase.

Enthusiasts want to explore all aspects of what effects sound in their vehicles. I simply pointed out four.


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## cajunner (Apr 13, 2007)

The fact that there are no car audio DSP currently using the expensive, phase-neutral FIR filters, means that the filters that all car audio DSP uses, do mess with phase. But, just how does this matter in the acoustic response?

Andy has used higher grade processing power than probably any of the readers of this thread, since he's right there in the research labs and working with the people who have the software and the computers to run it, that dwarfs any consumer-level product available to installers, even those who have 20 zillion installs to their credit.

I also find that understanding those differences between latency in a product's computational process and latency between processors, is a separate thing and it makes sense to me, that a DSP device is only going to have a time delay of whatever amount is necessary to compute for every possible circuit change in the digital domain, and that's it.

Analog manipulation is different, the digital part of DSP is the time it takes to turn everything into 1's and 0's, and then back into a wave again. That's where most of the latency happens, but if Andy says he had a center extraction algorithm take 40mS then that software seems to be doing more than what a consumer DSP would be made to accomplish. 

I know there are some of you who know what the different DSP's need in latency correction, am I right in assuming that most of the consumer DSP products are say, between 5 and 20mS of lag behind the input?

keep the discussion going, fella's.


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## Hanatsu (Nov 9, 2010)

Even if there's severe latency between different channels using different DSPs or FIR's with different lengths or whatever it will still show up in the acoustic measurement.

A tune can look identical to another on a RTA with all speakers severely out of phase in respect to eachother. Of course phase matters, but there's too much focus on what happens in the electronics, most issues can be indentified through a simple acoustic measurement.

FIR filters are available in MiniDSP units and for the CarPC guys.

Sent from my Samsung Galaxy 3 via Tapatalk.


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## Justin Zazzi (May 28, 2012)

Andy Wehmeyer said:


> *With that said, if you're applying crossovers to speakers, then the ONLY thing that matters is the ACOUSTIC response--phase and magnitude. It doesn't matter whether your electrical filters are 12 or 24 or 36 or linkwitz Riley or butterworth. All that matters is how those filters combine with the EQ and the speaker's response to make the final acoustic response. *


Bolded for extreme emphasis. I have been exploring this concept over the past couple days independent of this thread and came to the exact same conclusion. I've updated my build thread with three posts (below). Also, I'm using a processor without any special features or magic, this is something everyone can follow along and try for themselves.

Lots and lots of pretty graphs in these posts:

http://www.diymobileaudio.com/forum/2020518-post247.html #247
http://www.diymobileaudio.com/forum/2020994-post249.html #279
http://www.diymobileaudio.com/forum/2021090-post251.html #251

The resources I read to learn all this stuff are below, and I can't recommend them enough if you want to learn more about everything in this thread.

Resources:

Siedfried Linkwitz's paper revealing the Linkwitz-Riley Crossover
JAES papers

Rane Corporation's excellent overview of the LR Crossover
http://www.rane.com/pdf/ranenotes/Linkwitz_Riley_Crossovers_Primer.pdf

Elliot Sound Products - Phase, Time, and Distortion in Loudspeakers
Phase, Time and Distortion in Loudspeakers

The Crossover Design Cookbook Chapter 4: Theory
The Crossover Design Cookbook Chapter 4: The Theory of Crossovers

FilterShop Application Notes - Analog Audio Active Crossover
http://www.linearx.com/files/pdf/FilterShopApp_05.pdf

Texas Instruments - Basic Introduction to Filters
http://www.ti.com/lit/an/snoa224a/snoa224a.pdfhttp://www.ti.com/lit/an/snoa224a/snoa224a.pdf


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## 14642 (May 19, 2008)

Speakermakers said:


> And the battle rages on.
> 
> Manufactures and their engineers refuse to accept that they can not solve all problems on paper and on the bench while spending minimal time in a car.
> 
> ...


This one I don't understand. I've been in the industry for nearly 30 years and spent the first 10 as an installer. 

I'm not suggesting that phase doesn't matter. I am saying that the basics of phase do matter, but that the idea that cars can only sound great if we apply the same rigor to the phase response as we must to the frequency response. 

What i am saying is that if you've never measured the phase of a car audio system and have never even seen a graph of the phase response, that's a good place to start.


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## 14642 (May 19, 2008)

My only objective here is to help all of you make efficient progress in making your cars and your customers' cars sound better. Sometimes that means attempting to redirect conversations back towards the useful and away from the theoretical, especially when the theory is COMPLETE supposition without technical foundation. 

With that said, there's plenty that I don't know.


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## Finesa (Aug 12, 2013)

Andy Wehmeyer said:


> 1. Crossovers, so long as they are properly designed, don't screw up phase. For a high pass and a low pass the phase of one leads and the phase of the other lags. Designed properly, the sum of the magnitude and the sum of the phase are both "0". Crossovers don't screw up phase, unless they are improperly designed.
> 
> 2. Phase from midbass drivers that are time aligned but not phase aligned? Hmmm...this is just designing a proper crossover between the subs and the midbass. Same as the first point above. One leads and one lags. The sum should be flat.
> 
> ...


Andy,

1,2 are incorrect. Crossovers always affect phase response. Having perfect ( impossible in real world ) phase tracking would still present phase anomalies around the crossover point. How sensitive we are to it - depends on the frequency, but in the 1-2Khz region - a 5 degree error may be well enough for some to say it sounds weird and for others to love it ( its still an error and should not be there ). The assumption of that an order adds 90 degrees is a bit flawed because it does not. Deviates around 90 but is not exact. Sum up to a 4th order which is quite popular for low quality drivers and there you go with enough phase error.

One of the things people often ignore is the fact the loudspeaker itself has a crossover in its very heart. Depending on its inductance, it will have a different phase response ( someone brought this up above, I believe ) as well as impedance.

3. Absolutely true. Not just in car, but anywhere. 


Cajunner - phase altering in amplifiers affects the whole signal, the output may be delayed but you will never notice it ( because it is not reproduced yet ). Once it reaches the loudspeaker, it would depend on the acoustic phase of the two/three drivers emitting sound. This affects imaging in a clear way, Altec-Lansing have a good paper on the AES on this. As already put up by Andy, I have the feeling that many people here accetp phase = polarity which is an incorrect equation ( very much indeed ).


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## 14642 (May 19, 2008)

Finesa said:


> Andy,
> 
> 1,2 are incorrect. Crossovers always affect phase response. Having perfect ( impossible in real world ) phase tracking would still present phase anomalies around the crossover point. How sensitive we are to it - depends on the frequency, but in the 1-2Khz region - a 5 degree error may be well enough for some to say it sounds weird and for others to love it ( its still an error and should not be there ). The assumption of that an order adds 90 degrees is a bit flawed because it does not. Deviates around 90 but is not exact. Sum up to a 4th order which is quite popular for low quality drivers and there you go with enough phase error.
> 
> ...


Finesa,
You're missing my point in #1 above. The crossover has to be viewed as a part of the speaker system that includes the frequency response and impedance of the speaker. Measuring the electrical response of the crossover is almost completely irrelevant. It's very similar to your suggestion about amplifier phase--you don't hear it directly. You only hear it once the signal is converted into sound in the speaker. The fact that the crossover CHANGES the phase doesn't mean that it SCREWS UP the phase and, as a result screws up the sound. The same could be said about an EQ--changing the frequency response changes the phase, but in many cases, that change makes things sound better.


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## roduk (Sep 19, 2008)

jtaudioacc said:


> where'd the OP go?


Exactly. Loved his first post - it was a breath of fresh air, well written and written in a way for noobs to understand too.. It's so fabulous to have new guys sign up and offer years worth of experience to an open forum. What the F happened? :freak:

Simply put, phase is one of the most important things in sound reproduction.


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## sqnut (Dec 24, 2009)

roduk said:


> Simply put, phase is one of the most important things in sound reproduction.


And yet your sig does not include dsp that would help with some phase coherence in terms of timing first arrival at your ears. In a car, to think of phase as anything other than correct polarity and arrival times is pointless. In a car response is king.


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## roduk (Sep 19, 2008)

sqnut said:


> And yet your sig does not include dsp that would help with some phase coherence in terms of timing first arrival at your ears. In a car, to think of phase as anything other than correct polarity and arrival times is pointless. In a car response is king.


Ah sorry my sig is a bit out if date  digitally I have used Alto, Alpine, Audison, JBL and analogue I have used Arc DXE and Ranes, these last crossovers were great as they had a phase 'delay' on them which you could use to great effect. After all those I'm back with the Alto UCS Pro atm. Sorry for the misleading sig. Before you even look at phase, levels are key though!! (Where does this end!!)


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## cajunner (Apr 13, 2007)

Andy Wehmeyer said:


> This one I don't understand. I've been in the industry for nearly 30 years and spent the first 10 as an installer.
> 
> I'm not suggesting that phase doesn't matter. I am saying that the basics of phase do matter, but that the idea that cars can only sound great if we apply the same rigor to the phase response as we must to the frequency response.
> 
> What i am saying is that if you've never measured the phase of a car audio system and have never even seen a graph of the phase response, that's a good place to start.


how often do you suppose, someone will offer as a helpful suggestion in a home audio environment, to switch polarity of one side of the stereo-producing equipment, and see if that makes a change for the better?

I think we're on the same page, but the huge, gross, un-corrected response in most cars is so coarse as a result of the environment, that doing a 180 on a driver is not only a possible, but is given regularly as a "first fix" in getting the response to even out; this is indicating what a mess of it, it is in the first place.

Anyone who approaches the car with the home audio mindset, that ringing in the higher order crossovers is going to trump near-field reflections, that diffraction and compartment resonances, is the same as dealing with enclosure bracing, that the weak metal door interior panel resonating is the same as needing acoustic resonance control on the "first bounce" of an enclosure with parallel walls, is probably not yet keyed into the horrid nature of the car interior as an acoustic reproductive space.

does that matter all that much? 

not really.

it's just that phase is at once, obvious and mystical, you want to replicate the timing and signature of a natural event but you're not using natural means.

any modification of phase that gets you closer to a true, natural-looking wave-front is probably a good thing, and most everything that you do to the signal is going to modify the phase.

being aware of phase changes, when doing frequency response modifications is good, but not being aware of phase changes and just "fixing the curve" with whatever means you have, may be the less stressful path.


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