# Ported enclosure and group delay? Fix?



## therapture (Jan 31, 2013)

When modeling different subs in WinISD, I see that group delay goes up dramatically for a ported alignment. Is there a method to improve on this aspect and get it closer to a sealed enclosure, and how much does this matter?


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## 14642 (May 19, 2008)

It doesn't matter.


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## Hanatsu (Nov 9, 2010)

EQ the response to the same as the sealed in WinISD (apply parametric EQ). Look at the GD again.

The anechoic model shows fantasy numbers, it will look like crap in the car anyway. Just EQ the response, it will fix most of the issues.


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## therapture (Jan 31, 2013)

The reason I asked is because I have a new 10" Dayton Titanic MKIII laying around...I have a small sealed box of .8 ft³ (net), but I was thinking of trying it out ported as well.


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## Hanatsu (Nov 9, 2010)

Vented is better generally if you got EQ 

Sent from my Samsung Galaxy 3 via Tapatalk.


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## Hanatsu (Nov 9, 2010)

Just make sure the T/S parameters allow for it...

Sent from my Samsung Galaxy 3 via Tapatalk.


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## Patrick Bateman (Sep 11, 2006)

Andy Wehmeyer said:


> It doesn't matter.


You're killing me Andy 

Of course it matters, I can heard the difference between a ported sub, a front loaded horn, a tapped horn, and a sealed box. The difference is impossible to miss once you've listened to the various box types for a few years. I can hear the difference in literally five seconds.

As I see it, here are the differences:

1) Ported
Relatively easy to build, medium sized box, high power handling. To my ears, the worst sonically. The phase rotates 180 degrees, which means that the harmonics and the fundamental are not in sync. For instance, if you have a ported box tuned to 30hz and a 30hz note is played, the 2nd and 3rd harmonic are about 180 degrees out of phase.

2) Sealed
Easiest to build, very forgiving. Small box, high power handling. To my ears, the 3rd best sonically, but can be the best if you use a lot of them. Well behaved phase response. It's achilles heel is distortion; due to the relatively low efficiency the next two types will generally have lower distortion. IE, sealed boxes don't have high distortion, but they have low efficiency, which means that the next two types can play with lower distortion because they get louder with less power. But this can be addressed by using very large sealed boxes with multiple small drivers or one large driver. (IE, four eights, one fifteen, two twelves, etc.)

3) tapped horn
Easy to build. Larger box than vented. It's main advantage over ported is more output and the phase rotation is half as much, just 90 degrees. This can give you the best of both worlds, a lot of output for a single driver, while retaining a phase curve that rivals a sealed box.

4) front loaded horn.
Difficult to build. Largest box of all. It has the following advantage over the other types:
a) Very high output, which can exceed all other types, depending on box size. (IE, Hoffman's Iron Law dictates that output is determined by box size, and FLHs tend to have the largest box size. If box size is the same for all four, output will be about the same.)
b) Excellent phase response. A FLH has a phase curve that is nearly as good as sealed, better than ported, and similar to a TH
c) The "icing on the cake" is that a FLH dramatically reduces distortion. This is what sets it apart from an array of sealed boxes; the chamber in front of the woofer drops distortion a significant amount. There is no way to replicate this electronically, a coupling chamber and horn are a physical filter that reduces distortion.


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## Hanatsu (Nov 9, 2010)

Patrick Bateman said:


> 1) Ported
> Relatively easy to build, medium sized box, high power handling. To my ears, the worst sonically. The phase rotates 180 degrees, which means that the harmonics and the fundamental are not in sync. For instance, if you have a ported box tuned to 30hz and a 30hz note is played, the 2nd and 3rd harmonic are about 180 degrees out of phase.


But around the tuning frequency the non-linear distortion is generally very very low making this a small issue. If Fb lies around 25-35Hz the phase response should be very similar to that of a sealed box a half octave above that. The main issue with horns is no low frequency output without building huuge enclosures.


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## Hanatsu (Nov 9, 2010)

Here's a simplistic model;





Chapter 5



> The results from this thesis research imply that phase distortion is of secondary importance as compared to frequency response irregularities, which is in agreement with previous research results.


How a typical group delay measurement looks;

Case Study: Sub Alignment using REW v5 - Home Theater Forum and Systems - HomeTheaterShack.com










The issue isn't absolute group delay, it's the non-flat delay over a given frequency range as the fundamentals and harmonics are delayed towards each other as you said. Any lowpass function as we always use in cars will increase GD as well. The non minimal phase areas will screw up GD too... I don't think GD is that audible unless it jumps really high, 1-1,5 cycles or so. The important thing to get right is the frequency response first off, imo.


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## cajunner (Apr 13, 2007)

GRAPHS!

Hanatsu, killing it...

group delay of 20 milliseconds, in a wave 30 feet long, or 1/30th of a second, doesn't account to much smear.

it's the change in linear response as the musical information travels through the beat, that does make a difference, we're responding to the sound of the speaker handing off the wave to the vent, at different times.


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## Patrick Bateman (Sep 11, 2006)

In this thread I demonstrate that ported boxes are just beyond the limit of audible group delay:

http://www.diymobileaudio.com/forum/2052295-post105.html

I will concede the following:

1) Sealed works quite well
2) Ported works well, if you don't mind screwed up transients
3) Ported can work nearly as well as sealed boxes and horns, *but only if the tuning is very low.* Basically the group delay gets worse as you get closer to the tuning frequency. So one way to offset the crummy group delay of ported boxes is to simply tune them to a frequency that's lower than the musical content. For instance, 25hz.

But a ported box tuned to 30 or 40hz? Oh hell no. We've all heard these boxes; it's how most people built their ported boxes.

Horns still have an efficiency advantage if you can't afford a lot of drivers, and a FLH can easily have the lowest distortion.


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## minbari (Mar 3, 2011)

Andy Wehmeyer said:


> It doesn't matter.


x100! you cant hear it unless it gets to obscene levels, much larger than you will ever see.

If you want to play with it, port area is what will control GD most.


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## Hanatsu (Nov 9, 2010)

Patrick Bateman said:


> In this thread I demonstrate that ported boxes are just beyond the limit of audible group delay:
> 
> http://www.diymobileaudio.com/forum/2052295-post105.html
> 
> ...


But still. There's no low frequency output from horns. I want one that can do 30Hz with authority 

From your standpoint and mine a 4th order BP would be the best compromise. The graph in that post looks like it's nearfield outside the car. I argue that it's the space outside the enclosure that dominate the FR and phase response. It won't look that good as soon it's in the car. 
I believe that the main reason some alignments sound better than others simply can be described by non-linear distortion and SPL vs Freq (FR). I had major success eliminating the ringing modes by delayed active "anti-phase" subs. Removing bass note lingering for 500-1500ms does so much more than reducing GD imo.

Tapaaatalk!!


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## Hanatsu (Nov 9, 2010)

I think vented generally sound best among the "normal" enclosure types. Jugding from some of the other threads you made, it seem you like run subs much higher in frequency than I do. I tend to cross mine in the 50-60Hz area normally. Horns might generally sound better 60Hz+, idk. Not much experience with that.

Tapaaatalk!!


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## teldzc1 (Oct 9, 2009)

Hanatsu said:


> I had major success eliminating the ringing modes by delayed active "anti-phase" subs. Removing bass note lingering for 500-1500ms does so much more than reducing GD imo.
> 
> Tapaaatalk!!


That's pretty interesting. Is that for all frequencies that the sub is reproducing?




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## Hanatsu (Nov 9, 2010)

teldzc1 said:


> That's pretty interesting. Is that for all frequencies that the sub is reproducing?
> 
> Sent from my SAMSUNG-SM-N900A using Tapatalk


Only required at modal peaks so the lingering frequencies are even over the entire frequency range the main sub is reproducing. I only used mine at the first peak at ~45Hz, I haven't experimented much with this yet. It's one of my upcoming experiments in my project thread...


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## Hanatsu (Nov 9, 2010)

Ok. Did some measurements. I can conclude that the phase discussion is indeed silly. Look at this:

*An "anechoic" measurement of a sealed enclosure-subwoofer.*



*Same enclosure inside the car;*



*Same driver in a vented box with Fb 33Hz (inside car with EQ);*



*The vented box only have a 100deg acoustic phase shift in its limited passband. Not 180deg.

*Compare the two first measurements, see how much the phase changes and how dependent it is on the frequency response. The EQed vented box looks more behaved.


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## therapture (Jan 31, 2013)

Well....very informative guys!


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## 14642 (May 19, 2008)

Sorry. My answer was too short. 

As Patrick has suggested, tuning a vented box low is a really good idea. Avoiding a big PEAK in the group delay is good. (See the red curve in one of the graphs above). 

As far as fundamentals and harmonics being in phase, I can't say this is something I've ever paid attention to. I've always to tried to minimize distortion in subwoofers I've had a hand in designing. If it's harmonics of the fundamental that are included in the recording, then those are most likely going to be reproduced by another speaker anyway. 

I've never built a TL, tapped horn or front loaded horn for use in a car.


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## audijay (Mar 18, 2014)

Wow the information here is awesome, hopefully when I build my ported I can tune the group delay down with the EQ


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## sqnut (Dec 24, 2009)

Even when a ported and sealed box measure close, they do not sound similar. For about six months I modeled and tried 4-5 ported boxes, all tuned low and with every box I ended up with a lower end that was a touch fatter and a bit diffused. I could not resolve this with response and timing, much as I tried. I just gave up and went back to the sealed box.

Whatever is causing the difference is not being captured on a response curve. I would get the response from the ported close to my sealed box just like in one of Han's graphs, but the two boxes sounded and felt very different. 

At the risk of asking a silly question, what if there was a timing difference between the sound from the port and cone? That is something that can't be cured. I'm a bit curious about the topic.

Patricks point on the phase issue between fundamentals and harmonics is probably from the standpoint of higher xover points and shallower slopes on the sub. With the sub cut at 50 on a 30db acoustic roll off, you're hearing the harmonics from your mid bass like Andy pointed out. 

Whatever the issue I think it's related to timing, cause blurring in the image is normally a timing issue.


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## minbari (Mar 3, 2011)

sqnut said:


> Even when a ported and sealed box measure close, they do not sound similar. For about six months I modeled and tried 4-5 ported boxes, all tuned low and with every box I ended up with a lower end that was a touch fatter and a bit diffused. I could not resolve this with response and timing, much as I tried. I just gave up and went back to the sealed box.
> 
> Whatever is causing the difference is not being captured on a response curve. I would get the response from the ported close to my sealed box just like in one of Han's graphs, but the two boxes sounded and felt very different.
> 
> ...


that is not a silly question , there is absolutely a phase shift between the cone and the port. (90° if memory serves) you are right there is nothing to fix it.


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## sqnut (Dec 24, 2009)

minbari said:


> that is not a silly question , there is absolutely a phase shift between the cone and the port. (90° if memory serves) you are right there is nothing to fix it.


If I understand that correctly a 90 deg shift means quarter wavelength, which for 50hz is 5.63 feet. That means a timing difference of about 0.8ms, that's big.


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## ATOMICTECH62 (Jan 24, 2009)

If the group delay is 20ms it will make the sub sound as if it was 23 feet away.I have heard some people will delay the sound of all the other speakers by the same amount to over come this but since the group delay varies with frequency other frequencies will arrive sooner then the longest group delay.I have not tried it.


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## cajunner (Apr 13, 2007)

and 20 mS is the Haas effect, and we ALL know what that means, so...


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## Hanatsu (Nov 9, 2010)

In my car, the modal ringing lingered for 1500ms before it hit "inaudible" levels. The 20ms delay is nothing compared to it. A vented will have much lower non-linear distortion because of less excursion, which can be several percent at higher volumes. 

Everyone is hung up on phase. As I've shown in my measurements, the group delay and phase response will be altered by the environment. In fact, these parameters will be dominated by the "second box" you place the subwoofer in, i.e the car's interior volume. I can change, phase response and frequency response just by opening the window. It will be altered by moving the sub. Everything here is "it depends on...". I disagree that vented is somehow slower, muddier or something than sealed. In fact, I'm convinced that vented alignments are better than sealed in that area because of lower non-linear distortion, both HD and IMD. The new frequencies the HD and IMD introduce will be very close to the fundamental, so close that they will show up as a lower Q peak if you watch the response on a 1/3oct RTA if you play a sine wave through a low distortion and a high distortion device. The non-linear distortion destroys tonal accuracy imo. Every single vented alignment I've built is far far from "sluggish", if you wanna refer to such term. 

I actually believe what this guy's saying;



> While it is widely accepted in the industry that non linear distortion at low frequency is not particularly audible I am convinced that it is audible. In fact I believe that non linear distortion is the most audible at low frequency. One look at the equal loudness curves of human hearing should convince anyone that non linear distortion is least audible above about 3 KHz and most audible below 3 KHz. Above about 10 KHz it should be completely inaudible because the 2nd harmonic is exceeding the human hearing range. At 20 Hz the human ear will hear an equal level 40 Hz tone as 20 dB louder based on a 90 dB reference level. This means the 40 Hz harmonic must be 20 dB below the 20 Hz fundamental just to sound as if it is the same level or 100% 2nd harmonic distortion.
> 
> It's very difficult to achieve low distortion at frequencies in the 20 Hz range. Consequently I wonder how test were conducted to produce the widely held belief that distortion is inaudible at low frequency? Since most woofers produce at least 10% distortion at 20 Hz which sounds like 100% distortion at the 2nd and far higher at the 3rd, they have no clean reference to determine if induced distortion is audible. The test platform is starting with too much distortion to begin with for anyone to assess that an induced distortion is or is not audible.


mfk-projects subwoofer


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## Hanatsu (Nov 9, 2010)

Note that tuning a vented alignment requires more relative delay of the the rest of the system. I my previous build I had the midbass, midrange, tweeters delayed for about 20ms compared to the subwoofer. Also the type of midbass you're using is crucial for good integration. As most experienced people know around here... "speed" lies in the mibbass/midrange area.


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## Hanatsu (Nov 9, 2010)

sqnut said:


> If I understand that correctly a 90 deg shift means quarter wavelength, which for 50hz is 5.63 feet. That means a timing difference of about 0.8ms, that's big.


It's really not... Group delay is frequency dependent. It has the lowest audibility threshold at about 1ms at 2kHz. 

http://en.wikipedia.org/wiki/Group_delay_and_phase_delay


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## cajunner (Apr 13, 2007)

but overhang/ringing is what causes that "speed" to appear slow, it's the muddiness that we're fighting and modal ringing from the car's chassis is NOT helping.

pretty much why I think the car manufacturers who incorporate road noise control methodologies, (feedback/servo/active cancellation) could be piggybacked for the audio where not only is the chassis counter-measures used in handling, it could be used in fidelity, to negate the reverb of the metal body's resonant characteristics.


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## Hanatsu (Nov 9, 2010)

cajunner said:


> but overhang/ringing is what causes that "speed" to appear slow, it's the muddiness that we're fighting and modal ringing from the car's chassis is NOT helping.
> 
> pretty much why I think the car manufacturers who incorporate road noise control methodologies, (feedback/servo/active cancellation) could be piggybacked for the audio where not only is the chassis counter-measures used in handling, it could be used in fidelity, to negate the reverb of the metal body's resonant characteristics.


We can negate the ringing with an active subwoofer playing out of phase at selected frequencies delayed in time. Fix the big issues first, then worry about (imo) insignificant things like few ms delay around the subwoofer frequencies. Even if you don't agree with me, you can always T/A the system to compensate for the GD, as long as it remains constant over the passband you're using the sub.


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## cajunner (Apr 13, 2007)

Hanatsu said:


> We can negate the ringing with an active subwoofer playing out of phase at selected frequencies delayed in time. Fix the big issues first, then worry about (imo) insignificant things like few ms delay around the subwoofer frequencies. Even if you don't agree with me, you can always T/A the system to compensate for the GD, as long as it remains constant over the passband you're using the sub.


most sub's passband has a varying GD according to frequency, that is why vented is sometimes less distinct, and sometimes it's as good as sealed.

it's really not that I'm trying to argue this point, but you're saying that vented can be better and I'm saying it can come close but we're almost at the discussion of acoustic suspension vs. bass reflex.

you get more output from the bass reflex, which is why it's so commonly used.

it's always been that way, pretty much and that has a bigger appeal than the usually slightly less defined dynamics of short duration transients in the signal.

a longer note, shudders through the car and you can't really tell what is GD from what is the car's modal signature, but the human's ability to detect realism will, over time, come to appreciate the sealed box's accuracy.

will it overcome the need to wang? maybe that's more important, maybe the vented design and it's mild to middling GD issue is really justified for the extra db's of output that save on everything else.

3 db of gain, still sounds good and if you're not doubling speakers or amp power to get it, then it's an acceptable compromise.


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## sqnut (Dec 24, 2009)

Hanatsu said:


> It's really not... Group delay is frequency dependent. It has the lowest audibility threshold at about 1ms at 2kHz.
> 
> Group delay and phase delay - Wikipedia, the free encyclopedia


I have read that and some other stuff, which also gives similar results. But sometimes what you read is counter intuitive from what you experience. I wasn't talking about GD per se but more about a timing/phase delay between the sound from the cone and the port, when using ported enclosures. 

So while data suggests a threshold of 1ms at 2khz, with the drivers in phase you can evaluate better/worse with a couple of clicks of TA (0.02-0.07ms depending on what you're using). 1ms just seems like an awful lot. With speakers in phase, one normally has ~ 0.04-0.06ms play before the drivers start falling out of phase. 

Phase is basically about location of sound. I'm not sure how the thresholds were tested in all the studies, but if it was done using headphones, that could be one reason for the high numbers. Based on the way our ears and brain process sound, we need to hear some degree of cross talk for our 'location tool' to work properly.


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## sqnut (Dec 24, 2009)

Hanatsu said:


> I disagree that vented is somehow slower, muddier or something than sealed.





Hanatsu said:


> Also the type of midbass you're using is crucial for good integration. As most experienced people know around here... "speed" lies in the mibbass/midrange area.


For the record I'm not talking about slow and fast bass.


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## Hanatsu (Nov 9, 2010)

Download foobar2000, use the convolver plugin to import an impulse response from rePhase. rePhase is a FIR generator with parametric phase EQ to be able to use linear phase filtering. Throw phase off by say 180deg at 50Hz with a Q2 or something, listen again. I think you'll be surprised that the change was almost completely inaudible. 

Put two woofers in an isobaric configuration which reduces 2nd order HD by a big amount, listen and see if you hear any difference. I promise it will be a large difference. It sounds off at first, a little of that velodyne-servo sound. We so used to the high distortion that we disregard it as inaudible, but once you get used to low distorted bass you soon realize that the bass from sealed enclosure at moderate/higher volume is nothing but mud. With a vented alignment you basically trade better non linear performance for worse linear performance (as in worse phase response, not FR). Above 200Hz or so the linear performance is more important but below the non-linear performance is more important imo, it's the only parameter you really can't "fix" with processing.


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## sqnut (Dec 24, 2009)

Thank you for your patience. 



Hanatsu said:


> Download foobar2000, use the convolver plugin to import an impulse response from rePhase. rePhase is a FIR generator with parametric phase EQ to be able to use linear phase filtering. Throw phase off by say 180deg at 50Hz with a Q2 or something, listen again. I think you'll be surprised that the change was almost completely inaudible.


I can do that by flipping the polarity of one of the mids while playing the 50hz tone. The image will move cause you've just added/reduced 10ms of delay. Or while sitting in the sweet spot you've moved one speaker 11 ft closer/further. I guess precedence effect will kick in at some point, even in the ears poor sensitivity zone. One looses a lot of dynamics if the fundamental and harmonics are out of phase. 




Hanatsu said:


> Put two woofers in an isobaric configuration which reduces 2nd order HD by a big amount, listen and see if you hear any difference. I promise it will be a large difference. It sounds off at first, a little of that velodyne-servo sound. We so used to the high distortion that we disregard it as inaudible, but once you get used to low distorted bass you soon realize that the bass from sealed enclosure at moderate/higher volume is nothing but mud. With a vented alignment you basically trade better non linear performance for worse linear performance (as in worse phase response, not FR). Above 200Hz or so the linear performance is more important but below the non-linear performance is more important imo, it's the only parameter you really can't "fix" with processing.


Muddiness is in your mid bass . My take is very simple. As long as I'm playing the sub in a very narrow pass band and using steep roll off, I can use response, timing and box size to manage the 'muddy' linear distortion. 

As I understand it, non linear distortion is the difference between input and output as a signal goes through a device. The non linear distortion that can't be corrected is when you input a 50hz tone to your sub and it plays 48,50 and 52 hz. In a zone where the ears sensitivity sucks, can you hear that pitch difference? Distortion on a sub is less critical than that on a tweeter imo.


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## Hanatsu (Nov 9, 2010)

sqnut said:


> I can do that by flipping the polarity of one of the mids while playing the 50hz tone.
> 
> Muddiness is in your mid bass . My take is very simple. As long as I'm playing the sub in a very narrow pass band and using steep roll off, I can use response, timing and box size to manage the 'muddy' linear distortion.
> 
> As I understand it, non linear distortion is the difference between input and output as a signal goes through a device. The non linear distortion that can't be corrected is when you input a 50hz tone to your sub and it plays 48,50 and 52 hz. In a zone where the ears sensitivity sucks, can you hear that pitch difference? Distortion on a sub is less critical than that on a tweeter imo.


No it's not the same thing. The phase relative towards other drivers is easy to detect. That's not what I meant. Take a pair of headphones, push 50Hz 180deg out of phase with phase EQ. See if you hear the difference, that will create a group delay peak and I argue that it's very audible at all. I can however easily hear the difference in 0,5Hz steps around 50Hz - so should you 

*Non-linear distortion is distortion that introduces new tones not present in the signal, harmonic and intermodulated distortion. 

*Linear distortion is frequency response, phase response and energy storage (CSD). 

The distortion non-linear distortion of a tweeter playing above 7-8kHz can be mostly disregarded, the first harmonic ends up at 14-16kHz, the rest is inaudible. IMD can still end up lower in frequency however, these two are linked. Low HD almost always equal low IMD and vice versa. I mainly look at linear performance when I look at tweeters, found that it correlates more to what we perceive as transient and clean than HD/IMD. It's the opposite for the lows. IME -<


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## sqnut (Dec 24, 2009)

Hanatsu said:


> No it's not the same thing. The phase relative towards other drivers is easy to detect. That's not what I meant. Take a pair of headphones, push 50Hz 180deg out of phase with phase EQ. See if you hear the difference, that will create a group delay peak and I argue that it's very audible at all. I can however easily hear the difference in 0,5Hz steps around 50Hz - so should you


Headphones and the car are two different listening environments. I'm concerned by what I can or can't hear in the car. Headphones don't give you any cross talk. Without some degree of cross talk your 'phase detection tool' does not work very well. I'm saying the difference that you don't hear with the headphones is audible in a car when you're hearing sound from multiple drivers. Do you have the link to test pitch sensitivity?



Hanatsu said:


> *Non-linear distortion is distortion that introduces new tones not present in the signal, harmonic and intermodulated distortion.
> 
> *Linear distortion is frequency response, phase response and energy storage (CSD).
> 
> The distortion non-linear distortion of a tweeter playing above 7-8kHz can be mostly disregarded, the first harmonic ends up at 14-16kHz, the rest is inaudible. IMD can still end up lower in frequency however, these two are linked. Low HD almost always equal low IMD and vice versa. I mainly look at linear performance when I look at tweeters, found that it correlates more to what we perceive as transient and clean than HD/IMD. It's the opposite for the lows. IME -<


Non linear distortion on speakers is mostly moot according to Ms. Geddes and Lee. Both THD and IMD are as useful as damping factor on amps, unless you listen at 100db all the time. It's 80-85db when I'm cranking things. When I said I look at a tweeters distortion, I'm talking about linear distortion. Things that I look for are, FR just like you do. The only non linear distortion I look at is 3rd vs 2nd order spl. Drivers where the 3rd order is close to 2nd order never sound right and always sound harsher. I am not an expert in speaker distortion, but I think we need to take it with a pinch of salt.


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## TOOSTUBBORN2FAIL (Jan 31, 2011)

I have to agree with hanatsu on non linear distortion being far more audible than people give credit for. I say this based on my own push-pull experiments with bass drivers. I think people are just used to distortion at low frequencies and think its supposed to be there, because it always has been.


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## thehatedguy (May 4, 2007)

A aubwoofer is or should be the only speaker in the car (or out) that is played not only close to but below it's resonance. Keep the speakers away from their resonance, and then nonlinear distortion is moot. And/or keep the excursion levels down too.


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## Hanatsu (Nov 9, 2010)

Many people push the mids below Fs. If a driver is put in an enclosure, Fsc will move upwards. It's pretty common we play drivers through Fsc. It's not a good idea playing any form of midrange/tweeter domes close to resonance though. 

The non-linear distortion is more important than you think. It's the least understood part of psychoacoustics. Gedlee's paper on non-linear distortion is inconclusive imo. Some of the numbers there are simply weird. 10% distortion is very much audible in the lows, as I said it destroys tonal accuracy.


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## Hanatsu (Nov 9, 2010)

sqnut said:


> Headphones and the car are two different listening environments. I'm concerned by what I can or can't hear in the car. Headphones don't give you any cross talk. Without some degree of cross talk your 'phase detection tool' does not work very well. I'm saying the difference that you don't hear with the headphones is audible in a car when you're hearing sound from multiple drivers. Do you have the link to test pitch sensitivity?
> 
> 
> 
> Non linear distortion on speakers is mostly moot according to Ms. Geddes and Lee. Both THD and IMD are as useful as damping factor on amps, unless you listen at 100db all the time. It's 80-85db when I'm cranking things. When I said I look at a tweeters distortion, I'm talking about linear distortion. Things that I look for are, FR just like you do. The only non linear distortion I look at is 3rd vs 2nd order spl. Drivers where the 3rd order is close to 2nd order never sound right and always sound harsher. I am not an expert in speaker distortion, but I think we need to take it with a pinch of salt.


I don't know if we're supposed to bring crosstalk into the discussion or what relevance it has. If the group delay ain't audible in headphones, it's not audible in the car either. It should be MORE audible in a controlled non-reverberant environment. Geddes and Lee's tests are inconclusive, I've seen those papers. Some tests were interesting, but some of the stuff is simply incomprehensible, IIRC people couldn't hear 80% IMD or something like that... WTF? Do the klippel listening test, I can spot at 2% non-linear distortion easy. 

I agree that HD2 should be clearly separated from HD3 for natural sounding drivers and that linear performance is more important with tweeters.

Download RoomEQ/REW, it got a sine wave generator if you wanna hear the pitch change.


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## 14642 (May 19, 2008)

sqnut said:


> At the risk of asking a silly question, what if there was a timing difference between the sound from the port and cone? That is something that can't be cured. I'm a bit curious about the topic.


yes, the port and the woofer are 90 degrees out of phase. At Fb, the port plays and the woofer doesn't. As you move away from Fb, you hear the combination, so the sum of the two signals is less than it would be if they were in phase. They sum like any two signals that are 90 degrees out of phase. We can equalize frequency response anomalies that result from a 90 degree shift. 

Group delay is like any other discussion of absolute phase. It doesn't matter, UNLESS the relative delay between one signal source and the other causes them to play the same thing at different times, or, as Patrick has suggested, to play different parts of the same event at different times. 

We don't hear those two events as separate events until the relative delay reaches about 30mS WITH SPEECH. At low frequencies, it becomes even more difficult. Below 30mS, we perceive the location of the sound based on the signal that arrives first. This is convenient in a car because the impact--the transient--part of percussive bass comes from the midrange speaker. This is WHY we can put the sub in the back and make it sound lie it comes from the front, even without adding delay to the front. 

Now, if you're building home speakers and you have a woofer that has to play 20 Hz and 2k, and your room has a mode that causes the bass to ring and ring and ring. You'll undoubtedly hear a smear. If you seal the box, you'll reduce the bass that energizes the mode and you'll hear an improvement. Voila! The sealed box sounds "faster". Then, you go to your box program and look at the group delay and you say, "Eureka! I've found the culprit."

Han's graphs are instructive because he's measured this IN A CAR. And his findings are that it doesn't matter in a car because at super low frequencies, the car behaves kind of like it's minimum phase. Getting the phase away from a -180 degree condition and the frequency response right between the sub and the midbass in the front is important. I find that we tend to ascribe a lot of poor performance caused by problems in that objective to other system traits. 

Try this: Find your favorite musical track that includes some bass with lots of attack. Open it in Audaacity or some other program and apply a 3rd order LP filter at 70Hz. Listen. See, there's no impact. It sounds like "bloam bloam bloam". Then, slowly move the filter up until you hear the snap. See, that's not bass. Then, guess what, Your subwoofer doesn't play the transient that causes the snap. Good news. You can put the sub almost anywhere and it will sound like it comes from the location of the midrange and tweeters.

Unless, of course, you boost the level of the sub so much that you overcome the precedence effect. Then, you'll hear the sub.


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## cajunner (Apr 13, 2007)

so, is it like Acoustic Resonance Control, where you have the internal standing wave inside an enclosure, being offset by a small port in the enclosure?

Acoustic Resonance Control<sup>®</sup> (ARC) | Technology & Engineering | Polk Audio®

is this where the car is the enclosure, and the subwoofer box is the driver, and the internal standing wave is acted upon by the box's output, both port and driver?

and ARC in this case, would be either a Helmholtz trap, or using active wave cancellation?

then you could stiffen up that vented box's harmonic distortion, which would back-end the 'smear' that comes from excessive group delay..

haha...

no, really, that was fun.


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## 14642 (May 19, 2008)

^^No, that's not it.


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## cajunner (Apr 13, 2007)

Andy Wehmeyer said:


> ^^No, that's not it.


right.

you can't fix group delay in a vented system, correct?

you can't hear it either, since it doesn't matter, is that also correct?

you can have as much group delay in your set-up as possible, but you won't hear it either way.


this is the gist of what I'm getting out of this thread.


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## 14642 (May 19, 2008)

No, that's not it either.


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## thehatedguy (May 4, 2007)

It's just that it gets harder to hear the lower you go...and in the bass in a car you probably wouldn't hear it if it isn't some crazy high number.

You could minimize it by using a smaller ported enclosure...but at those sizes you aren't gaining much if anything over sealed.


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## cajunner (Apr 13, 2007)

la la la la la...

group delay doesn't matter!





obviously, you can fix group delay.

you turn off the stereo, and voila! 

no more group delay.


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## cajunner (Apr 13, 2007)

and if you can appreciate that, then you'll see how I'm the group delay in this thread.

LOL


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## sqnut (Dec 24, 2009)

Appreciate your comments as always, Tks! This whole debate started cause I mentioned that I could never get a ported box to sound like a sealed one even with extensive control over time and response. 



Andy Wehmeyer said:


> yes, the port and the woofer are 90 degrees out of phase. At Fb, the port plays and the woofer doesn't. As you move away from Fb, you hear the combination, so the sum of the two signals is less than it would be if they were in phase. They sum like any two signals that are 90 degrees out of phase. We can equalize frequency response anomalies that result from a 90 degree shift.


No issues here. 



Andy Wehmeyer said:


> Group delay is like any other discussion of absolute phase. It doesn't matter, UNLESS the relative delay between one signal source and the other causes them to play the same thing at different times, or, as Patrick has suggested, to play different parts of the same event at different times.


Isn't the sound leaving the port and cone a case of playing the same thing at different times?



Andy Wehmeyer said:


> We don't hear those two events as separate events until the relative delay reaches about 30mS WITH SPEECH. At low frequencies, it becomes even more difficult. Below 30mS, we perceive the location of the sound based on the signal that arrives first. This is convenient in a car because the impact--the transient--part of percussive bass comes from the midrange speaker. This is WHY we can put the sub in the back and make it sound lie it comes from the front, even without adding delay to the front.


Yes, imaging cues are typically ~300hz and up. In most 2 ways and three ways 300hz and up is upfront in any case. We don't localize sound very well around the 30-50hz mark. The brain just places this sound with the rest of the frequencies that it is localizing. The snap and rightness of mid bass comes from the midrange. You need a correct response at 500-600hz, 1-2khz and ~5khz. You need to ensure that 80, 160-300 aren't bloated. Then you will get that tight snappy mid bass. 50-60hz from the sub gives your low end the mass.



Andy Wehmeyer said:


> Now, if you're building home speakers and you have a woofer that has to play 20 Hz and 2k, and your room has a mode that causes the bass to ring and ring and ring. You'll undoubtedly hear a smear.


In which case you should be able to identify the frequency and cut it at the eq. I tried to eq a ported box to sound like the sealed one and got very close~+/- 1-2db in the 30-60hz range. I got close but I could still pick the difference and I don't know why.



Andy Wehmeyer said:


> If you seal the box, you'll reduce the bass that energizes the mode and you'll hear an improvement. Voila! The sealed box sounds "faster". Then, you go to your box program and look at the group delay and you say, "Eureka! I've found the culprit."


 well, that's pretty close. 

I, along with countless others here really appreciate the time you take in educating us. So I'll try and take some more of your time.

1. As an audio engineer what are the distortion parameters (linear / non linear) that are critical while designing speakers? A post on distortion and it's relevance to what we hear would be great. 

2. For our brain to accurately perceive location of a sound it needs both direct and reflected sound, since location is based in part on timing. If we hear only direct sound eg headphones or an anechoic room our ability to locate sound goes for a toss (for one no imaging). Hence environments that give both direct and reflected sound are better for picking phase differences than say a headphone. Is this accurate?


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## Hanatsu (Nov 9, 2010)

Why do we need the reflected sound to perceive location? For a stereo image we need two sources, nothing else... reflections act like phantom sources that muddy up imaging.

Tapaaatalk!!


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## 14642 (May 19, 2008)

No, SQnut, the port plays at Fb and the woofer doesn't. It doesn't play mucg because it has trouble overcoming the higher pressure in the box at Fb--the radiation impedance is MUCH higher at resonance, so the power sent to the coil is much more efficiently transferred to the air. That's like the difference between a person swimming and flying. You can flap your arms easily in the air and go nowhere, but it's much harder in the swimming pool and less flapping results in more propulsion.


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## Hanatsu (Nov 9, 2010)

sqnut said:


> Appreciate your comments as always, Tks! This whole debate started cause I mentioned that I could never get a ported box to sound like a sealed one even with extensive control over time and response.
> 
> In which case you should be able to identify the frequency and cut it at the eq. I tried to eq a ported box to sound like the sealed one and got very close~+/- 1-2db in the 30-60hz range. I got close but I could still pick the difference and I don't know why.


I have managed to do this several times. A year back I did a little blind test. Used a mid EBP driver in a sealed Qt 0,7 box and the other driver in a QB3 vented alignment (Fb 30Hz), EQed both to the same, used the same crossover settings, gain within 0,5dB etc. I had 5 different people listen if they could tell the difference. At low and moderate levels we all failed hearing any difference with a wide set of audiophile grade recordings, various electronic and rock music. At higher volumes all of us picked the vented as better sounding. The only explanation would be the so called "inaudible" non-linear distortion. At Fb the non-linear distortion can be extremely low. I've measured sealed and vented enclosures using the same drivers, the sealed had 10-15% THD, while the vented had 1-2% THD at the same output (around Fb)... It's clearly audible!! Recently I've noticed that high non-linear distortion masks pitch change, the lower distortion, the easier it is to pick pitch changes* (*The interesting thing here is that the effect is even more pronounced using two-tone signals). My theory is that IMD cause this, as 2nd/3rd order IM products will be very close to the fundamental. What I think happens is that a high "noise floor" of distortion actually masks "details" in the linear domain. I have to research this more before any real conclusions.

Tapaaatalk!!


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## sqnut (Dec 24, 2009)

Thanks for saying they sound different, lets leave better or worse aside.

I don't believe I spent two days talking about phase distortion. Reliving the frustration with the ported enclosure masked out the common sense about phase. Normally, I never think about phase issues. In a car phase is nothing but correct arrival times and correct polarity. Occasionally I will play with the delay between drivers and that is the extent of my focus on phase. Even when I'm tweaking the TA I'm either trying to pan the image a bit or correct a response issue that is not getting addressed via the eq. That's as far as phase goes. 

In a car trying to maintain phase coherence is a futile exercise imho. Move your head by an inch and your drivers are out of phase. Yet with the right response, none of this matters. It will sound great and image well every time you get in the car, even though you're sitting slightly differently each time. In a car it is about the right response.


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## cajunner (Apr 13, 2007)

Hanatsu said:


> I have managed to do this several times. A year back I did a little blind test. Used a mid EBP driver in a sealed Qt 0,7 box and the other driver in a QB3 vented alignment (Fb 30Hz), EQed both to the same, used the same crossover settings, gain within 0,5dB etc. I had 5 different people listen if they could tell the difference. At low and moderate levels we all failed hearing any difference with a wide set of audiophile grade recordings, various electronic and rock music. At higher volumes all of us picked the vented as better sounding. The only explanation would be the so called "inaudible" non-linear distortion. At Fb the non-linear distortion can be extremely low. I've measured sealed and vented enclosures using the same drivers, the sealed had 10-15% THD, while the vented had 1-2% THD at the same output (around Fb)... It's clearly audible!! Recently I've noticed that high non-linear distortion masks pitch change, the lower distortion, the easier it is to pick pitch changes* (*The interesting thing here is that the effect is even more pronounced using two-tone signals). My theory is that IMD cause this, as 2nd/3rd order IM products will be very close to the fundamental. What I think happens is that a high "noise floor" of distortion actually masks "details" in the linear domain. I have to research this more before any real conclusions.
> 
> Tapaaatalk!!



what I'm getting from this post, is that you find at higher output levels, the vented box sounds cleaner.

the issue, (or point?) with this to me, is that the mechanism of excursion control that a vent provides, is not available in the sealed box.

if you can keep excursion down at higher input levels, you should theoretically be able to get clean output, or more clean output from the vented box.

the problem with higher input levels is the structure, or mechanical stress peaks of extra power start to decouple the materials, the cone beings to warp, the coil begins to rock, the suspension does it's little deformation dance, and distortion creeps in.

but, since most subs are operating well inside their pistonic region, you can get that extra output and you see this with most conservative power input setups, (outside of burp zones) but that does not address the sound of vented, or the fact that no matter what you do, the time it takes for a cone to move air, and the subsequent "rotation of linear sine" into the group delay for the port/passive radiator/horn of whatever degree, is audible.

I've never done the comparison you have, however. Like the amplifier discussion, I've never pushed two enclosure schemes into a highly equalized, ABX comparison scenario.

I don't even believe it is possible inside a car, unless drastic techniques are used because if you put a sealed box and a ported box inside a vehicle together, when you go to test them, one is going to act more like a helmoltz trap than the other, and affect the response audibly in the small, higher pressure volume of a car's interior.

and as long as I believe there is a difference, perception bias doesn't allow me the opportunity to accept that I can't distinguish between sealed and vented, it's humbling to accept that I would eschew the ported dynamic based on conventional wisdom. A wisdom I've come to accept as truth.

so, any idea that sealed boxes are higher in distortion, anywhere, but especially within the confines of the car interior and against the higher static pressure of a small volume listening space... I have no proof, no evidence, and I just have my belief that it isn't so. I find I can hear a sealed box's signature, and it sounds like it is beating that higher interior pressure when in fact, it may not be, in comparison.


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