# 2 Questions I have trouble with: Subbass direction, and pathlelengths



## ErinH (Feb 14, 2007)

A while back, there was a thread that touched on location of subwoofer and the effects of output. I've done some experimenting myself and I've found that in my trunk, the subwoofer right next to the trunk (rear of the car) and facing that direction provides the most output. The further the sub is moved toward the car's cabin, but still facing the rear, the less output there is, and finally if the sub is faced toward the cabin the output is the least of all. All these scenarios are with the rear seat up. I know this is what happens, but I'd like to know _why_. Is there cancellation issues somewhere, is it due to the bass at the rear of the car having no room to dissipate and be pushed back forward due to immediate reflection, what is it? I'm a visual learner, so if there's any pictures or the like to explain this it would help. 


Secondly, what is the importance of pathlength differences in our situation (car audio)? I rarely see this topic brought up here, but in other forums it's almost a guarantee you'll see it brought up. So, what's the big deal?

Hopefully this will help me get a better understanding and allow me to tune my car a little more efficiently. right now I feel as though my subbass is easily distinguished as coming from the rear, and my midbass could be better, but I'm not sure if it's due to the pathlength phenomenon.


----------



## durwood (Mar 7, 2007)

Think of the peaks of a sinewave, where are they located in the timedomain? SPL guys use 1/4 wavelength theory and some of the super loud guys try to position the subs 1/4 of the total distance of the wavelength for the frequency they like to use to burb at from the mic location. This doesn;t exactly work for us guys who listen to music vs "burps". Then there is the 3db gain of subbass for each surface the subbox uses for waveguides, so corner has essentially 6db of gain over being set in the middle of a room.

Try working with the time delay of your midbass and sub only. Note this time value Then once you find the magic spot, you can readjust your midbasses so they align with you midrange/tweets. then you might need to delay your subs by the difference you found from before and the new value.

Example (values are fictious).

midbass delay is 1.5ms (drivers side) and 0.4ms(pass side) to get it to blend with sub.
Then you adjust the midbasses to lets say 2ms/0.9ms to blend with the rest of your front stage.
Then delay your sub by 0.5ms difference.

I've been trying to solve my midbass/subbass integration issues for a long time and I am finally tried the example and it worked great. I don't know why I never had thought of it or tried it before. I felt kind of like a noob.:blush:


----------



## 89grand (Nov 23, 2006)

Bikinpunk, check this out. It sure seems to a be a logical and accurate description of what the problem is and why.

http://www.teamrocs.com/technical/pages/aiming.htm


----------



## durwood (Mar 7, 2007)

Here are some useful formulas you can play with.

v = 331.5m/s + 0.6T

where
v = velocity of sound (m/s)
T = temperature (°C)

then 

lambda = v/freq

lambda is the wavelength of sound in meters.

for example

@20C
v=343m/s
freq=80Hz

lambda = 343/80Hz = 4.28meters
lambda/4 = 1.14 meters

40hz is twice the distance.

Finding the right way to line up your midbass to sub can be tricky, since frequencies change, so IMO there will always be some type of phase cancelation between midbass/subbass when they are located so far apart from each other.


----------



## ErinH (Feb 14, 2007)

durwood said:


> Here are some useful formulas you can play with.
> 
> v = 331.5m/s + 0.6T
> 
> ...


lol...is that referenceing pathlenght?

My main question with pathlength is how does it effect what we hear? Is pathlength used to discuss cancellation? 

89, thanks for the link. Those drawings are perfect for me.


----------



## Whiterabbit (May 26, 2006)

question:

I read that the results of a volume test match the widely accepted theory as shown on eddie runner's page.

.....so which SOUNDED better?


----------



## ErinH (Feb 14, 2007)

Whiterabbit said:


> question:
> 
> I read that the results of a volume test match the widely accepted theory as shown on eddie runner's page.
> 
> .....so which SOUNDED better?


Now, you know that can't be answered legitimately. Sound is relative, you silly.


----------



## durwood (Mar 7, 2007)

bikinpunk said:


> lol...is that referenceing pathlenght?
> 
> My main question with pathlength is how does it effect what we hear? Is pathlength used to discuss cancellation?


I am taking about wavelength and how the pathlength difference between your midbass and sub plays a part in what gets canceled out. It helps show why your sub can sound louder in particular places in your trunk. It varies by frequency, temperature, humidity, and the transient response of your speakers used so I recommend experiementing to find the best compromise of your desired end result.


----------



## MarkZ (Dec 5, 2005)

Whiterabbit said:


> question:
> 
> I read that the results of a volume test match the widely accepted theory as shown on eddie runner's page.
> 
> .....so which SOUNDED better?


If you guys want a laugh, dig up the Eddie Runner and Tom Nousaine threads from google groups...


----------



## 89grand (Nov 23, 2006)

MarkZ said:


> If you guys want a laugh, dig up the Eddie Runner and Tom Nousaine threads from google groups...



I've read that. It was great.


----------



## Kuztimrodder (Jan 10, 2007)

Bikin, is there a way you can try downfiring just for ****s n giggles? Maybe even try different distances from the driver to the trunk floor. I'd like to hear how this compares to the rest of your tests. Might surprise you


----------



## ErinH (Feb 14, 2007)

Kuztimrodder said:


> Bikin, is there a way you can try downfiring just for ****s n giggles? Maybe even try different distances from the driver to the trunk floor. I'd like to hear how this compares to the rest of your tests. Might surprise you


Might be able to sometime. I'd have to put it up on some 4x4's to get it to sit high enough that the subwoofer won't hit the floor.


----------



## stryke23x (Jun 22, 2007)

MarkZ said:


> If you guys want a laugh, dig up the Eddie Runner and Tom Nousaine threads from google groups...


The sad thing is that Eddie Runner's measurements are very flawed and though they basically show what he wants with no validity. It really frustrates me when people start posting this kind of information as fact when it is completely wrong, and then they get other people believing it. Both in theory and in measurement Tom Nousaine is correct in what he states. The measurement method of using a sweep as Eddie Runner did will not measure anything accurately in a vehicle because you are giving a false reading. You need to use a short MLS tone that you can time gate to remove reflections. Otherwise you are falsely adding to the level as the reflections are read. 

I've given the extreme example before of shouting across the grand canyon. If you measure with his method, you would read a certain level, then as the echo came back it would be added to the signal and slightly increase the level. The fact is that the echo didn't make the original shout any louder, it just measured that way, hence the flawed method. The correct method is to measure the original shout and stop the measurement before the reflection comes. This is called time gating which you need to do in any kind of measurement like this.

The problem is that he is using a sweep, which you CAN time gate, but it gives wrong results. Lets take this graph here:

http://www.installer.com/tech/neonresponse.jpg

The green is the box firing directly to the mic forward. The purple is the box firing backwards. Now keep in mind his sweep goes from 120hz down. What happens if you start the time gate too late? The beginning of the sweep isn't measured making the reading on the top end much lower than it should be. Hmm.. that explains why it is measuring something like 25dB lower at 100hz and then it increases in level as it goes down. The peak going up from there to 120hz is typical of some kind of resonance in the vehicle cavity. The reason that measures is because a resonance stays around longer, that's why it's called a resonance. 

So that all said, why does the purple measure higher? Simple, turn the box around and you have about another 6ft of path length. That is a time difference of pretty close to 5.3ms longer that it takes the sound to get to the mic bouncing off the back of the trunk first. Most likely he time gated for this first so it didn't chop off the front of the sweep in the measurement, hence measuring the whole sweep and showing the higher output level. 

To do this all accurately would require the MLS tone and you have to examine the impulse response and time gate each measurement accordingly to compare. Then you would want to look at a quick waterfall(decay) plot without any time gating to actually view your in vehicle resonances so you can get rid of them. 

Now, in regards to Eddie Runner's drawings there is also another flaw even if his method were correct. He shows how the frequency is "out of phase" by a certain amount based on the distance difference between reflected and direc path. That may be true, but only at ONE frequency as the wavelength is different at ALL frequencies. Lambda is the wavelength which is 1130/frequency to get in feet. So while he could claim that at 100hz the direct and reflected wave would be out of phase, at other frequencies they would be in phase, 5degrees out, 10 degrees out, and on and on. The same goes for when the wave reaches the front of the vehicle and then bounces towards the back again. At 20hz the wave needs to bounce back and forth from the front of the vehicle about 5 times to even complete a single 54ft wavelength. 

Anyway, if anyone wants to see a real, valid test, let me now. If people want to come up to Green Bay, we can have a get together and we'll do the measurements with a bunch of people present. And at the same time do an ABX test to see if anyone can pick out whether the sub is firing forward or backwards to see if it "sounds" different too. 

John


----------



## 89grand (Nov 23, 2006)

Well, I didn't perform the test so I can't say much about it in that regard. What I can say is that with my very first sub from nearly 20 years ago, a Kicker C10 (the good ones) that I had almost zero bass in the car with the enclosure firing up towards a hole in the rear deck. When I mounted the sub enclosure facing towards the rear of the trunk, the bass improved 10 fold. That's when I came to the realization that something was up.

Why? If not what Eddie Runners experiment suggests, then what?

Really, I'm curious.


----------



## MarkZ (Dec 5, 2005)

stryke23x said:


> The sad thing is that Eddie Runner's measurements are very flawed and though they basically show what he wants with no validity. It really frustrates me when people start posting this kind of information as fact when it is completely wrong, and then they get other people believing it. Both in theory and in measurement Tom Nousaine is correct in what he states. The measurement method of using a sweep as Eddie Runner did will not measure anything accurately in a vehicle because you are giving a false reading. You need to use a short MLS tone that you can time gate to remove reflections. Otherwise you are falsely adding to the level as the reflections are read.
> 
> I've given the extreme example before of shouting across the grand canyon. If you measure with his method, you would read a certain level, then as the echo came back it would be added to the signal and slightly increase the level. The fact is that the echo didn't make the original shout any louder, it just measured that way, hence the flawed method. The correct method is to measure the original shout and stop the measurement before the reflection comes. This is called time gating which you need to do in any kind of measurement like this.
> 
> ...


I haven't read his site (or that thread) in a long time... I thought his point was that it was about position and not aiming? That seems consistent with what you said.

As for the phase, the sub is often used in a rather narrow band. The "useful" bandwidth (at least what we usually care about) tends to be less than an octave. So cancellation and coherence can most certainly be a factor across the sub's entire bandwidth without the effect being humpy, which is what you might expect if the same phenomenon was occurring across a couple octaves. I fail to see how that's a blow to his position.


----------



## Abmolech (Nov 2, 2006)

To achieve a reflection you require a surface that has blocking ability. A car panel can only block 12 dB, the rest simply goes through it. (The reason why you can hear it outside of a car). The primary factor in this phenomenon is acoustic impedance mismatch. We are trying to transfer electrical to mechanical to acoustic.Because there is such a great mismatch between mechanical and acoustic impedance, less than 5 % of the energy is transfered (The remainder transfers into heat)

The major reason for bass boost in a car is a far better impedance match compared to a small room etc. (Usually about 12 dB) This is because it is loaded, IE it is forced into a smaller area than it is capable of. A flat baffle of appropriate size will cause a 6 dB gain in output. (Assuming a proper baffle capable of blocking, for most people with their SD reduce this to 3 dB)This is the most basic form of a wave guide, and is the principal driving force behind better mid bass by "sealing" up the doors, IE making a baffle.

Logically a wave will propagate in a spherical form unless it IMPEDED. Therefore a baffle will cause it to propagate in a half sphere. This is rated in steradians (Solid, where we get the word "stereo" from) The more we impede the shape the GREATER the acoustic load, and the higher the gain. Obviously the baffle must be large enough to cause this action. When we achieve around 8 times the load we normally call them horns.

http://www.trueaudio.com/st_spcs1.htm

Point
The sub wave will radiate out in a spherical pattern, the most gain will come from loading it.

Mid bass integration is a function of time (In phase) and matching group delay. (effected largely by the complexity of the network to produce the slope, weather that be acoustic and/or electrical)


----------



## stryke23x (Jun 22, 2007)

The whole situation is all about the transfer of energy. Energy can't be created or destroyed, only change form. There are other things to take into account also in the vehicle that may cause differences in measured output at certain points that have nothing to do with the pathlength. For example, firing directly into a back seat in close proximity will cause an extra mechanical resistance on the woofer. If it can't move as easily with the same given power, it won't produce as much output. The extra energy in this case is wasted as heat in the VC. 

The more solid the surface you are firing at, the less energy will be lost. The back of a trunk is more solid than the trunk lid, so firing upwards towards the trunk could use up a lot of energy in making the body panel resonate. Many practical examples illustrate this idea of losing energy. Try holding onto a 2x4 and hammering a nail into it and you move the 2x4. Most of the energy is used up in moving the 2x4 around and not driving the nail into it. Now set it down on a concrete floor. The 2x4 doesn't move like it does in your hand and the energy is transfered to the nail. If you ever see someone who has a trunk lid that jumps all over, have someone sit on the trunk which will quickly make it more rigid. You'll notice quite a difference in the output inside the car vs when the trunk lid is bouncing around.

John


----------



## 89grand (Nov 23, 2006)

And that means...what? That bass response does or does not improve based off its location in the trunk?

I don't want to come across as an ass, but I'm not following you at all.

This question is for Abmolech


----------



## chad (Jun 30, 2005)

Abmolech said:


> Point
> The sub wave will radiate out in a spherical pattern, the most gain will come from loading it.


Hot Damn! I did not have to post it this time


----------



## stryke23x (Jun 22, 2007)

Abmolech said:


> To achieve a reflection you require a surface that has blocking ability. A car panel can only block 12 dB, the rest simply goes through it.


Actually how much the panel blocks is a correlation between the thickness of the panel and the wavelength of the frequency. At 10KHz a car panel will absorb much more than at 10Hz. You'd need to find the absorption coefficient at the given frequencies to say exactly what kind of level it will block. The absorption coefficient would also only be valid if the panel was completely rigid. As the panel moves, more energy is used up in the resonance and less energy will be transfered to the outside of the vehicle, just like less energy is reflected inside the vehicle. In any case, yes, at low frequencies a lot of energy is transfered through to the outside world.

John


----------



## Abmolech (Nov 2, 2006)

> For example, firing directly into a back seat in close proximity will cause an extra mechanical resistance on the woofer. If it can't move as easily with the same given power, it won't produce as much output. The extra energy in this case is wasted as heat in the VC.


If it changes the "Q" of the driver, then, this is true. Otherwise it is false.



> The more solid the surface you are firing at, the less energy will be lost. The back of a trunk is more solid than the trunk lid, so firing upwards towards the trunk could use up a lot of energy in making the body panel resonate.


Half correct. It is how well it blocks a wave. The reason for echo (distance aside) is mostly due to the ability to block.

The less of a sphere it forms, the higher the loading, and the better the energy transfer. This is primarily determined by baffle shape and size ratio to the lowest wave length expected. (IE a minimum of 1:1)and to baffles blocking ability.

This is the reason why horns (and to a lesser extent) wave guides are so efficient.

12 dB at 125 Hz on a steel panel 1/16 of an inch thick. 
Yes I took liberty in assuming we were only talking about sub bass my apologies.


----------



## stryke23x (Jun 22, 2007)

Abmolech said:


> If it changes the "Q" of the driver, then, this is true. Otherwise it is false.


It doesnt' need to change the Q of the driver, just the Q of the system. The same as shrinking the box size in half. The driver Q isn't changed but the system Q is changed. This means more power needs to be input to reach the same output level and in turn more power is being wasted as heat. 

John


----------



## Abmolech (Nov 2, 2006)

Agreed,
I again took the liberty of assuming the only "Q" we were concerned with was the electrical to mecahnial transfer and therefore I did not specify this.

I apoligise for taking liberty again.


----------



## stryke23x (Jun 22, 2007)

no need to apologize at all. I love to have discussion with others where others can learn something, we can be challenged, and hopefully learn something ourselves as well. 

John


----------



## Abmolech (Nov 2, 2006)

If we take the highest expected frequency to be produced by our sub woofer (Well as high as we want to listen to it)as being 80 Hz we have approximately a 14 foot wave. To cause standing waves we require a particular set of circumstances.

First, and possibly the most important, is the actual distance between the reflective surfaces (blockers)
Second is the angle of incidence of those blockers. 
Third is the measuring position.

Do we have a correct distance to obtain a standing wave in a car at 80 Hz?
Since they can only occur every quarter, it would require a MINIMUM length of 3.5 feet. So yes we could achieve a standing wave if we consider distance in isolation.

Do we have blockers in parallel? (Or close to it)

We have a flat boot area, and an angled front windscreen, with possibly a return rear screen (angled) These provide 22 dB reflection at 125 Hz. Problem is they aren't in parallel.
How about the doors windows? Possibly.

Measuring position.

If we assume the only standing wave of concern are the door windows, we could achieve a significant null there at a specific frequency. (Centre distance) This could be measured and calculated (God forbid ) or we could wind down a window .....


----------



## t3sn4f2 (Jan 3, 2007)

So would mounting the sub box on a properly installed and sealed from the trunk baffle that fires into the cabin and putting that baffle at a distance where the bass sounds the loudest at the listening position be optimum for a trunk mounted sub box? Assuming I don't want to rattle my car to death by firing the sub right into the rear of the car.


----------



## Astral (Sep 6, 2005)

One thing I noticed about Eddie's plots is that smaller cars tended to have a lot less difference starting from a higher frequency and going lower than the bigger cars/SUVs. Some conversation was that below a certain frequency, you start getting standing waves. So the shorter the trunk/smaller the car, the higher this frequency is. Is this correct?

If that's correct, then subwoofer aiming matters less if you play only lower freqs.

I measured bass response once in my older car with the sub facing towards the rear, the front, the side and with the trunk open (chart). These were sweep measurements, so it's measured similarly incorrectly to Eddie's (not using MLS). At any rate, the yellow line shows sub facing into the back, green into the side and orange into the rear. Purple is trunk open (shows loss of cabin gain). It seems that all the major differences were above 65Hz. Since these bass frequencies are easy to hear and are what most people consider "bass", it's understandable why most people correlate bass output with location. However, for me, subbass is most important, so 60hz and below seems to be barely affected.


----------



## Guest (Aug 8, 2007)

I've found that simple, first-order reflection theory ... image sources behind reflecting surfaces, and destructive interference from delayed sources (which forms the basis of comb filtering. Yes, the comb is not just restricted to midranges and tweeters) ... does a remarkably good job of predicting bass cancellation for trunk-mounted drivers. Sure, we are really talking about standing waves, and reflecting surfaces that absorb/transmit a significant fraction of the incident energy ... but simply modelling the back of the trunk as a reflective surface, beyond which there lies an acoustic "image" of the driver, will give you _more_ than just a simple, qualitative feel for what's going on 

In other words, visualize the following : whenever you place a driver (particularly one whose dimensions are short compared to the radiating wavelength, so that it tends to radiate uniformly in all directions) near a reflective surface, the easiest way to analyze the resulting acoustic propagation (primary plus reflection) is to imagine an "image" driver _behind_ the surface ... same distance behind as the real, primary driver is in front ... and then imagine the reflecting surface removed  So the resulting acoustic propagation is from _two_ drivers : primary plus image.

What happens when two drivers are playing the same frequency ... but one is delayed from the other (the delayed one is the image ... it's a couple feet behind your trunk lid)? Constructive and destructive interference, of course  Bass loading, cancellation depending on distance from back of trunk ... are all readily explained by this simple model. More math details can be provided, of course ... but this alone should be enough to explain why the first bass null moves _down_ in frequency as the sub is moved _farther_ forward from the back of the trunk (assuming that the cabin is not acoustically isoalted from the trunk, of course).

Lots more details can be found in the ECA tech section, in the infamous "Dashboard Domes and Reflection Combs" thread. We even accurately predicted the first bass cancellation null for subs in the trunk of a civic


----------

