car acoustics

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car acoustics

Postby Brian Dayton » Sat Jul 17, 2004 8:08 pm

i know this is a bit off the normal topic, but it's a question i've had for a decade now.

in my college days i was a car audio buff, and the going theory concerning in-car acoustics was this:

at some frequency sound in a sealed car stops being a wave and becomes a simple pressure change. i read the word hydraulics used in context with this once, and phrases like "the frequency of pressurization" and so forth.

below this frequency the sound level is boosted by 12db/octave with decreasing frequency. typically this occurs at ~60-80hz, and the theory at that time was that if one built a sealed box subwoofer it's 12db/octave rolloff would match the 12db/octave cabin boost and yield a flat response to essentially DC.

at the time i used the equations i was presented with and designed a sub box to match. what i got is described in the attached pic, data taken with a retailers RTA, which was said to be calibrated/accurate/etc., but which i cannot speak for.

anyway, some ancient curiosities of mine, perhaps someone has a thought

1. this is all based on pressure in a sealed "box". why, then, does the low frequency volume in cars not plummet when the windows are open. it is fairly common for car stereo buffs to state the opposite.

2. is a pressure phenomenon like this ever discussed in room acoustics?

3. the spike/dip and less than theoretical boost are things i've hypothesized about in the past, but perhaps a formal answer could be had here.

the woofer would have been almost exactly at the rear of the car, aiming at the rear, the measurements were taken at ~ ear height near the front (drivers position, but centered in the car from side to side).

take care, and i hope you don't find this a waste of board space.

Brian

PS: i asked this a year ago or so on a car forum, and my query didn't garner much feedback, hence i ask here.
Attachments
in car response.gif
in car response.gif (5.27 KiB) Viewed 12740 times
Last edited by Brian Dayton on Sat Jul 17, 2004 8:37 pm, edited 1 time in total.
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Postby Bob » Sat Jul 17, 2004 8:16 pm

I thought that in cars that LF goes right through the walls/floor/roof anyway, so opening a window wouldn't make that much of a difference.
Whereas HF is reflected within the car and opening the windows (ignoring the outside ambient noise) would be a 10% change in difuse HF in the car.
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Postby bert stoltenborg » Sat Jul 17, 2004 8:29 pm

No Bob,

A loudspeaker enclosure is only a MDF/plywood box with a bad sound isolation, but sound behaves logaritmic, so a 20 dB isolation is a factor 100 of sound power decrease. When you are in the noise this is quit a lot. Else the out of phase sound radiated of the back of the speaker would hardly be damped by the enclosure and you would have hardly any bass in a room.
Have to think about Brians question (interesting), but now I must leave to play the old guitar.
Maybe the real smart guys come up with an answer.
See you tomorrow.
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Postby Brian Dayton » Sat Jul 17, 2004 8:32 pm

bob,

well, something out of the ordinary occurs in cars

a "sport" of sorts, or an engineering excercise, rather, is to design cars for max low frequency SPL. 170db+ has been recorded inside of the vehicles with a single woofer, which is probably 50+db more than the woofer would register in a large room.

my old car would do 145db @50hz with 2 10 inch woofers. the same woofers/boxes/power wouldn't do >100db at 50hz in my living room when i got the clever idea to find a way to get evicted from my apartment.

nobody is in the cars when this is tested.

i presume that if all the sound was passing out of the car, we'd wouldn't get 40db of gain...

Brian

PS: my 100db living room would have been based on a radio shack meter. it wouldn't have occured to me to test for a mode/dip as i'd never heard of one. but it was tens and tens and tens of db louder in the car regardless.
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Postby Scott R. Foster » Sun Jul 18, 2004 12:04 pm

170 dB? - Yikes!

Just put the shotgun barrel against the roof of your mouth and pull the trigger with your toe... a less complicated approach to a pressure wave that'll blow your mind. Or maybe make yourself a pair of ear muffs with two of these:

http://www.smarthome.com/8248.html

Just imagine 370 lbs. of force driving into each side of your skull with every beat... be sure to glue AND screw for the full effect.

And yeah, no doubt the car interior leaks LF [you can hear the thump from hip hop lowrider at the trafiic light above all the higher frequency components well enough to know that much] but no way does it all just pass thru... surely the air in the can is pumping up as Brian describes.

To some extent I would reckon it's simply the ratio of air volume in the enclosure to air displaced by the transducer's cone excursion.... while sub woofer "X" may achieve displacement of an impressive fraction of the total volume of air in the car, doing the same in a typical room would require an extensive array of same type sub... most folks don't do that, so you don't have the same phenomena arising... well not typically... remember the guys with the masonry folded horn sub vaults in their floor covered with marble slabs?

Also, aint the car body itself induced to vibrate and reradiate [?] so that in some sense, in a hyper-sub automotive application you are "bass shaking" the whole house?

And what about the distance the propogated pressure must traverse [a few feet at most] both when radiated, and as to any re-radiation / resonance vs. the inverse square law diminishment encountered in a room sized space?

In short, it's a whole different world inside a can... and no you don't have the same results in a normal room. Seems to me it is kinda like talking about why a ceiling fan behaves so oddly in your car... I would answer: "the odd behavior is because it's a FREAKIN' CEILING FAN AND IT"S IN YOUR CAR!

All kidding aside... I trust we will get the real answer once Bert finishes thinking about not what a speaker does to a room, but rather what happens inside a speaker... seems to me that is the proper paradigm.
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Postby Philip de Haan » Sun Jul 18, 2004 12:14 pm

Brian,

Think of a the box in the car as a giant bandpass system. The box as the fist chamber with the second chamber (the car interior) tuned to 0Hz. As pointed out by Bert this is a leaky system, but this won't hurt much the efficiency boost. So you will gain a lot of extra SPL compared to a normal living room. Wich is much bigger so the boost will start at a much lower frequency.
I guess the at very frequencies the car gets so leaky that the boost is lower than expected from the simple pressure model. Another deviation from the expected flat response from 0Hz up, could be the microphone and preamp setup. Both having a high pass of maybe at least 10Hz.
Opening the window will tune the second chamber bandpass to a very low frequency (maybe 10Hz) or so. This will only boost the frequencies around 10Hz and leaving the pressure model above 10Hz unaffected.

Philip.
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Postby Brian Dayton » Sun Jul 18, 2004 7:35 pm

Also, aint the car body itself induced to vibrate and reradiate [?] so that in some sense, in a hyper-sub automotive application you are "bass shaking" the whole house?


well, typically for high-output systems, the panels are damped (products liek dynamat and brown bread, etc.) to prevent that vibration, which deletes from output. a marketing example:

http://www.dynamat.com/
click on technical
click on get more bass
click on the pdf file

ok, the small volume ... i wonder if a formula exists to predict spl based on woofer displacement in a given room with a given volume?

philip commented on the open window acting as a vent of sorts, to maintain the pressure model, which is interesting.

thanks for the thoughts, fellas
Last edited by Brian Dayton on Mon Jul 19, 2004 7:39 pm, edited 1 time in total.
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Postby Scott R. Foster » Sun Jul 18, 2004 11:32 pm

well, typically for high-output systems, the panels are damped (products liek dynamat and brown bread, etc.) to prevent that vibration, which deletes from output. a marketing example:


Nonetheless... at anything like the sound pressure levels you mention I have doubts that a viscoelastic layer is gonna keep the car from shaking.... dampen the metal panel resonances somewhat? Ok, sure, but with a sub operating at 140 dB I bet the funky beats would tickle your ass if you leaned on the car! Forget the metal panels, at those pressures levels I would expect the windows in their pliable rubber mounts to act as transducers.
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Postby Brian Dayton » Mon Jul 19, 2004 2:34 pm

Nonetheless... at anything like the sound pressure levels you mention I have doubts that a viscoelastic layer is gonna keep the car from shaking.... dampen the metal panel resonances somewhat?


yeah, i think that is probably just what happens - damp the resonances somewhat. perhaps the same ratio applies - 10db below the sub level at 100db = 90db, 10db below at 170db = 160db, so it's shaking like crazy, but enough below the signal to let spl go up a tad.

?
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Postby Dan Nelson » Mon Jul 19, 2004 4:03 pm

I think Philip is correct. One thing that sticks out in my mind, is cars are not really sealed. They have extensive ventilation and climate contol systems. I think you will find there are one way vents to allow the pressure in the car from closing doors, fresh air intake, and extreme spl levels, to release to the outside. I think this is the reason you do not see the results you expect.

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Postby Brian Dayton » Mon Jul 19, 2004 4:11 pm

I think Philip is correct


yeah, i think the window-as-vent idea is a great concept.

One thing that sticks out in my mind, is cars are not really sealed. They have extensive ventilation and climate contol systems. I think you will find there are one way vents to allow the pressure in the car from closing doors, fresh air intake, and extreme spl levels, to release to the outside. I think this is the reason you do not see the results you expect.


very sensible. what about the pig peak at 80hz? the lowest frequency standing wave before the semi-pressurized system takes over?
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Postby Dan Nelson » Mon Jul 19, 2004 4:33 pm

I have no idea on the 80hz peak. It could be the ventilation system tunes the the car to 80hz in your case. I think a test would be to see if the peak moves if the windows are open. As Scott is known to say it little more then a "wild ass guess". Do you have any links to explain the pressure theory. Car acoustics are very complex and without some very expensive software and a few years (well for me anyway)to learn how to use it "a wild ass guess" might be the most you can hope for.

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Postby Brian Dayton » Mon Jul 19, 2004 4:55 pm

this was widely discussed online as recently as the mid/late 90's, but a search earlier this week before posting this thread didn't lend me much in the way of references to aim you at.

some of the things i did find:

http://www.infinitysystems.com/caraudio ... K%20om.pdf - page 2, left column mentions the 12db/octave boost with dropping frequencies

http://www.diysubwoofers.org/projects/cartf/
this link has a graph not unlike mine

i didn't search broadly, but in my searching i didn't find any of the pointed explanation that used to abound online.
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Postby Bob » Mon Jul 19, 2004 5:36 pm

Have you sent an email to Ford? Sometimes they'll forward it off to the right people.
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Postby bert stoltenborg » Mon Jul 19, 2004 7:17 pm

Brian,

have you measured or calculated the speaker/box system behaviour in free field?
you can easily tune a speaker/cabinet combination to a peak in the bas area.
what speaker was it and how big was the enclosure?
as Flip correctly described, a very big enclosure like the cabin of an american car with some vents on it (ventilation etc), would probably still be tuned very low, much under 80 Hz.

Philip's name was spelled incorrectly and I know how he hates that, so let's hope he's not to pissed off to give us more to think about. :-) :-)

regards,

bert

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Postby Brian Dayton » Mon Jul 19, 2004 7:38 pm

The curve above would reflect the predicted bass response of the sealed enclosure with publish T/S parameters. no ports to miscalculate and cause such excitement.

incorrectly spelled names and soggy breakfast cereal are unfortunate. :P

further measurements aren't possible, unfortunately, as the car is long gone.

thanks again, folks
Last edited by Brian Dayton on Mon Jul 19, 2004 8:30 pm, edited 1 time in total.
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Postby bert stoltenborg » Mon Jul 19, 2004 7:53 pm

you don't need a vent to create a peak in your response, this can be done with a closed system.

bert
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Postby Brian Dayton » Mon Jul 19, 2004 8:31 pm

it can, but the calculated response based on published T/S parameters combined with the low-pass filter are shown above. it was not a hi-Q sealed box.
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Postby bert stoltenborg » Thu Jul 22, 2004 4:06 pm

Philip offered to trow a couple of 10 ''s in his car and do some measurements, just to get a feel for the real world situation.
How about it, Flip? :-) :-)
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Postby Philip de Haan » Sun Jul 25, 2004 3:37 pm

What to do on a rainy sunday afternoon.

Here are the results of my measurements (that were promissed by Bert).
Conditions:
Ampeg 4x10" enclosure (closed box) measured half space and calculated to full space.
This is the dotted line in mercedes.pcx.
Next I measured this box lying on its back in the trunk of a Mercedes 200 stationwagon. The microphone was lying on the driver's seat. This the solid line in mercedes.pcx. The car interior length is about 3m. This gives the 1st standing wave at about 50Hz (The interior damping will slow down the speed of sound a bit).
The file windows.pcx shows the infuence of opening the windows. Solid line: all 4 windows open. Dotted lines: no, 1, 2 windows open. Closing and taping the ventilation system didn't shoe any differences.

Philip.
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