# Acoustic foam...NOT sound deadening...inside car



## ZeNmAc (Sep 11, 2010)

Does anyone use acoustic foam to get rid of reverb or excessive reflections inside their cars? Acoustic foam and bass traps are used in home theater/listening rooms which already are a better environment for speakers. Anyone use them in cars?

If so, where and how much?

I've noticed I always have a nasty midrange peak(s) in vocal range somewhere between 1k-2k. I thought it was my old speakers but after putting in new ones I still have my eq set to -6dB at 1.2k (Q is 1).

I haven't even finished yet, so that was by ear since I haven't had a chance to take measurements of the current speakers. I was thinking about getting wedge or pyramid type acoustic foam and putting it on the sides of the center console which is right across from the speakers in the door. I'm sure I'm not the first one to do this, in fact I think I remember seeing someone do something similar.

Any comments from you pros ?


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## chefhow (Apr 29, 2007)

I use it BEHIND the speakers in my car and have seen it used by MANY competitors in various places depending upon the car and the install/need.


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## Hanatsu (Nov 9, 2010)

The 1kHz-2kHz range is probably caused by window reflections, try lowering your side windows and see if the peak disappears. I've damped my roof with absorbing foam, did have a measureable decrease in the 100-300Hz area due to less resonances. I never place speakers towards windows, I try to place them onaxis and as far away from reflecting surfaces as possible. This way I got rid of the nasty 1-3kHz peaks in my latest build. Before I rebuilt my A-pillars, I had my midranges face the window. This caused 10dB-/+ dips/peaks in FR. When I placed them onaxis away from windows, the freq response measure almost flat between 500-6000Hz (3dB+/-)


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## Hanatsu (Nov 9, 2010)

Damping does most good when placed behind driver. Place a layer of Dynamat (or something alike) then some soft absorbing foam-mat on top. Will have a noticable effect in midrange!


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## ErinH (Feb 14, 2007)

talk about timing...

I actually was looking in to this today. I've got a resonance/mode at about 71hz that I've managed to tame but was considering using some of my roxul to create bass traps in the corners of the car to see what effect it has. I'll make sure to document my tests and post them up for those interested.


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## Hanatsu (Nov 9, 2010)

I have a massive amount of FR/Phase graphs of the effects of damping the car. Can't post them at the moment, don't have access to my laptop for a while :/

The resonances I've come across in my car (VW Passat Touring) is 50Hz, 500Hz, 1,1Khz and bad one at 3,2kHz. The 50Hz must be due to cabin gain or something, but the 500Hz was the door midbass's distance to the doors outer layer. 1,1kHz was due window resonance, 3,2kHz I never figured out really but it disappeared once I added a dedicated midrange. Basicly all resonances exept the 50Hz one, vanished. These resonances was like 10dB dips/peaks, worst was the one at 500Hz, think it was a 25dB dip between 490-505Hz. And the door's really well damped. Not a problem anymore though, since I have midranges in A-pillars.

71Hz could be a cabin gain issue, if your car's smaller than mine that could count for the offset. Dampen roof was a major improvement in midbass response in my car. Flattened out some of the irregularities between 90-150Hz, these wasn't as bad as the others though...


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## ErinH (Feb 14, 2007)

the funny thing about deadening a car... you didn't remove the resonance; you just moved it. how much would it suck to move it from a non-offending frequency to one that causes you more troubles.


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## ISTundra (Jan 3, 2009)

I have mids in my kicks and added sound absorbing foam to the underside of the dash in an attempt to calm early reflections. I had a lot of protrusions and odd shapes on the passenger side and opted to use thinner foam as I thought it would make it be easier to form and cover those features with the thinner foam. I used 1/2" thick material, but after it was done probably could've used 1" thick and still managed to make it work.

In the end though, I can't say it made much difference, maybe from 5khz and up it helps, but I don't play my mids up that high. You would probably need ~2" thick foam to quell issues down around 1khz.


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## ErinH (Feb 14, 2007)

4.68" of the correct foam would be needed to absorb down to 500hz. And that's accounting for the air losses.


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## Hanatsu (Nov 9, 2010)

I experimented with 1/3" foam around tweeters. Minor differences in the 12-13kHz area but it did help with staging a bit, better center than before. Also 1,5" soft closed cell foam behond the mids, cleaned up response beyond 2kHz a bit. Damping of the floor and panels were minor, took some of road noise away, also some minor rattling. Didn't affect freq response at all though.


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## ZeNmAc (Sep 11, 2010)

bikinpunk said:


> 4.68" of the correct foam would be needed to absorb down to 500hz. And that's accounting for the air losses.


This is for complete absorption though right? Depending on the situation, wouldn't less than that be ok? I mean you want some reverb. Not that too little reverb will EVER be a problem in a car .

Relevant link. The only downside is you have to order in largish quantities.


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## ecbmxer (Dec 1, 2010)

Hanatsu said:


> Dampen roof was a major improvement in midbass response in my car. Flattened out some of the irregularities between 90-150Hz, these wasn't as bad as the others though...


Really? Now I might have to pick up another pack of RAAMmat and foam in the spring.


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## adrenalinejunkie (Oct 17, 2010)

Ok, so the thicker the foam the better. What if we use 1/4 Neoprene foam and apply two layers on top of the CLD? Will that still be effective or do we have to use a thick sheet?


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## ZeNmAc (Sep 11, 2010)

So should I put some open cell foam behind the drivers in my doors? I have some sonic barrier from PE that I could use, although if it doesn't work I'd rather save it for whenever I get around to building speakers (inside ).

I've been reading around and I keep reading conflicting things about OC foam. Some say CC doesn't absorb much sound and OC does, others say OC doesn't absorb much and it's the shape that matters (i.e. break up the backwave).

To me this doesn't make sense. If that was true, why bother putting acoustic foam in recording studios?  Last time I checked, foam absorbs sound although thickness is proportional to frequencies absorbed.

Can anyone confirm this? All I really need is the midrange cleaned up. I think an inch or two should be able to do this right?


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## Rudeboy (Oct 16, 2005)

Closed cell foams absorb very little sound for exactly the same reason they don't absorb water. Open cell materials are much better absorbers of sound - and water. The reason you don't want to use them inside doors.


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## Hanatsu (Nov 9, 2010)

I think OC is more suitable to absorb and CC to insulate/reflect. I have OC inside the whole door, you will notice a difference in midrange. It will dampen out reflections made my backwaves between the hard surfaces inside a door. Will also clean up staging in some cases. Place OC on a vibration dampener and NOT on the door directly, other than audio, it will absorb water 

... someone beat me to it :<


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## Rudeboy (Oct 16, 2005)

Attributing any acoustical properties to CCF is guaranteed to lead you to its improper use. Its function is purely mechanical. It keeps one thing from making contact with another. There's a huge range of what those two things can be, so it is a very useful thing to do.

It doesn't have enough mass to block anything meaningful. Its structure means it won't absorb or change the direction of reflected sound in any meaningful way. Closed cell foam is a pile of sealed and connected bubbles that are impervious to moisture and air. Neither can pass beyond the top layer.

Absorption occurs when sound moves the air inside the absorbent material. Friction between the air and the material convert that energy to heat. Since air can't penetrate beyond the first layer of cells, the only possibility for absorption occurs there. Since frequency absorbed is directly tied to the thickness of the absorbent material, even CCF with large cells (generally bad for compression set), will only absorb sound way above the audible range - way, way above.

The same relationship between thickness and frequency exists when we want to alter the direction of reflections.


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## nick650 (Feb 7, 2011)

There is also the hardness of the material as well right?


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## ZeNmAc (Sep 11, 2010)

Rudeboy said:


> Closed cell foams absorb very little sound for exactly the same reason they don't absorb water. Open cell materials are much better absorbers of sound - and water. The reason you don't want to use them inside doors.


That's at least the second time I've read this analogy today. TBH it kind of bothers me to compare absorbing sound to absorbing water. MDF soaks up water like a sponge. Does it absorb sound? Not so much.

But yea I see your point.



Hanatsu said:


> I think OC is more suitable to absorb and CC to insulate/reflect. I have OC inside the whole door, you will notice a difference in midrange. It will dampen out reflections made my backwaves between the hard surfaces inside a door. Will also clean up staging in some cases. Place OC on a vibration dampener and NOT on the door directly, other than audio, it will absorb water
> 
> ... someone beat me to it :<


Thanks, I'll probably try it next time I take my doors apart .

It doesn't rain too much here, plus from what I've read OC foam by nature can drain once it's soaked up water.



Rudeboy said:


> Attributing any acoustical properties to CCF is guaranteed to lead you to its improper use. Its function is purely mechanical. It keeps one thing from making contact with another. There's a huge range of what those two things can be, so it is a very useful thing to do.
> 
> It doesn't have enough mass to block anything meaningful. Its structure means it won't absorb or change the direction of reflected sound in any meaningful way. Closed cell foam is a pile of sealed and connected bubbles that are impervious to moisture and air. Neither can pass beyond the top layer.
> 
> ...


Thanks for clearing that up .


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## bobduch (Jul 22, 2005)

I used this:

Owens Corning: Making the World More Energy Efficient with Quality Building Products & Fiberglass Composites


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