Sound Deadening FAQ

I posted this over on ImprezaWRXSTI:

Some information on sound and vibration that some of you might be able to use.
I've noticed that a lot of folks who apply, or paid to have applied, a Dynamat type product are getting a bad deal. They either just don't know how noise attenuation works or they are maximizing profits.

Here's a semi-technical explanation, from a Master's level Mechanical Engineer, on the proper way to install the stuff.

There are two ways to attenuate noise in a car.

1. Sound wave Absorbsion
2. Resonance reduction

Absorbsion
This is when you have a fiber batt that actually traps and prevents sound waves from propogating. The thicker the batt the lower the frequency that will be absorbed. Thin batts absorb high frequencies. (The thickness of the batt absorbes up to the freqency that is 1/2 the wavelength of the batt thickness) The denser the batt, (normaly ~5 times as dense as fiberglass insulation) the more of the frequencies that are to be affected are absorbed. So dense, thick batts will absorb the most sound.

Body cavities or deap space will collect and potentially amplify the noise, much like a wind instrument or guitar. This is where this stuff works best.

Resonance Reduction
This is what Dynamat and its clones are trying to do. Every panel in the car will have a natural frequency that it will vibrate at then excited; Doors, firewalls, pillars, roofs, etc.

simply: Frequency = SQRT [ Stiffness / Mass ]

Dynamat essentially adds mass to the panels to alter the vibration charateristics of the panel. They also break up the standing vibration waves in the panels. (denser the better)

It doesn't really add stiffness, unless you make a structure and glue or bond it to the panel to increase the cross sectional area.

This is the main acoustic differnce between the tinny sounding doors of a stock subaru and the bank vault doors of a lexus.

What does this mean?
You don't need to plaster the entire inside of the car with dynamat. Simply put 1-2" strips of the stuff over about 20-30% of the surface of the panels, in a chevron, or checkerboard pattern. This will get you 95% of the sound improvement of a full application without all that wasted mass or expense. (Dynamatt is EXPENSIVE!)

In the body cavities (firewall, behind the cosmetic peices in the trunk and package shelf, etc.) is where you want to put the batt. This will prevent a megaphone effect.

You can also get a drum effect, where a closed cavity (trunk) will make a panel (seat back) vibrate. Applying either one will work there, although Dynamatt is normally easier. The batt will also stuff between a hard structure and a cosmetic panel to keep the panel from humming (like the rear parcel shelf)

All strips should be about 1-2" wide, AT MOST

So, for the most cost effective solution I would:

1. Chevron the interior wall of the door skin with about a 20-30% surface area application.2. Put a few strip on the interior door structure around the speakers.
3. A few 1-1.5" strips of the stuff on the rear shock mounts from floor to top in sort of a spiral pattern.
4. A few strips on the underside (or topside if you remove the cosmetic shelf) of the package shelf.
5. A few diagonal strips on the rear seat back and the structure around the shock towers.
6. A few strips in the trunk bottom and side walls, but not too much.

Probably 20-30 square feet AT MOST for all of this. (it goes fast) Which will add like 6 pounds to the car.

This should get you 90% of the benefit of a full application with minimal cost and added weight.

If you are brave you can take out the headliner and the dash to get to the firewall. An X or >> pattern in the roof would be enough. Adding two, 1", strips in an X or // pattern under the carpet in each footwell won't hurt either.

If you think you can quiet it down more, you can always add more later. But there is no sense in plastering the whole car with $200 of dynamatt if $50 will get you 95% of the benefit.

These guys have the products I'm talking about, but I have no affiliation with them, nor have I purchased from them.

http://www.b-quiet.com/index.html

Sorry, I don't have pics, but I've done this on 3 cars so far with outstanding results.

Thanks! I will sticky this for you.

Would such an application be what i need for the rattling in my rear shelf due to the sub installed there? I dont want to spend the money on something similar if my rattling wont go away. Im thinking this will do the trick? And if this is the right way to go, do i do the strips application or do the whole area?

Theres some sort of sound-deadening paint out there too.

I thought this was a pretty interesting article.

I have a problem with the auto dimming rear view mirror. It's not that I mind the mirror vibrating when the sub hits a low note, but there's times when that big fat plastic housing resonates like I magically hit its 1st harmonic or something.

Does anybody else have a problem with their rear view mirror sounding out bad vibrations? Any fixes other than turn down the radio?

Just a suggestion, it would be very helpful to this thread if someone could post how to remove the various trims in the car to apply sound damping - how to remove door skins, seats, center console, dash, etc. etc.

Thanks

I have purchased products from B-quiet.com in the past and had no issues. The brown bread product (since updated to b-quiet ultimate) is just as good if not better than dynamat for less $. I found in my experience the most significant noise reduction was by putting their vcomp product behind the door panels. Haven't tried any noise reduction on the LGT yet though I plan to in the near future.

from personal experience is modding cars and doing sound competitions Hushmat is by far the best sound dampening material out there. It sticks to any surface unlike most where you have to clean clean clean. A full car install will run roughly $1,000 USD and is so quiet its like riding in a lexus and I was pushing5000 watts of power in my last car and you almost couldn't hear my system with the windows and doors shut. Hushmat was originally used by GM in specialty vehicles proven to stick and stay even to the inside of dirty greasy doors in the factories.

OK, so what is 'hushmat'? Sounds kinda pricey. The v-comp from b-quiet.com is non adhesive, no cleaning necessary before installation.

Out of curiosity, is dynamat really that different form the rubber electrician's mastic tape for sale at the local Home Dumpy?

Probably. Check www.b-quiet.com for their 'extreme' product. As good as or better than Dynamat, but cheaper. Dynamat is overpriced IMO. Also, if you're looking for a noise reduction solution, seriously consider b-quiet's v-comp product. I installed it behind each of my doors and it's pretty effective in reducing road noise. Here's a link/walk through http://legacygt.com/forums/showthread.php?t=20069

Here's a link to an install that has some neat info on sealing some of the larger hole in the door to improve the audio quality of the door speakers. I think if I ever put speakers in the car I'll go this route http://legacygt.com/forums/showthrea...n+part+install also.... and the dynamat route.

for those of you who are interested in reducing volume levels from aftermarket exhausts, especially in a wagon, I have found that sound deadening material applied under the rear seat and in the spare tire well does NOT make much of a difference. Note that I have not yet applied the material (b-quiet) to the sides of the trunk area, where a lot of the resonance that you hear presumably comes from. At any rate, I doubt that using this stuff can effectively compensate for a loud exhaust - at best it might tamp down the resonances such exhausts produce. A resonator might do something similar while also reducing the overall volume level, I'm not sure.

Try the v-comp product. Much more effective at reducing noise IMO than anything similar to dynamat (i.e. the b-quiet 'extreme' product), though I don't have experience with reducing noise from aftermarket exhausts. It's possible it is generating resonances throughout your vehicle and not just at the rear.