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.

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:

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.

http://speed-society.com/george/images/damping.gif

 

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/showthread.php?t=11345&highlight=wagon+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.

Where does the sedan "need" Dynamat besides the doors? I'll have a 10" sub in the trunk.

If it were me, I'd put it on the floor and in the trunk as well.

I did this to quiet down the Perrin TBE and it help tremendously. A also had to do my STi. That car has no sound deadening material at all.

For floorpans and the tranny hump, your best bet for overall road noise reduction is to use a "barrier" type material.

 

Unlike damping sheets, which essentially add additional mass to panels and thus lower the frequency of vibration, barrier materials act to acoustically isolate/decouple.

 

The idea is to have a high density amorphous material which absorbs incoming acoustic/vibrational energy, and suspend this material between layers of closed-cell foam. The inner material is thus unable to retransmit any vibration to the cabin interior. This is similar in principle to the thermos.

 

The newer generation Legacies already have asphalt-based mats applied to the floorpan areas and foam/fiber absorbers glued to the underside of the floor carpet. Adding more damping materials here is already testing the law of diminishing returns.

 

I laid Cascade Audio VB3 and VB4 from the front footwells back (floors, tranny hump, wheelwells), which significantly reduced road noise. These are non-asphalt-based materials (no smell) and are only 0.25" thick.

 

The Subaru uses "quiet steel" for the firewall, which is just two steel panels sandwiching a central butyl rubber absorber; barrier material behind the dash would probably have been overkill.

 

I had also tried this on my previous econobox (no factory damping), and the difference was much more noticeable.

 

 

For doors, trim panels, and the roof area, damping materials are the way to go, mostly because 1) these areas have much tighter space tolerances, and 2) you're now dealing more with panel vibration issues as opposed to "noise from without."

So, where did you get the material...

How hard was it to pull up the carpet?

 

I want to do my doors, floor, and definitely the floor of the wagon bed, but I have never done this at all, and I am a little scard of doing it...

 

Ted

Further info on Cascade damping/barrier materials:

 

www.cascadeaudio.com

 

They have application guides, how-to's, etc.

 

Where to buy:

 

Plenty of online retailers (partsexpress, mmx, etc.) sell this stuff. Your best bet is to search by product, as different retailers will have the the best price at any given time.

 

How to install:

 

I have to 'fess up and admit that I have a previous (BH) gen Legacy wagon, so my experience may differ from that of a BL/BP owner.

 

For the complete soundproofing treatment (floor, doors, roof), you will have to gut your interior. The process is not difficult, it's just time consuming - this is why a shop will charge $$$ to do this for you. You can break the job up into two stages: 1) passenger seating area 2) rear cargo area

 

It is vitally important to have proper disassembly/reassembly instructions beforehand. This is especially true if you have side airbags in the front seats! If you have not already done so, go to techinfo.subaru.com to download the complete service manual. For $35 you get a 72 hour subscription, which may be enough time (over a long weekend) to poach the entire thing. You can browse the TOC without a subscription; you just can't see the actual PDF files.

 

General hints:

 

Take the time to "understand your trim fasteners." Some can be pried out, some must be unscrewed. Most screw type plastic fasteners take a #0, #1 or #2 Phillips head screwdriver. Use the proper bit to avoid mangling fasteners. Have several thin-blade flat head screwdrivers on hand to help with prying. A right-angle (ratchet is best) or stubby #2 Phillips screwdriver may come in handy for tight spots.

 

Vinyl barrier material (VB4) is half the price of the lead/foam stuff (VB3), but it's really only useful for FLAT surfaces. Any kind of curvature (wheel wells, tranny tunnel, rear underseat, etc.) requires the flexibility of the VB3. Plan on two packs of VB4.5 and 3-4 packs of VB3.5. Get a single pack of VB2 (not the thicker VB2HD) for "taping" pieces of barrier material together. With damping materials, the adhesive gets "old" if it sits too long (6-12 months) so buy it as you need it.

 

Covering the entire cabin floor is a good 2-3 day job for 1 person. Either budget the time, or find a competent friend or two to help. It is possible to drive around a gutted car, but it is not pleasant, and the car will be vulnerable to water leaks in the door pillar areas.

 

To avoid wasting material, you should ideally construct templates using large sheets of paper. Cover the target surfaces with the paper. Hit flat spots first, then progress to the curvy surfaces. The paper will help you figure out the proper piece geometry for covering curved areas. When you have your templates figured out, arrange them on the barrier material like you're completing a jigsaw puzzle so as to minimize gaps between pieces.

 

You must try to achieve near total coverage when using barriers, otherwise the noise will "find a way around" the barrier. Try to minimize the number of cut pieces, and make sure that each piece "mates" with adjacent pieces. This is largely a "trim to fit" process, which can be minimized by good templates. Cover each joint with VB2 "tape". For funky curves, and to hold pieces in place, use 3M double-sided foam tape periodically to "tack" the barrier to the body.

 

The floor carpet is heavily padded in some areas, and the padding plus the barrier may be too thick in some spots. This may prevent trim panels and even the car seats from fitting properly once reinstalled. Likely trouble spots are the seat mountings, the rear cargo area quarter panel trim, the tranny hump between the front seats, and the cargo floor. TEST FIT PANELS AND SEATS EVERY NOW AND THEN to make sure that you remove enough barrier and/or carpet padding in the tight spots for proper reassembly.

 

THE SEAT MOUNTING LOCATIONS ARE TRICKY - MAKE SURE YOU CAN PROPERLY THREAD THE SEAT MOUNTING BOLTS, OR YOU WILL STRIP THEM AND THE NUT WELDED TO THE BODY ($$$ TO RETAP OR REWELD A NEW NUT).

 

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

 

http://www.quietcar.net/ .. I want some feedback from this stuff! They sell it too you quite nicely on their site

Seriously guys - Even at 80 MPH, the car is very quiet, with the only noticeable noise coming from the A-pillar area from crosswinds and from tire noise. (Yes, I used Dynamat Extreme on the front doors when installing the stereo - but even before that there was no noticeable noise from the doors.)

 

I'd rather put effort into fixing the wind noise & reducing tire noise. Any suggestions on how to effeciently fix these areas?

I've read this thread 3 times...this is interesting. I'm going to read it again. I would like to get the best material to do a project like this...I think it would be worth it to kill some road noise.

has anyone done the wagon "door"? I was looking for instructions, especially on how to remove the spoiler, and didnt find any. any help would be appreciated. TIA