Introduction

Most likely you’re reading this article because you want to quiet down your beast of a ride.  Loud cars with a nice tone is music to the ears.  That music turns to noise when you have to listen to the same sound again and again.  So here you are to view your options and see what they do.

We going to do budget oriented sound insulation and deadening.  We are using materials from hardware stores that are very similar to professional-grade sound insulation material.

Common sound deadening now-a-days is made from butyl or asphalt, up to 80mil and average of 50mil (1 mil=0.001 in) thickness.  Deadening works by increasing the panel resonance frequency. What this means is that higher frequency sounds such as road noise will not pass through, but the lower frequencies will. Window flash rolls, 25 mil thick, will be used for sound deadening. Do not use this stuff where it gets hot, like under the hood, it will melt and become a big mess.

Sound absorbers do what the name suggest. They are thick pads usually made from foam-like material. As sound tries to pass through, it has to take alternative routes, and in that action absorption is done. Sound insulation basically resists sound from coming in. Cushionaire carpet pad will be used for sound insulation. We got it from a carpet supplier, just ask anyone working there.

To make work easier, we can use a simple device called a sound dB meter.  With this widget we can place a number on the loudness.  With the car in its original form (no modifications as of yet), we use the meter to see which floor panels emit the most noise, and concentrate our efforts there. If we don’t use the sound dB meter, we’re going to have to work on the whole car and we do not want to do that

dB meter at work.

Cleaning Up

Before we get to apply the materials, we have to deal with the floor first.  In our example car, Audi 90, we have to remove the seats, the carpet trim, and other small things.

Let us start with the seats. The rear seat goes first as it’s the easiest.  In our case, the seat is held in by two screws. Once those are undone, we can lift the seat off and put it somewhere else. In many other cars, the rear seat just lifts off with no screws involved.

Now, we going to remove the passenger seat. Audi made it super simple, remove the limiting bolt and nut. Remove the plastic panel that is in the back. Slide the seat back all way so the rail comes off, tilt the seat back, and undo the heated seat connector (on this example car). Lift the seat off and get it out of the car.

bolt that has to be removed.

rail cover.

super reclining seat

Remove carpet trim which is held in by one screw in the back and by a bunch of clips throughout.  Gently pry it out, if it does not go easily, DO NOT take your chances.  If the carpet trim breaks, you would have to replace it. So take your time, try different angles, and check that you have no screws along the carpet trim.

Remove the air duct.  We now have access to the carpet.  Lift it and there you have the car’s original absorption mat and source of deadening.  Now we have to do same thing on the other side.

It is most likely no one has ever been here since the car rolled out of the factory.  So before you start working, let’s clean it all up. An extra step to make it extra nice. Like extra butter on your pop corn.

arrow pointing at dirt.

Deadening

Deadening is not required to cover the whole panel of the floor, 50% of the panel is enough, and only the flat surfaces. Any corners or fancy metal work does not need deadening.  Our local hardware store carries window flash.  Window flash is a butyl or asphalt strip with aluminum backing and a sticky side. They are perfect alternatives for expensive deadening mats.  Cut but DO NOT peel the strips. Lay them out first.  After you’ve done that, peel and stick them. Use the heat gun to make it more malleable to conform to all the small gaps.  You want it to stick securely to the panel, any gaps would mean that the window flash would have no effect. You should lay more in the center than the outside.  A good way to find where you should lay the strip is to simply knock on the panel.  Knock around with a blunt object like the back of screw driver. The area where the sound of the knock is the loudest, you should place the strip there. Once we are done with deadening our panels, we saw a drop in sound level by 0.5 dBs. This 0.5dB drop is not much but it helps overall (note: every 3 dB drop, the volume drops by a half.)

so clean, so nice

laying it out

sticking it on

Insulation

The sound insulation is next.  We picked up cushionaire carpet pad for 48cents/sq ft.  You must cover as much area as possible using the padding to maximize the insulation.  Before you go scissor happy, use a ruler and measure the area where you want to put the mat.  It is hard to cut it with regular scissors or a knife.  Cut and trim to fit the panel nicely outside then place the mat inside. Extra padding is placed above the area where the muffler exits, or the rear right. The rear left side had water damage to the carpet and it smelled like dead fish, so we cut out the bad part, and substituted in with fresh padding. The front left had worn padding, so we added extra padding on top of the old padding and then covered it all with more padding. The front right did not need any work, so we left it alone. Your car WILL vary (e.g. no bad fish smell), so use discretion when placing the padding.

Tip: If you cannot get full access to the floor that you working on, you can place your mat on top of the carpet and trim it accordingly.  When you are done, slide the mat under the carpet.

carpet padding

original padding

carpet in, vacuumed

dead fish

extra padding

trimming to fit, easier outside that inside

Results

In comparison to the naked stock car (no carpet or padding), the stock car mat drops the sound by 3dB. The new padding (Cushionaire carpet pad) on top reduces it by 2.5dB.

The stock car carpet drops noise by 1dB. The deadening (windows flash roll) reduces it by 0.5dB on top of that. So overall, in comparison with the car’s stock carpet and mat, the $15 of materials drops sound by an additional 3dB.

So sound levels inside the car before and after the deadening and insulation are:

Area	Stock, dB	New, dB	Change, dB
Left rear	62	63	+1
Right rear	70	66	-4
Left front	69	64	-5
Right front	66	66	0

Tools:
Heat gun
Sound dB meter
Window flash roll
Carpet padding