Sound Proofing

by: Andrew Krause

One of the most critical things you can do for a car is to sound proof it. Whether you have a Mercedes Benz, or a Hyundai Excel, all cars can benefit from extra sound proofing (especially the Hyundai). The first thing to understand about sound proofing is that it can never be accomplished in a car. The important thing is that you can reduce the amount of external noise, and vibrations. Sound proofing will make any car feel like a million dollars. A quieter ride, less distraction, and an overall more comfortable environment are what you can expect when you start sound proofing your car. For car audio enthusiasts, this topic is critical. Nothing will lose points faster, or spoil your sound more than a resonating panel, or vibrating frame member.

Sound Proofing starts with some means of damping. A number of products are available for this, and they all have various degrees of effectiveness. The best results are always obtained from using a combination of these products. There are mats, sprays, foams, and insulation available from a number of manufacturers

Mats are usually either Styrene-Butyadine-Rubber or asphaltic sheets backed with an adhesive of some type (although other materials are used in some cases). Installing mats in your vehicle is a simple way to reduce vibration, and is effective as well. The way mats work is that they are used to cover panels. The material they are made of absorbs vibrations in the panels, and turns them into heat, or it may simply lower the resonant frequency of the panel. Mats can also be placed between panels to reduce the amount of vibration between the two panels when they are in close contact. Many times, the mats will also have a metal foil backing to improve the heat resistance of the matting (thus allowing you to use it in an engine compartment). The matting also adds weight to a panel, reducing it's tendency to vibrate in the first place. Some of the more popular mats are Dynamat and Road Kill, but there are alternatives. One of those alternatives is a material known as Ice-Guard. Ice Guard is normally used as a roofing material, but is essentially the same material as commercial mats, except that it incorporates an asphalt material for water proofing. Ice-Guard is much cheaper than the other commercial mats, up to 80% less. It has to be bought in large amounts, however, and minimum purchases are usually 80 to 100 dollars for an entire car, compared to $400 using Dynamat or Road Kill. Ice-Guard also has to be aired out for a couple of days after installation before you can put your interior back in, and it is recommended that you spread some activated charcoal powder or baking soda before laying your carpet back down over it.

Sprays are also used for damping. These sprays normally come in a professional can, which require a compressor and paintgun to apply, but many companies are starting to market aerosol cans of sound deadener spray. The spray is often used in places where matting would either be too difficult, or would add too much weight/bulk. Door panels are the most common application for sprays, as well as highly irregular crevices (like inside kickpanels). Sprays are suitable for large panels as well, but they tend to be messy, and require taping/masking off of upholstery and windows.

Foams come in two forms: Sheets of foam, and foam sprays. The sheets of foam are used much like mats are; They are laid over panels to reduce and absorb vibration. Unlike mats, which absorb the vibration and convert it to heat energy, foam sheets disperse the vibrations throughout, reducing its total energy. Foam sprays are used to fill in crevices. As they dry (or rather, cure), they expand slightly, pressing against the nearby panels. The individual cells help to disperse energy away from the vibrating panel, and absorb them. Foams can be expensive as well, and there is a low cost alternative here, as well. The first is Styrofoam©, which can be obtained in a spray can. Styrofoam© is the brand name for the polystyrene foam we are all familiar with (and somewhat annoyed by at times). The fumes given off by Styrofoam© are noxious, and many communities have laws banning its use due to environmental concerns. Another alternative is insulating foams like Great Stuff©, which is used in home construction. Great Stuff© is cheap, fireproof when cured, and readily available at any hardware store for about three dollars a can. Great Stuff© is also shapeable when it cures, and can be used to smooth sharp corners. The downside to Great Stuff©, like Ice Guard, is that it is messy. Once Great Stuff© is sprayed on upholstery, your clothes, your skin, etc, it's all over. You hands will be stiff and sticky for days, if not weeks, and your clothes are forever ruined. Great Stuff© also expands voraciously, so spray it carefully.

Finally, there is insulation. Jute is the most common insulation. It is laid under carpets in both cars and houses, and is basically a thick mat of fibers which absorb sound. Though less effective than the other methods, it adds a plushness to carpets, and has very good thermal insulation. Micro Jute is recommended, because it's much thinner than jute, and has about the same level of effectiveness. Jute or Micro Jute can be gotten from a number of manufacturers, and is available at any carpet supply store.

When using these methods, you will invariably use at least two, if not more. The one opportunity I had to sound deaden someone's car for them (I did it as a favor, since it wound up costing them about $250), was a 1986 Monte Carlo SS, which took me three days. I used Ice-Guard all around the trunk, floors, side panels, rear deck, doors, and roof. Great Stuff© foam was used in the less accessible A/B/C pillars, kick panels, inside bottom of the doors, and rear fender wells. I used the foam to spot a few places in the dash as well, such as the blower housing. I finished up with a layer of microjute on the floor, rear deck, and lining the floor of the trunk. Some Dynamat was used on trim panels because it was less messy. The reduction was incredible. The guy for whom I did this first remarked that he wasn't sure his engine was started because he did not hear it. He was also surprised at the quietness of the cabin, and compared it to his wife's Lexus, which he said may not even have been as quiet. During driving, we noticed a "whooshing" sound, which we traced to a defective door seal. We replaced the seal, and tightened up the windows on day 3, and the car was as quiet as a tomb at highway speeds, with only the sound of his monster exhaust making it through. This car was a whim, for me, but it secured in my mind the importance of sound proofing to a car.