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Introduction to Car Audio: How to Tame That Road Noise

June, 2003

Brian Weatherhead

 

Introduction

A very important step in any good car audio installation is to apply sound deadening to the car. The following article is a primer on sound deadening, and should help in deciding how it best applies to your environment.

Available Products to Do the Job

There is little difference in the sound deadening industry between products. So, no matter what vendor you choose, it's almost guaranteed to be similar material. I highly suggest looking for the best price, as all will perform within 15% of each other. I contacted several vendors, and looked at several products. I was greatly impressed with B-Quiet (http://www.b-quiet.com), as they offered a superior product at half the cost of the standard brand. As a result, we ordered both damping types described below from them.

Damping Material Type 1: Viscoelastic Deadener

This is often referred to as Dynamat, but sold under many labels. B-Quiet's product is called Brown Bread. The product is an aluminum constraining layer, having a unique composition (tar, minerals, fibers) bonded together, with an adhesive backing. The goal of this product is to convert the vibration of the panel that it's applied to into heat. This transfer function can be measured and is referred to as acoustical loss. The measurement that is frequently used is the ASTM acoustical loss factor @ 200 Hz. This B-Quiet product offers an acoustical loss factor of 0.24 - 0.39 @ 680F - 860F.

There are variations of this basic product. The 'Extreme' versions are lighter and offer a little less in the way of damping. The best way to compare this product between manufacturers is to compare their ASTM loss factors, and the higher the value, the better the product is at controlling noise.

Damping Material Type 2: Noise Barrier Composite

Where a deadener tries to stop the vibrations of the panel, the composite barriers try to stop the sound once it's created by the panel. These two products together can make an astounding difference. B-Quiet's L-Comp is a 1/4" absorber foam with a black urethane film facing, a lead barrier, and a 1/4" thick foam decoupler. The product is applied with the black urethane facing up. This product alone can greatly reduce road noise, engine noise, and tire noise. The table below shows how much dB reduction the barrier composite can give. Although the noise reduction for the material is high, there are still plenty of surfaces that you can't treat, such as the windshield, so the overall noise reduction in the total car environment will be less than these numbers.

Frequency (Hz) Noise Reduction (dB)
125 17
250 21
500 24
1000 32
2000 34
4000 36
STC* 30

* STC means Sound Transmission Class

Installation

The most important thing to remember when using a Viscoelastic Deadener is its adhesion to the surface. Here you can see that we removed the entire interior of our project car. (The carpets, seats, insulation, and plastic trim was removed.) It took roughly 4 hours to disassemble the interior.  After the interior was removed, it was vacuumed to remove large debris. All surfaces were then wiped down with a wet rag and dish detergent to remove grease and grime. Once dry, the surface was then wiped down with mineral spirits to remove any residue.

At this point the surface was very clean, and ready to receive the sound deadener. The sound deadener came in a 70 sq. ft. roll, 2 feet wide. I measured sections starting at the driver's firewall. The material is VERY sticky, and easily applied to all surfaces. Once laid down in place, it was trimmed to fit, and holes were cut for seat mounts and trim mounts.

The material is very flexible and easy to contour to odd shapes. In areas where possible, I recommend two layers, running the seams in opposite directions. Two layers will yield three times the performance. You might as well do this now, since the car is already torn apart. Keep in mind that while multiple layers are great, it can create a few headaches while re-installing trim. So try it on one spot and see if the trim can be put back on without having to force it, before you do this throughout the car.

After spending a day working on this, I had applied over 70 sq. ft. to the car and car doors. The doors now closed with a different sound. We affectionately called the project car the "Easy Bake Oven." I also highly recommend using a wallpaper seam roller to help seat the product to the car, and work out any lifted areas.

The L-Comp was then laid down on horizontal surfaces, and we used four cartons (4 x 70 ft2). This was much easier than the Brown Bread, and only took 30 minutes.

Results

So how did we do in noise reduction? With the interior re-installed, and cruising at 65mph (same stretch of road, same gear) I measured 85 dB, while at idle I measured 78 dB. In our first introductory article, we said the noise level in the untreated car was 93 dB, so we got an 8 dB reduction. What you really notice while driving is the expansion joints on the freeway are no longer noticeable, and there is no drone from the tires. The products also eliminated all the rattles, buzzes, and other audible noises in the car.

Overall, it cost $160 for the Brown Bread and $165 for the L-Comp. It's hard to beat these kind of results, in a convertible no less. Remember, the dB scale is logarithmic, so 8 dB represents a huge reduction in noise.

 

- Brian Weatherhead -

Related to the article above, we recommend the following:

Car Audio Intro - Our Project Car

The Corvette C5 Factory Audio System Evaluation

The Digital Link

What we Hear

High Fidelity

Accuracy, Distortion, and the Audiophile

 

Copyright 2003 Secrets of Home Theater & High Fidelity
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