Sound Solutions: Preventing Unwanted Cabin Noise

Jul 3, 2010

Here’s a typical scenario: You’ve just finished a work of hot rod art for your customer. The car’s paint is jaw-dropping and the lusty engine makes more horsepower and torque than any sane street driver could possibly need and the stainless exhaust system is full-custom and glistens in the sun. At first glance, this custom rod (or impeccably restored classic) has it all.

Even though the car may look and feel great, if, when it rolls down the road, your customer hears nothing but an orchestrated array of hums, road noise and wind whistles, they’ll surely be less than happy. What good is a state-of-the art, high-dollar audio system if you can’t hear it over all that buzzing and road noise?

Controlling and subduing the noise level of a car can sometimes offer a challenge, especially if this concern isn’t addressed early enough in the build.

Here are a few places on a car that a builder should focus on and adjust to reduce unwanted cabin noise.

Body & Frame

In a build that features a full frame, consider new isolator bushings at the body mount locations. For builds with a body that mates fully to the entire length of the frame rails, consider applying a layer (about 3/16-inch thick) of dense closed-cell rubber foam. This will help to insulate the body from harmonics that may be transmitted from the frame. Be sure to use a closed-cell type of rubber if you’re adhering dense rubber foam strips to the top of the frame rails so that it won’t soak up moisture.

Suspension

When considering the suspension pivot areas, you have a choice between “hard” and “soft” materials such as rubber, polyurethane and spherical rod ends (heim joints).

Spherical rod ends offer non-compliant pivot points but are better suited to race cars, where precise control takes priority over noise and comfort.

Polyurethane and graphite-impregnated composite materials improve handling characteristics over rubber and are less harsh and noisy than spherical rod ends. For maximum interior sound control and ride comfort, a soft-durometer urethane or rubber would be the better choice.

If aftermarket bushings are the suspension isolators of choice, check into their level of lubricity. Even if a bushing package says that the product requires no additional lubrication, some are very squeaky when they dry. Others feature grease fittings or are graphite-impregnated. It’s very important to pay attention to all of these issues. Depending on the brand/model that you prefer, if you have a choice of non-greasable bushings or those that feature grease fittings, spend a couple more bucks and get the versions that can be easily lubed.

This holds true for any urethane pivot bushing, even those that mount anti-sway bars.

Tires

This is a very broad subject, but be aware that road-noise levels can differ depending on tire make and model. Granted, you or your customer will probably be choosing tires based on required size, as well as appearance. However, you won’t know if a specific tire’s road noise level will be objectionable or not until you try it out.

An excellent source of information is the Tire Rack. Its technical staff performs extensive testing and evaluations on all tire makes and models that it carries. You can look up the tires that you’re considering and check out their rating for noise level. Tire Rack performs its own testing instead of relying only on what the tire maker says, so you can be pretty confident in their ratings. This will speed up your tire selection decision, regardless of where you buy the tires. It’s a great resource. Go to  www.tirerack.com for more information.

Electric Fuel Pump

Unfortunately, most electric fuel pumps create some sort of buzzing or humming noise. If the pump is in the tank, there’s not much you can do about it to completely eliminate the noise. However, if the pump is mounted externally and is anchored directly to the frame or body, it may be magnifying the noise. Consider using compliant mounting bushings between the pump and mounting surface. Of course, you’ll need to make sure the pump is properly grounded. Read the pump instructions to see if an external ground is needed.

Fender

An excellent way to reduce noise caused by rocks hitting the inside of fenders is to apply an insulation material to the underside surface of the fender where it’s exposed to the tire rotational path. This also eliminates the chances of star fractures, dings and chips on the outer painted surface caused by rocks being spit out from the tire tread.

One way to do this is to purchase dense rubber strips from a rubber products supplier (you’ll need strips that are about 3/16-inch thick). You can also do this by spraying bed liner material onto the fender surface.

Either way, the area to be treated must be cleaned first. Simply apply your chosen material to the entire length of the fender underside in a width that most closely matches your tire tread width. Granted, this will add a bit of weight to the fenders, but the typical resto or street rod isn’t a professional race car, so a couple of extra pounds won’t be an issue.

Exhaust System

This may be a sticky point for some, but eventhe coolest and best-fitting aftermarket exhaust system may be a source for harmonic resonance that can sometimes transmit unwanted buzz, hum or other noise into the cabin.

When choosing exhaust hangers, try to pick ones that feature a rubber or urethane isolator bushing, or a reinforced rubber strap. This can help reduce any harmonics being transmitted through the pipes. After all, you should never anchor any exhaust system rigidly. The system that extends from the primary pipes or manifold must be allowed to move somewhat, both to allow for engine rock and thermal expansion of the pipes, and to reduce the chance of annoying resonance.

Depending on the specific exhaust system you have, it may be responsible for generating a resonance that transmits to the interior. In general, it’s usually more common for thin-walled systems to create more resonance than thicker-walled systems. When you buy an inexpensive system, one of the potential reasons for the lower price might involve pipe wall thickness. Usually, the thinner and cheaper it is, the greater the chance of resonance issues.

One method of eliminating exhaust system resonance is to attach a hefty rubber damper to one or both pipes. There are several ways to accomplish this, with one way being to bend a piece of rod into a 180-degree or Z-shape. Weld the rod to the pipe and install a tight-fitting bushing or damper with a center hole to the exposed end of the rod. This will theoretically serve as a harmonic absorber and hopefully dampen the resonance noise issue as the heavy rubber piece serves to absorb and “bleed off” any present harmonics. There’s no guarantee that this will be effective in every case, but it’s another approach worth considering.

Cabin Insulation

This is perhaps the most obvious area to try to make improvements on. Before the interior is installed, apply a quality sound-deadening material (acoustic barrier) to all cabin surfaces, including the floor, firewall, cowl, rear bulkhead, roof and doors.

Sound insulation may be applied using either an adhesive-installed mat, such as Dynamat’s Duramat, or a spray-on material, such as LizardSkin. These materials will improve cabin climate as well as reduce noise levels. As an added bonus, installing an acoustic barrier in the cabin also serves to dramatically improve the quality of any sound system. (For more information on climate control, see page 24 of the June issue of Hotrod & Restoration.)

Wind Noise

There’s no need for us to provide a lengthy description of this problem or the solutions. Make sure that all potential sources of wind leaks are addressed in terms of panel fit and the use of the appropriate weatherstripping/seals. Also, make sure that no holes or gaps are left in the firewall area that would allow engine and road noise to travel into the cabin.