Structural noises originate from the exhaust, tires, and other external noise sources before entering the vehicle and transmitting through body panels and other metal components. You hear these car noises as vibrations, buzzing noises, roaring, droning, humming, rattles, squeaks, and groans. It's particularly bad in older and antique cars, and of course, any vehicle that has a killer audio system.
It gets worse. The metal body panels will actually amplify your car's structural noises. That's why you hear those aggravating buzzing noises with the audio system's deep bass sound. That's also one of the reasons your car is much louder on the highway – which is often when people notice the worst road noise. Highway road noise can be so loud that you can't hear the person in the passenger seat. Soundproofing
your car will solve these problems.
The best way to reduce structural noise is by applying damping materials to the vehicle's body panels. A damping material is a resilient material that constrains the motion of the panel. "Loudness" correlates to the sound waves' amplitude, so by reducing the amplitude, we reduce the noise level. The less a car's body panel can flex, the softer the sound will be that it radiates.
If you'd like to take a detour here and learn more about
dampening vs damping
vs deadening, feel free. We'll wait on you.
Airborne noises are sound waves that travel into the car through the air and then
bounce back and forth inside the car's cabin. The best ways to soundproof airborne
noise is to block or absorb it. You either want the sound waves to never reach you
or, once they're in the car, to stop them from reverberating. Then you get to enjoy
the quiet, calm environment you've created.
Think of a recording studio. The studio's walls and ceiling are lined with materials
that block, trap, and absorb unwanted airborne noise, so only the music is heard and
recorded. Soundproofing a car uses the same principles.
In most cases, some sound will still be transmitting through the body even if you did an excellent job damping the metal and dampening that structural noise. Blocking the airborne noise that does make it through is the next step. Reducing sound transmission requires mass. When working in a building, that mass is typically added using gypsum board (drywall) or concrete. For an automotive sound barrier, mass loaded vinyl is the best option to achieve high mass with good flexibility and a limited thickness.
Once airborne sound waves are inside the car cabin, the last line of defense is to absorb them. The OEM insulation
and upholstery in your car do most of the work here. If you're looking to add on, two high quality options are
hydrophobic melamine foam and jute insulation.