Sound Dampening Guide
Eliminating unwanted noise in a vehicle is a multistage process. This is because there are many ways that noise can enter a vehicle. The most common types are road noise, engine noise, panel vibration and rattling. The process and materials that are used to quiet these sources of noise are listed below.
Vibration Damper (CLD or Constrained Layer Damper)
Let’s start with Panel Vibration. This can be caused by the tires reacting to the surface of the road, the engine and drive train or even the sound system of the vehicle. The most effective way to eliminate this type of noise is to stop the vibration. The best way to do this is to add to the rigidity of the panel that is vibrating.
Constrained Layer Damper or CLD is the most common type of Sound Deadening that is used in Car Audio. CLD products are usually constructed with a layer of Butyl Rubber that is attached to a constraining layer such as Aluminum. CLD products are attached directly to the vehicle by a layer of adhesive on the Butyl Rubber side.
CLD products add mass to the panels of the vehicle in order to increase the rigidity of the panel which in turn stops the vibration. In most cases 50% coverage of a panel is enough to eliminate unwanted vibrations.
How To Install CLD Sound Dampening
Sound Barrier (MLV or Mass Loaded Vinyl)
Road noise and engine noise are a completely different issue. CLD products do very little to stop radiated sound from entering a vehicle. This is where you will need to use a sound barrier product such as Mass Loaded Vinyl or MLV.
MLV products are made of a dense but flexible Vinyl that has a very low resonant frequency which makes it ideal as a sound barrier. When used properly this layer of MLV will encompass the entire vehicle from the window line down to create an environment that is isolated from the outside world. You need to make sure that you minimize any cracks or seams in this layer of MLV that would allow radiated sound to penetrate the cabin. This requires that you overlap the seams and use either tape or a vinyl adhesive to complete the seal.
Sound Absorber (CCF or Closed Cell Foam)
Road noise and engine noise can also be minimized by using a sound absorber. It can be very difficult to get MLV to adhere to an inverted surface such as the roof, hood or trunk of a car. The weight required to qualify MLV as a great sound barrier becomes a problem when trying to keep it from peeling away from the surface.
Closed Cell Foam or CCF is a good choice for these surfaces. Due to the size constraints of the vehicle CCF is not as effective as MLV at minimizing lower frequencies but it still does a good job at reducing higher frequencies. For any material to be effective as a sound absorber it needs to be 1/4th the thickness of the frequencies wavelength. You would need to use a sheet of CCF that is 4 feet thick to absorb an 80Hz tone. This is just not feasible in a vehicle. CCF that is 0.25” thick will only be effective at limiting frequencies above 13 KHz. CCF that is 0.5” will lower that to 6.5KHz while 0.75” and 1.0” will be 4.4KHz and 3.4KHz respectively. This will definitely help with high pitched sounds such as wind and tire noise.
Some sound absorbers have an added layer of protection from heat that makes them ideal for under the hood. They can also be used on the transmission tunnel and firewall to repel heat that is radiated by the engine and transmission.
Sound Decoupler (CCF or Closed Cell Foam)
The last type of noise is rattling that occurs when two or more vibrating panels make intermittent contact with each other. CCF can also be used as a vibration decoupler or cushion between two panels to stop the rattling. CCF is a great choice for this because the closed cell structure will not absorb moisture or odor from inside the door panel or liquids that may be spilled inside the vehicle.
Some products that are available in the Car Audio market combine two or more of these products in to a single product.
For example Stinger RKCP12 and Accumat AMT250 are a combination of CCF and MLV in one convenient sheet. These are ideal for installation in areas such as Doors and Quarter panels where rattles can occur.