It depends how serious you are.
Your main target passband is between 50 Hz and 250 Hz for road noise.
At this frequency range sound absorbers are impractical.
Weapons of choice.
Decoupling
Essentially this is what Dynamatt etc seek to achieve (more on that later). A decoupler is a reducer of energy of transfer, its primary function is to turn (or limit) the amount of energy transfered, by dissipating it as heat. (In fact most sound barriers do the same but in different ways). A very good example is your engine mounts, and various exhaust attachments. Less well known, but almost equally important is your suspension and steering linkages. The electrical equivalent is a resistor.
First route is to replace or upgrade these as necessary. Serious gains can be made there.
Sound deadener's.
These are a combination of a decoupler and blocker. (aluminium backing) The best sound deadener's have these properties.
1/ Heat range useful for maintaining adhesion
2/ A suitable elastic material that resists deformation, and more importantly returns to original shape.
3/ A heavy backing that helps with blocking, and creates an acoustic impedance difference between the elastic material and the metal panel. (sandwich)
4/ Fumes, and gases not harmful.
You may well note that spray on deadener's cannot decouple, and provide very little blocking ability. They do however help reduce stone chip noise, and applied correctly, inhibit rust. I suggest this should be its primary use.
To maximise the cost advantage of sound deadening they should only be used on large flat panels (doors roof etc) away from the panel edges (6 inches). There are cheaper and better alternatives for blockers.
Putting sound deadening on a firewall is a VERY costly use of sound deadening, that can only block. (The firewall is too convoluted for decoupling advantage)
Blockers
Best bang for the buck. Aluminium, copper and lead are the best. 1 mm thick lead can attenuate 125 Hz wave 24 dB. To give you a clue, your car panel attenuates it by 12 dB and your glass by 22 dB (6mm)
Point.
Glass will be the biggest limiter, therefore use a blocker to equal glass attenuation.
Aluminium foil,vinyl and rubber are your "best friends" 2mm thick aluminium would take 4 layers of Dynamatt to equal. You can layer it up for ease of install. Use it everywhere. Blockers use reflection as their primary sound reduction method, and are superior to all else in matters of efficiency of sound attenuation. 0.5 mm thick lead on metal panels will exceed 6 mm glass. 2 mm aluminium will do the same. Or 90 mm thick wood. (Just trying to give you a perspective of just how good these are)
Save yourself some money and forget chant etc. These 'diffusers' are nothing short of a sad joke. The best they can hope for is changing the panel resonance frequency by adding mass.
Diffraction.
If an object is equal to or larger than the one quarter of frequency length it will diffract a wave. This causes the wave to "go around corners", and energy is lost in the form of heat in this action. This is the primary function of absorbers, to diffract a wave. (IE to cause it to go through a labyrinth) If a wave is large in comparison to the absorber thickness, very little will be lost to heat. Hint a 250 Hz wave is approximately four feet long, and therefore would require an absorber to be 1 foot thick, to have any realistic chance of diffraction. If will let you ponder the thickness of attention for a 50 Hz wave in your vehicle.
Your primary source of noise is the motor and tyres.
The firewall can often benefit from a blocker, however your primary sound ingress is your door panels and roof. Next is the floor. Remember you are limited by glass attenuation. Wind the window down to hear just how good it is.
Once you have maximised these areas, the only choice is to reduce the noise at its source.
Edited by abmolech, 01 October 2007 - 06:25 PM.
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