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Originally Posted by MTT  so, I've got my car completely covered in butyl mat, and am aware of its benefits and deficiencies, whats can I add in the back half to quiet down the muffler ?????(A new muffler is not the answer) |
when I use the term dynamat please substitute the name of your preferred damping product
the dynamat is part of it but it is not complete at stopping the intrusion of noise. the damping material will minimize the radiation of noise produced by the sheet metal of the car and do a fair job of stopping noise at certain frequencies. Actually in the case of a muffler the dynamat will be one of the most effective solutions due to its low frequency performance (everything else commercial works best at frequencies over 500 Hz and engine/ road noise is all over the 50 to 3000 Hz range)...Damping ASTM 756
Mass loading is a tried and true method, it is a way to increase the transmission loss of sound entering the cabin. You can pour concrete (SPL cars do this for several reasons) into the offending area, you can put down a second or third layer of dynamat, basically anything that can spread out and add mass. if you use a rigid material it will stiffen the area and raise the resonant frequency, which might help quite a bit OR it could be a problem when the frequency is hit, so a soft compliant material is generally used like vinyl or asphalt to lower the resonant freq. Adding mass means for the material to get excited by the incoming sound wave more energy will be required.... Transmission loss ASTM 90
To lower the level of reverberant noise that still enters is the area of absorbent materials (ASTM 423). This is the stuff we are most familiar with; fiberglass insulation, rugs, foams drapes are all absorbent (the stuff you see in a theater). They generally work best at frequencies over 1000 Hz but by adding a greater thickness it increases the lower end of the functional range. Using expanding foam in hard to get at places is a good trick (while only fairly absorbent it fills air gaps and increases attenuation while adding rigidity. you can use Thinsulate, urethane foams and any one of a number of commercially available products. For the exhaust issue I recommend jute (typically sheets of recycled blue jeans) since it works very well in the automotive environment (low frequencies) and it is cheep. I also use 3M marine sheet thinsulate for the 2000 Hz and up region. Filling ALL the air gaps is fairly important here so be meticulous
As you tear apart your car to make these additions or to run 0 gauge wire you will find damping materials and foams and jute, it is not a coincidence since inexpensive products that work well are in the manufacturers best interest too.
