Crash info course on thermoacoustic automotive insulation, stage 1 - stage 3, long post sorry! This is the rear gate of an 06 #Acura #MDX, its structure is a great example of what you might find behind interior panels when you are adding #sound #deadening and insulation.
Modern auto structures, particularly doors, have an outer sheetmetal skin (the other side of which is the paint and exterior) and an inner skin (to which the wires, switches, speakers, plastic panels, ect are attached to). "Stage 1" sound deadening is adding sheets and pieces of foil-backed butyl rubber to the inside of the outer skin whenever possible, or to the inner skin. Here it is added liberally to the outer skin and also applied strategically to the inner skin. Good coverage is the goal. It does three things: dampens stray resonances and vibrations, physically blocks outside sound from getting in and inside sound (audio) from getting out, and physically blocks outside temperatures while making inside temps more efficient (AC/heater). Thickness matters a lot here, "pro" grade would be thicker than 80mils, I'm using ultra at 110mils. #gtmat "Stage 2" is adding medium-density closed-cell foam in tiles to strategic locations, atop the first deadening. You've seen this foam on walls of a music studio, music room, or theater. Doesn't need full coverage. It does three things: mainly it physically traps sound waves in the little closed cells, does block and absorb some outside noise, marginally helps AC or heater efficiency as a general insulator. Perfect on wheel well humps, firewall, rear, and a few tiles on the roof skin. Thickness matters, I'm at 10mm thick, about 4 or 5 sq ft used here. "Stage 3" is adding #Polyfill batting to key places. Polyfill comes in two main forms: on a roll like padding and loose like stuffing. It's used inside speaker boxes, subwoofer boxes, and in pillows and quilts. It does two things: physically slows down sound waves, and absorbs some sound waves (converting it to heat). While it is inorganic, thus won't mold, CANNOT be placed where it can get wet. I've used 1 bag of roll and 1 bag of loose here. #caraudio #audio