[FC] Sound Deadening

[wild8bill at aol.com]

Forgive the length, but I wanted to include all info
 
Someone hit it on the head (pardon the coming pun ), the metal panels  act 
like a drum. To keep an area quiet, you need to stop the metal from  
vibrating, as well as absorb the soundwaves from the source. 
 
The metal side panels can be removed and a dense rubber/foam/tar-like  
material applied to them from behind to prevent them from vibrating. Products  
include Brown Bread, Dynamat and others designed for this purpose, but most 
are  expensive. I have used the 1/8" thick rubber/tar self-adhesive building 
material  tape designed for sealing window and door frames during 
construction with some  success. The dense, soft rubber floor mats designed for use at 
workstations on  cement are excellent at absorbing soundwaves, and can be 
cut and attached with  adhesive to the inside of the side panels, and can 
also be fitted as a mat to  place on top of the engine compartment or used 
under carpet. It should be noted  that to effectively stop metal panels from 
vibrating any material needs to be  securely affixed to the metal, usually with 
adhesive, and not just screwed,  taped or propped against it.
 
To reduce noise from the source (engine), another little physics  lesson 
needs to be understood. Soundwaves are made by the movement of air, and  move 
from the source like ripples on a pond when you toss a rock in. You can  
seal off movement of air from the area of the source and that will 
significantly  reduce the decibel level. However, the material used will absorb the 
vibrations  and transfer them through to the other side and release them. The 
denser and  less rigid the material the less it will vibrate sympathetically. 
Multiple  layers that are separated so they don't directly transfer the 
vibrations to each  other will further reduce the decibels. Higher pitched 
sounds have less "power",  and are the easiest to stop. Low pitch sounds have 
much more "power", and are  difficult to stop with sound (air wave) absorbing 
material, a real-life example  of this are the cars you can hear "thumping" 
from blocks away when they have  their sound system turned way up.
 
So, after all that, the absolute BEST way to reduce noise inside a  
Greenbrier would be to strip every interior body panel, flooring material,  
headliner etc. out and completely line everything with Dynamat or equivalent and  
then re-install everything. Use the heat resistant material to line the 
engine  compartment and inside the tranny access panel, place new weather seal 
foam on  the lip of the engine compartment and tranny access panel and secure 
tightly.  Cut a piece of anti-fatigue floormat to fit over the engine 
compartment area. If  you do all this, your Greenbrier should be as quiet as a new 
$50k  SUV.
 
A cheaper way to do the same would be to attach the fore mentioned  
construction sealing tape to at least half the surface area of the back of  
interior panels, install carpet pad or in the case of rubber floors; the 1/8"  
thick foam used when installing laminate flooring, under the flooring material,  
and make a cover to fit over the engine compartment. If you haul stuff, you 
can  make a cover out of 3/8-1/2 plywood that is backed underneath by foam 
or other  padding that is easily removed and will withstand scrapes and 
bumps. If it's  REALLY a work truck, you could make side panels out of 1/8-1/4" 
plywood and back  them with foam that fit over the surface of the inside, 
making the screw holes  match the existing ones (window handles would need to 
be removed or  adapted).
 
Road noise is made up of tire noise and wind, some tires are louder than  
others. Blocky tread patterns are loud, where "highway" tread with continuous 
 rubber are much quieter. Wind noise is usually part of body design, and 
the only  way to reduce it is to line the interior of the cab. Large sideview 
mirrors will  cause wind noise.
 
I have a friend with a Unimog, and there is only a single layer of  sheet 
metal between the engine and the cab, it is LOUD inside. He installed a  
layer of heat resistant material inside the engine compartment and it reduced  
the decibels by at least half.