[T3] Sound Deadening

["Jim Adney" <jadney@vwtype3.org>]

On 14 Mar 00, at 21:12, Bob Perring wrote:

> Multiple layers of thinner materials having differing densities will 
> work well, also.
> Many times, a "thick" mass will just change the frequency of vibration 
> sounds, and do little to lessen the dB level.
> Alternating layers of thin sheets of material which will have differing 
> frequency resonances will often work quite well for sound deadening, 
> without adding much in the way of shell thickness or weight addition to 
> the overall structure.

I intensionally left just a simplified answer, but I think I want to 
respond to this.

It is true that just adding mass will not necessarily damp 
sound/noise. Damping requires some means of converting the 
sound energy to a less noticable form, usually heat. In general this 
is done by sandwiching a high hysterisis material between the 
original structure and a third, heavy material.

High hysterisis materials can be thought of as having a lot of 
internal "friction" which means that if you deform it and let it spring 
back, much of the energy that was required to deform it is lost and 
not recovered in the springing back. This is just the opposite of a 
good spring, or a good bell. We often describe such a material as 
"dead."

The massive object is required just to provide an "anvil" for the 
noisy object to beat the high hysterisis material against. You can 
think of this as the noisy part moving while the massive part 
preferring to stay still and the damping material caught in the 
middle.

For each such sandwich there is curve that describes damping as 
a function of frequency, and each such sandwich has a resonant 
frequency where it is actually poor at damping. For this reason 
sound damping sandwiches are sometimes done in multiple layers 
with different resonances to avoid any overall resonance. A good 
choice of damping material will also reduce the resonant peak, so 
that extra layers may not be necessary.

Tar, or pitch, if not the really hard stuff, is really pretty good at 
damping and makes good sound deadener because it is BOTH 
high hysterisis AND heavy. Kind of a 2-in-1 material. In fact, most 
people don't realize that what used to be called "undercoating" in 
the automotive industry had sound deadening as its MAIN attribute.

-
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054

-------------------------------------------------------------------
Unsubscribe? mailto:type3-request@vwtype3.org, Subject: unsubscribe