In article ,
Dr. Rev. Chuck, M.D. P.A. wrote:
Steve Knight wrote:

On Wed, 16 Jul 2003 08:23:15 -0500, Lawrence A. Ramsey
wrote:

Steve, I beg to differ. Look at your house. If you want to hear
EVERYTHING that goes on in the next room, leave the wall space empty.
If you want to deaden it, build a double stud wall staggered and weave
fiberglass insulation between. Used it in an college athletic dorm and
it works great! But merely empty space is NOT a sound barrier.

when I mean dead air space I don't just mean one space. remember all the
sheetrock and studs are tied together and transmit sound.
Yes I know about it because I studied acoustics when I was a recording
engineer. one of the primary sound deadening methods is to build two
walls that
don't touch.

Isn't fiberglass basically trapped air? If so, it's useful for
thermal insulation and a needless expense for sound insulation.

Well, yes AND *no*, applies.

There are two separate issues with regard to how sound (mechanical energy)
carries.
1) energy transmission _within_ a particular material
2) enerty transfer _between_ two dissimilar materials

Transmission _within_ any material is more efficient than "across the
boundary" to a dissimilar material.

_Within_ any material, losses increase with the _distance_ the energy
must travel. Introduce a long twisted path, with lots of dead-ends,
and the energy losses go _way_ up.

Those two facts are why products like styrofoam and foam-rubber are
such effective sound "insulation". *LOTS* of air-plastic interfaces
that attenuate the energy. And any path "just through the plastic"
is _far_ longer than the "straight line" distance through the foam.
Also, lots of 'false paths', where a fair part of the energy ends up
going 'sideways', or even 'backwards', rather than 'through' the
material.

Another effective technique for isolation involves using _lots_ of
small objects, rather than a single large one. Irregularly shaped
small objects work best. This -maximizes- the surface area for
contact with 'dissimilar' material (e.g. the surrounding air),
_and_ *minimizes* the contact area with 'like' material. Thus the
sound passes 'moderately efficiently' through that _small_ contact
area, spreads out in the next object, and only the small part of
the energy that reaches where -that- object contacts other objects
is passed on.

Sand is an amazingly effective isolator in this respect. Styrofoam
"peanuts", "popcorn", etc, are also highly effective. 'sheet' in-
sulation is good for this, too. Fiberglas bat is significantly
less effective, because the 'rigidity' is much lower (closer to that
of air). It's definitely better than an 'unfilled space', but
inferior to some of the 'higher density' filler materials.


Of course, *nothing* beats a hard vacuum. grin