"lbbs" wrote in message
...
It's a question that I posted on many news groups, but no one
is willing to answer my actual question. It is a simple question. There
are two different ceiling tiles to choose from. One is light, flexible
and made fiber glass insulation. The other choice is a heavy tile, that
is
hard (made out of asbestos or dry wall type of material, not sure though).
Which is better for sound insulation? Simple question right!!

I have asked a slew hard ware store to ask this very question and every
one a ask says the exact opposite. I have always heard that you need
mass and air space to have good sound insulation. My thinking is that
yes, hard objects carry sound, but since this ceiling is free floating and
supported only by wire, the sound will not travel through the hard tile.
Those that make sense.


PREVIOUS POST:
I was just wondering if I replace my flexible 48" x 24" ceiling tiles with
the hard type (much heavier) will I notice the difference in sound
proofing?

I call all kinds of hardware stores to see which of the two is better for
sound. The opinions are split 50/50 between flexible and hard type.
I
personally think the hard type are better, I wonder if replacing them
with
this type will actually make a noticeable difference to be worth replacing
them?




I don't think the answer is as simple as the question. Let's take it from
the
top:-

Sound can either be transmitted, reflected or absorbed. You want to
eliminate
the transmitted sound, so you need to reflect or absorb it.
To reflect sound you need to create mismatches in the compliance of the
mediums
that the sound travels through ( in electronics, analogous to an impedance
mismatch ).
The way to do this is to put dense materials ( or more correctly, massy ) in
the way
of the sound, as if the noise comes via the air then that has a low density,
so the mismatch
is high, and the reflectance is also therefore high.

Absorption needs an energy loss mechanism; the sound wave needs to lose
energy as
heat whilst passing through a medium, for it to be absorbed. Such a material
might
be rockwool, sand, or some kind of non-resilient material.

The best combination would be a material that is massive and energy
absorbing.
There are complications, low frequency sound will be difficult to defeat,
any airgaps
leaking noise must be sealed, there should be no ( or few ) ways for the
sound to
travel through the supports of the sound insulation ( and thus bypass the
airgap
and suffer only a small mismatch and thus not be reflected ). Also, any
sound that is
reflected will only try and return if it is not absorbed by something after
its first
reflection.

Your hard tiles have mass but probably no absorption; the sound will be
largely
reflected by them but bounce back up to the ceiling ( slightly attenuated
as some will transmit back up through the ceiling ) then bounce back for a
second
go at getting through your tiles.
The flexible tiles have less mass than the hard ( so reflect sound
less ) but may
absorb any transmitted sound better. I say may, as we have no info on the
material characteristics vis a vis absorption. So your question is
impossible to answer
on the data given. What I have written here, I hope will allow you to think
about the
problem in a more scientific manner, and hopefully you can ask for more info
on these
materials.

If you go wih the hard tiles, which are massive, you at least can be sure
that their
reflectance will exceed that of the lighter flexible tiles. You might then
think about
some absorbtion material in the airgap. Sand is no good with the suspended
ceiling you have, rockwool might work ( I have no idea of its absorption
properties,
it is suggested quite a lot though ). Othe things that might work would be
the audio
equivalent of RAM ( Radar Absorbtion Material ) tiles used in radio. These
are
carbon-loaded and have an exaggerated stalactite type appearance to them.
The
carbon absorbs the radio waves, and the spiky profile ensures multiple
reflections
( and hence opportunity for absorption ) when it is struck by a radio wave.
Sound waves behave similarly and although carbon loaded foam won't work for
sound, something that is non-resilient ( i.e. does not instantly spring back
into
position when disturbed ) will. The stalactite/eggbox/dimpled patern will
also work for
sound waves. IIRC, you could also buy sheets of material with lots of holes
drilled
in it, I think it was filled with rockwool; hey used it in 'phone booths,
that is an
absorption type material.

So, high mass tiles for reflectance, then load the airgap with rockwool or
glue some
kind of absorption tile or material on the ceiling on the other side of the
airspace,
preferably with the eggbox profile if they do it, and you have the
theoretical best
of both worlds. Seal all airgaps, between the tiles and at thje edges. Make
sure the
suspension wires don't transmit sound along their length, so bypassing the
airgap/
absorption material, and don't expect too much anyway.

Hope this leaves you less confused, rather than vice versa,

Andy.