--- Forwarded Message from Bob Majors <[log in to unmask]> ---

>User-Agent: Microsoft-Entourage/10.1.1.2418
>Date: Thu, 27 Mar 2003 12:10:21 -0800
>Subject: Re: #7097 acoustics in a media-screening room
>From: Bob Majors <[log in to unmask]>
>To: Language Learning and Technology International Information    Forum   <[log in to unmask]>
>In-Reply-To: <[log in to unmask]>

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> I have a new 36" Toshiba flat screen with speakers on the front frame. We
> do not have surround sound, as the room is fairly small. The tv sits in a
> corner against a brick wall, and backs up to cable conduits. The problem is
> how extremely well the noise carries through the brick wall and into the
> next classroom, and how varied reactions are to the noise. We've filled all
> gaps around the conduit with rubber caulking. Can anyone suggest
> *inexpensive* solutions that would redirect sound away from the wall
> without absorbing it? Or, how I might conduct tests to figure out what will
> work best?

You can test with various sound reading devices, you need the kind that test
at multiple frequency bands, can be calibrated, and have a very flat
(frequency-wise) omnidirectional microphone built in.  But you said
inexpensive, so maybe find out if some other dept. has one and will test for
you.  Or just assume the leaking sound is broad spectrum, and attack the
problem in a general fashion (use mass that is broadband).

I don't know what kind of rubber caulking was used; it is best to use a
non-hardening, permanently flexible, non-shrinking type, that will stay
flexible and in place for many years.  US Gypsum makes as good as you'll
find (acoustical caulk); there are other brands that your local
(professional) wallboard supply company may carry that would work.  The
stuff from your local home repair store may not be the best.  Use it very
liberally (you can place foam backing rod in behind to cut down on the
amount of caulk you'll need.)  Conduits should not be straight through walls
for good sound isolation, they should turn after entering the wall, go a few
feet, then come out the other side.  If the conduit is truly one of your
biggest problems (I don't know), and you can rebuild it, try that.

You're on the right track though, to first properly seal all cracks that
would allow air to flow (because where air go, so does sound).  If there is
a big enough pressure differential between the rooms, you might be able to
(on the high pressure side) move a stick of incense up close next to all the
surface areas, to see where cracks are, then seal those up (masonry sealer
if on the brick wall).  You could drill into the brick with a mortar bit,
then screw in Unimast-brand (I think they've been bought out by Worthington)
RC-1 deluxe (original US Gypsum design) resilient channel.  Since you're in
the midwest, you may have to specify the original one, since Unimast sells
another RC-1 that has been tested (see
http://www.jglacoustics.com/acoustics/resilient_channels.htm ).  Don't buy
RC from your local home building supply; go to a professional sheetrock
firm.  Then screw it into the brick wall, and apply two layers of 5/8"
sheetrock to it, following the instructions for screw length, type, and
installation, *exactly*, or else you've wasted your time and money.  There
is a limp mass barrier product, 1/8" and feels kind of like rubber, with an
STC (sound transmission class) of 27 (average across the spectrum, so less
effective at lower frequencies).  It is kind of pricy, but you could give it
a shot if you don't want to build a wall.  It won't do as much as the
sheetrock design I described.  The brick wall may be acting as a resonator.

You didn't describe the floor, other walls, and ceiling.  The sound might
easily be traveling up and over a drop (acoustical/tile) false ceiling.  Can
you tell by listening up close in the classroom?  The brick wall and/or
conduit may not be the only problem.  Whatever you do, you need to seal all
air gaps as per above, and decouple your structures.  Don't worry about
absorbing too much of the TV sound in that room, you'll still have plenty of
sound.  Also, if there is any physical coupling of the TV and/or TV
stand/mount to the wall and/or floor, you need to eliminate that --
structural transmission of sound causes lots of problems.  Try putting the
TV and/or stand on the proper kind of neoprene mat.  It's hard to know
exactly at a distance what your worse problems are.  Good luck.

Bob