From: Dan Nelson <dprimary@x??????xxxxx.xxxx
Date: Tue Nov 2, 1999 6:21 am
Subject: Re: acoustic treatment
A resonant panel or diaphragmatic absorber is used to absorb low frequencies.
They work by vibrating at these low frequencies and turning the sound energy
into heat. The drywall in wall construction acts as a resonant panel and
absorbs a considerable amount of bass. If you a ever been in a empty masonry
or concrete room you might have noticed the amount of bass build up when you
try to talk in the room. Compare that to an empty room in the average house
with sheetrock walls there will be some reverb but the bass build up like in
the concrete room. When you calculate the acoustic absorption of the room
remember to account for the bass absorption of the drywall.
The simple way to build a resonant absorber is to mount panel on a 2 by 4
frame with fiberglass batt (R11) or 703 inside of it. If you use 703 leave at
least a1/4" space between the panel and the fiberglass or else the panel won't
be able to resonant properly.
Approximate Plywood Panel absorption peaks on a 2 by 4 frame 3.5 " deep
1/8" = 150 Hz
1/4" = 110 Hz
3/8" = 87 HZ
You can also build them into the corners of the room which will give a
variable depth increasing the bandwidth of the absorption.
I had a recording room with plaster over brick walls that had a nasty problem
at 250 Hz well it just happens that R19 insulation with the paper facing the
room has a peak absorption @ 250 Hz. I built two 8' high by 4' frame in two
corners put in R19 with the paper facing the room. They fixed the bass problem
right up. I finish them by adding some 1/4" wood molding on top of the frame
and stretched some fabric across the frames to finish them. The molding is to
keep the fabric from touching the paper.
If Everest hadn't of pointed out the absorption of R19 in his handbook I would
never thought of it.
I made this a little long winded so I could have a good start on the subject
for the site.