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Acoustic treatments for
studios and control rooms fall into three
categories:
absorbers,
barriers and
diffusers.
Acoustical absorbing materials, often called
“fuzz”, are open, airy, fluffy, light materials
that do not reflect sound. Perhaps the most common absorber is open
cell
polyurethane acoustical foam.
This material is highly efficient and
yields absorption coefficients above the
theoretical limit due to the sculpted surface
pattern. Although the flat surface area
absorption may be limited to 1.00, the total
surface is increased producing more total
absorption than would otherwise be possible.
While sound will not bounce back from
absorbing materials it will pass through them
with minimal loss.
Barriers to sound transmission are heavy,
dense and massive materials.
They include multiple layers of drywall
(sheet rock), acoustical lead, and most recently
mass loaded vinyl.
The latter two materials are thin
(one-eighth inch) but have sound transmission
losses that exceed those of a two-inch solid
core oak door.
They offer the advantage of retrofitting
an existing wall without heavy construction or
loss of space.
Barriers do little to absorb sound.
Diffusers are a means of acoustical crowd
control.
The object is to scatter the sound, not
kill it.
They are particularly useful for
increasing the apparent acoustical size of a
room and for broadening the “sweet spot” for
mixing.
Early diffuser designs, such as the
polycylindrical seen on film scoring stages,
take this a step further as they become
bass traps below 500 Hz.
This is particularly useful to
small control rooms where sound can hit the
rear wall at the mid-point of its cycle and
return to cancel itself in the middle of the
room.
Other odds and ends that are quite useful
include sound
sealant caulk and
neoprene
isolation pads. A crack in the intersection of a wall can
cause a loss of 8 to 16 dB.
If air or
light passes through, then sound sneaks through
as well.
Resilient pads are necessary to decouple
structures, float floors and walls. Sound
travels through structures and particularly well
below 100 Hz, and isolation the structure by
floating may be the only way to prevent this.
While
acoustical foams are generally low cost
and safe for home use, there are some other
effective options.
Moving pads work well to attenuate
high-end leakage.
Absorber panels can be made from
compressed fiberglass ceiling tiles.
When placed across a corner and linked
with rock wool, they make an effective bass
trap.
These materials should be covered with an
acoustically transparent fabric to prevent
glass
particles from entering the breathing space.
Chair cushions and pillows are also
useful.
In the past, budget studios have used
egg
cartons; however, the acoustical properties are
not that good, and an open paper wall covering
won’t lower your fire insurance premiums.
There is little that can be done to stop sound
transmission through walls other than increasing
their mass with extra drywall or
barrier.
If the walls are solid, however, door
seals (including the gap at the bottom) and
caulking will sometimes provide significant
improvement.
Diffusers, particularly the
polycylindricals,
can be a good do-it-yourself project.
A piece of 3/16” Masonite placed between
two strips of 1”x2” wood across a corner or
centered on your rear control room wall will
provide both high-end
diffusion and bass
trapping.
A full 4’x8’ sheet that bows out at the
center 6-7 inches will
scatter sound over 180
degrees and have a center frequency of
absorption at 63 Hz.
The back of the panel should have
absorbing material to dampen the resonance while
the front surface will benefit from
acoustical
fabric such as
Guilford of Maine FR701.
The hard surface without the cloth
produces a scraping
fingernails-on-the-blackboard sound (not a lot,
but very unpleasant.
Other styles of
diffusers, such as the
quadratic, can be approximated by use of a
bookcase filled with books of different sizes
and shapes in a symmetrical left and right
patter.
A good selection of books can also
impress your clients.
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