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wrote:
i'm building the airflow on my iso booth. the part i'm working on is
the external sound trap for the fan.

the idea is to have the fan flow travel down a long, rectangular box,
then enter the wall.

i saw some circuit breaker boxes at home depot, but couldn't find any
that were really long and narrow.

those would be cool in a lot of ways. they have a top hole cut out for
air intake, a front door, and punch out areas....just not long and
narrow enough.

any ideas on a box i could rig up to build this air tunnel that would
go on the outside of the booth?

I've had to build several of these for control room and instrument booth
ventilation.

The basic principle is to build a wooden box (we've used 3/4" MDF) that
has a pair of baffles about one third and two thirds of the way along
its length. The inner faces of the box and the baffles are covered with
an absorbent such as expanded polyurethane foam.

The baffles extend just over half way into the box from opposite sides,
thus forcing the air to flow up and over the first baffle, and then down
and under the second. The baffles only need extend into the box enough
to disrupt a line of sight path for the fan motor sound.

As for fan, get yourself something like an 8" in-line fan and mount it
to the inside of the duct using 1/2" neoprene pads and an interference
fit. Do NOT screw the fan housing directly to the box - you'll get a
low frequency hum through the duct casing itself into your booth.

Make sure that the air flow created by the fan is not so large that it
becomes noisy on the inner grille.

The bottom line is that any ventilation duct should not offer a direct
line of sight from the outer to inner grilles. If you have a line of
sight, then even if the duct is lined with foam, sound will still travel
along the duct. Make sure the duct is foam lined, and put a dog-leg, or
some baffles inside the duct (or send it around a few corners), and this
will ensure that no sound is transmitted.

Chris W

--
The voice of ignorance speaks loud and long,
but the words of the wise are quiet and few.
--

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