[Jerry Steiger]

"Scott Dorsey" wrote in message
...
It's easy to do if you only care about high frequencies.

Since our little bitty speakers can't do much below 300Hz, I figured I was
getting into the range where it was doable.


The hard part is
the door.

I was planning to have small panels that I would mount with screws. That
will slow me down in testing, but this is something that we won't do all
that often or spend that much time on.

The lower the frequency of interest, the larger the chamber has to be.

Got any good rules of thumb or a likely source to find some? 300Hz means a
wavelength of 3.76 feet. Does the smallest dimension of my chamber need to
be greater than that or some multiple thereof?

Take a look at the IAC catalogue. They make portable chambers in all
varying sizes from very small to very large. You can try copying their
designs.

Thanks for the tip. I didn't find any that looked like a very good match for
what I am trying to do. They make a couple of small test chambers,the mac-1,
mac-2 and mac-3, but they only seem to be interested in isolating the device
under test from the outside, since they use sheet metal walls on the inside.

The mac-1 has roughly 2" insulation and they claim 23dB of reduction from
out to in at 250Hz, 29dB at 500Hz, and 36-38dB at 1KHz to 4KHz. The mac-2
has roughly 4" insulation and claims 40dB at 250, 45 at 500, and 52-57 at 1K
to 2K, so it does seem like my 4" cotton might be able to absorb quite a bit
of energy. More layers should be better, so maybe it isn't such a bad idea.
Or is it?

They don't seem to worry much about the "room modes". The mac-2 has inside
dimensions of 24x24x32 inches. Lots of reinforcing of nulls and peaks in
there!

I am thinking that Chris is correct, that if I get enough absorbtion at the
frequencies of interst, I don't need to worry much about the modes in the
chamber or the resonances in the walls. But can I get enough absorbtion?

Thanks!

Jerry Steiger