[Scott Dorsey]

Jerry Steiger wrote:
"Scott Dorsey" wrote in message

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?

Look at some of the room mode calculators on the web. If you can make it
large enough that all of the major room modes fall below the lowest
frequency of interest, that's a start.

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.

Right, I think you're going to need something much larger than any of these.

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!

Right, that's not the right tool for the job... look for the larger chambers.
You can get away without a walk-in but you are going to need something larger
than a fridge if you care about 300 Hz.

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?

If you want absorption at 300 Hz, you need several feet of fibreglass. And
yes, that is the solution.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."