We want an environment where we can play speech recordings and re-record to determine distortions added by the equipment. We'd like it to be reproducible, meaning if we repeated the test 10 times, we'd get similar results each time. The recording apparatus is very small, like 1 cm x 5 cm x 10 cm and the cone of the speaker to play the recordings is like 4 cm in diameter.

We're thinking a mini anechoic room, something as simple as a double-walled cardboard box with some bumpy foam inside. Suspend the recording apparatus with clips on the ends of thin wires. The speaker can be embedded in the foam with just the front exposed. We can put the box in a quiet room to limit external noise.

Is there a limit to how small the box can be, or should the box be large enough to accommodate the lowest wavelength of sound in speech?

Since all the sound energy eventually degrades into heat, would we expect the internal temperature to increase significantly? If so, should we look into foam with a low thermal expansion rate?


By all means try the box, measuring the baseline noise, and comparing against the component tests. If it meets your performance goals, great. A box is going to leak sound, a box within a box less so. Two things stop sound from propagating, mass and dead air gaps, preferably both used together.

Googling 'recording booth construction' will show you how isolation booths for vocal performers are made. You could reduce the size of an iso booth down to your requirement while using the proper construction techniques.

The editor of www.silentpcreview.com has a similar requirement for testing low-noise PC components. The story of how he converted a room into an anechoic chamber is here. anechoic chamber blog Even if you don't want to do a whole room, there's some good information there.

The box should probably be large enough for the lowest speech frequency, I'm not enough of an acoustic expert to say with authority but that seems right on the face of it.

Regarding sound to heat, there's not enough energy transfer there to affect the foam, the operating heat of the electronics would produce much more heat in my opinion. Keep in mind that foam does not soundproof, it reduces acoustic reflections in the space it is installed in.

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