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Simple Audio Test Chamber
It's been quite a while since I stopped following this group. I stopped
being the recorder for the local youth symphony and ran out of time to keep up with audio matters, bu now I want to build a small audio test chamber for work. I'm hoping the good folks here can help me a bit. We make rugged hand-held computers for outdoor use. Our products are all water proof. One of the challenges we always have is getting good speaker output and microphone input. Good for the speaker means easily heard and understood in somewhat noisy environments, meaning we need something like 80-90 dB SPL output. Good for the microphone means that we can record easily intelligible voice input. Both are at a distance of a foot or two or three with a frequency range of about 300 to 3400 Hz. In the past we have tested speakers and microphones very simplistically, using a sound level meter to monitor speaker output in a normal office environment, but we would like to get a little more professional about it. We had a summer student do some preliminary work, so we now have a reference speaker (Event Tuned Reference 8) and microphone (Audio-Technica AT2020), an Alesis RA 150 amplifier, an Edirol UA125 USB Audio Capture System, a small box to link them all together, and some Matlab programs on the computer to drive the system and plot the data. I was thinking that it would be best to have a small enclosure in which I could mount either a speaker (either the reference speaker or a test speaker or one of our hand-held units acting as a speaker) and a microphone (either the reference microphone or a test microphone or one of our hand-held units acting as a microphone). I would line the inside of the enclosure with sound absorbing panels so that we would be measuring mostly the direct response of the speaker or the microphone, since that is what we would mostly be getting in our outdoor environment. Am I on the right track? Is this something reasonably easy for me to do? How big should the chamber be? I was thinking of making the inside dimensions 25"x59"x96". I chose those dimensions so I wouldn't have any double or triple resonances but could use standard sizes of sound absorbing panels. How much effort should I put into making the side dimensions to spread the resonances? How close together can resonances be before they become too problematic? I would line the walls with 4" thick 24"x48" cotton panels filling most of the sides and ends and 12"x12" foam corners on two of the long edges to cover the gaps. I was thinking that I would put a hole in the long side roughly 3 feet from one end that I could put removable plates in that hold the reference speaker or the speaker under test. I would put another hole and plate one meter from that hole on which I could mount my reference microphone or the microphone under test. That would put my microphone roughly 2 feet from the other end. Is that a reasonable way to position the pieces? Should I fill the space behind the speaker and microphone with more sound absorbing material? Would stacking sections of the pads work? Would stacking the pads on the sides to leave just a narrow space a bit bigger than our hand-held be worthwhile? Would 1" MDF be stiff enough for the walls? Would I need to stiffen it with some outside ribs? I can readily calculate the resonant modes of rectangular plates. Do I need to worry much about the resonances? How many modes would I need to look at? Will the cotton panels (B.A.P. Bass Buster from www.acousticalsurfaces.com) absorb enough sound to make this very roughly like an anechoic chamber at the frequencies of interest? I don't need super precision, but I would like to have measurements that correlate fairly well with what the user would hear in actual use. Jerry Steiger |
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