problem with microphone in small enclosure

[k2panman]

I need to design a device with a microphone in a very small (2"x2"x2") enclosure. My problem is, whatever I do, I seem to be over modulating the microphone. I checked SPL with a CheckMate SPL meter, and both A and C weighted give me SPLs in the low 120s.

I've tried many variations on placing the microphone (put it inside tubes, shielded it with baffles and membranes, put it in a separate chamber), and I've tried about 6 different microphones (all condenser), and several headroom circuits, including a 4 wire mic and headroom circuit that is suppose to be good to 136db, but it still sounds like I'm over modulating - slamming the mic membrane to the wall. If I open the enclosure, the microphones all sound fine.

Any suggestions?

[mickeymoose]

Have you tried sound insulation material or lining inside your enclosure? E

[k2panman]

I did try stuffing the whole space with some foam, but it wasn't acoustical foam - it didn't make any noticeable change.

[Enzo]

So then just what is doing the driving? In a sealed box, what was sound waves can become pressurization.

[k2panman]

I have the enclosure vented with an open 1/2 diameter hole, so it isn't pressurizing. I would try to put some kind of acoustical dampening material in there, if I knew what to use. I was looking around for some cork, but being I filled the whole thing with open cell foam (the regular packing foam/sponge - polyethylene or polyurethane?) I'm not sure that is the problem. I am reading about room acoustics - anyone know what happens when you shrink a room to 2" x2" x 2"?

[sofaspud]

When I first read the original post, I thought of "echo chamber." Maybe a pic or a little more info will stimulate some solutions.

[k2panman]

I have trouble drawing this - but - Take a paper cup (coffee cup), poke or cut a hole in it big enough to stick your finger through, hold it up to cover your mouth and chin, and talk into it. I want to stick a microphone in the cup, and record it.

When I try this - I get what sounds like totally over modulated recording. If I pull the cup away from my face 1/2 inch or so, the recording sounds fine. I need the recording to sound good with the cup sealed against the users face.

I think I've seen this type of a setup in a courtroom - I think there was a "court reporter" that was speaking into a cup - I couldn't hear her, but I think that what she was doing.

[Ron E]

Small electrets are going to have issues with SPL's more than ~110dB. 1/2" or 1" electrets might work better. Radio shack sells a cheap omni dynamic microphone that might also work fine for this. Even a headphone element might work well as a microphone in this situation.

[k2panman]

Thanks Ron. I don't suppose the microphone manufacturers rate their mics on what SPL they handle, especially the cheap ones. Is there anything to look for that may reveal a higher SPL capability?

[k2panman]

I just ordered a Sure SM58 microphone and I'll test this. It is a dynamic cardioid microphone. I also got a pop filter - a double screen that stops the direct air strikes on the microphone. I'll post how this works in my enclosure.