microphone problem in a 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?

[mediatechnology]

Are we at liberty to say what instrument you're attempting to mic?

I would defer to either emrr or Roger with your question.

My hunch is that smaller "1.5V-powered?" low-voltage FET-buffered condensor mics are going to overload in this situation. Not sure why it's worse in a sealed box.

[JR.]

I have done some work with small cheap microphones in my drum tuner fractions of an inch from a drumhead (not being played).

You need to determine if the mic is overloading mechanically from SPL, or electrically.

IS SPL meter placed identically to mic?

Is mic performance frequency sensitive? A response curve measurement might reveal resonance or other trends,

JR

[mediatechnology]

Thanks John. I overlooked an obvious resource on that one.

[k2panman]

I stuck the SPL meter in the same location as the microphones, although I moved the microphones all over the place, including behind barriers of various materials, but the results were always the same. I am trying to get some new audio equipment setup to record some, so I have something to look at.

This is for recording voice - it is a cup like thing that is pressed against the face - like a court reporter microphone if you've ever seen that.

I'm also going to try to use a dynamic microphone, as I've been trying various condenser elements.

I build steel drums, the musical instruments that originated in Trinidad and are now found worldwide, especially in the Caribbean. I plan to try to mic these also, but that is a future project. I haven't heard anyone who has done a great job mic'ing steel drums except in a studio - I hope to come up with something that works good on a gig.

[ricardo]

Did you have the SPL meter in the same sealed box?

What 6 condensors did you try?

[JR.]

If this is pressed against the face does it make a sealed enclosure or is it vented to the air? I wonder if the person is pressurizing it? Not that I know how that would effect the mic, shouldn't bother it if the back of the diaphragm is vented.

Breath noise could saturate the mic with LF if closely coupled.

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I worked with guy back in the '70s who was from Trinidad and he made his own steel drum too (doesn't everybody?). ... I don't know if they're hard to mic, or you just don't realize how bad they sound when you play them.. Just kidding. I had to do a drum industry trade show with a steel drum booth right behind me. The cloth curtain was not much of a sound barrier.

Seems a mic mounted under the drum pan perhaps with some selective EQ might help key in on the resonances you want and down play what you don't.

Note, my drum tuner does not tune steel drums.

JR