I'm using this MEMS microphone circuit: mic circuit]([![enter image description here

and I'm not getting a good frequency response. The microphone's datasheet shows a good flat response from 200 Hz-4 kHz (datasheet:mic datasheet). I played tones from 200 Hz-4 KHz, and the amplifier output did not respond well outside of the range 650 Hz-1000 Hz. I think there must be something about the amplifier circuit that's causing a bad frequency response. Any suggestions on what the problem could be or how to improve the circuit?

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    \$\begingroup\$ Is your sound source perfe tly flat? \$\endgroup\$ – PlasmaHH Nov 3 '15 at 7:54
  • \$\begingroup\$ It's a frequency generator phone app, the sounds are played at the same intensities. \$\endgroup\$ – cheeto Nov 3 '15 at 8:24
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    \$\begingroup\$ If you're using a phone spekter as a calibration source, what makes you think that has a decent frequency response? \$\endgroup\$ – Brian Drummond Nov 3 '15 at 10:56

R3 and C1 form a high pass filter that has a 3dB point at 22 Hz so this isn't causing your low frequency problem. Maybe you need to explain what you used to stimulate the microphone?

At the high end of the spectrum C3 and R4 (if the components used are shown correctly) produce a high frequency low-pass 3dB point at about 16 kHz so there's no real problem here. Again, I ask what you used as your audio source.

I've just seen your comment that you are using your phone to produce sound stimuli and I hang my head in despair that you believe that your phone can adequately produce accurate sound pressure levels to calibrate a microphone with. Go do some research on what your speaker can output.

Here's the frequency response of a speaker that is suitably sized to fit in a cellphone: -

enter image description here

Between 200 Hz and 1.2 kHz the SPL output changes about 50 dB. Taken from this report by Murata. Now clearly a speaker like this is going to sound tinny and there will be some attempt to offset this by electronic circuits that can emphasize the deficiencies but you are still not going to get anything like a flat response that is at all usable to calibrate your microphone.

Your microphones specification is 60 Hz to 15kHz +/-3dB is far better than what your cellphone speaker will be.


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