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I'm not sure that's the right forum to ask this question but I'll do it anyway .

in my project, I've to listen to different machines using an MEMS mic by fixing the mic directly on the machine , the problem is that I get, is that all the records that I need are overchaged so useless, my question is how should I choose the mic for really loud measurements.

UPDATE here is a link to the device:

thanks in advance for any hint.

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closed as unclear what you're asking by Leon Heller, Keelan, Matt Young, placeholder, Vladimir Cravero Sep 17 '14 at 14:08

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ A link to the device would be useful as would the anticipated SPL of the machine. \$\endgroup\$ – Andy aka Sep 17 '14 at 8:57
  • \$\begingroup\$ @aka it's not only one machine it can a car on motorcycle \$\endgroup\$ – Engine Sep 17 '14 at 9:07
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    \$\begingroup\$ This question was cross posted to dsp.stackexchange.com/questions/18234/… with all typos intact. \$\endgroup\$ – JRE Sep 17 '14 at 9:08
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    \$\begingroup\$ Normally you would post the question in one forum. If you don't get an answer there, you could delete it and ask in another forum or have one of hte moderators move it for you. There's something about that in the forum rules. \$\endgroup\$ – JRE Sep 17 '14 at 9:23
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    \$\begingroup\$ @Engine, Well here's the digikey search results for microphone. digikey.com/product-search/en/audio-products/microphones/…. You pick one :^) \$\endgroup\$ – George Herold Sep 17 '14 at 13:22
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You could lower the gain on the amplifier by changing R4 to a lower value. That should help a lot.

Right now, the amplifier has a gain of 67. If you change R4 to 1.5K then the gain would be one. I would start by reducing the gain to 10 (R4=15K.)

How to calculate gain: Gain = R4/R3

ETA: The pictures don't show the locations of R4 and R3. You will need to see which is marked 105 (100K) - that will be R4. You could also download the eagle files and check the layout.

Building on Andy aka's information, you should probably set your gain down to 1 (R4=1.5K)

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  • \$\begingroup\$ thanks for your answer, but the problem isn't the amplifier the problem is that the mic itslef generated overloaded signal \$\endgroup\$ – Engine Sep 17 '14 at 9:56
  • \$\begingroup\$ Are you sure? Have you checked what comes from the microphone before it goes into the amplifier? \$\endgroup\$ – JRE Sep 17 '14 at 9:59
  • \$\begingroup\$ yes I boght the mic and design my own circuit , and put a potentiometer to adjust the amplifier but it did't change anything \$\endgroup\$ – Engine Sep 17 '14 at 10:03
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    \$\begingroup\$ If the microphone itself is causing distortion, then you need a microphone with a higher maximum sound pressure level. The ADMP401 is specified for 120dB SPL - you would need a higher rating for that value like maybe 160dB SPL. \$\endgroup\$ – JRE Sep 17 '14 at 10:14
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    \$\begingroup\$ Best I can come up with is this: invensense.com/mems/microphone some are up to 130dB. I also find that electret mics can take a higher SPL, but most of them don't specify how high. \$\endgroup\$ – JRE Sep 17 '14 at 12:50
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Following your link I read this: -

The amplifier produces a peak-to-peak output of about 200mV when the microphone is held at arms length and is being talked into at normal conversational volume levels

The link also said: -

1.5 to 3.3VDC supply voltage

Normal conversation at 1m (from here) is 50 dB SPL

A passenger car at 10m distance is 80dB SPL (also from same source) so I would expect an engine mounted microphone to receive at least 30dB more sound pressure (110dB SPL).

That's an SPL that is 60dB greater than normal conversation. If normal conversation produces 200mV RMS then a car engine mounted microphone might produce a voltage that is 1000 times higher (60dB is an amplification of 1000) and so your 200mV RMS turns into 200 V RMS and of course this is a completely impossible situation given that the the MEMs module you purchesd can run from 3.3 volts.

CONCLUSION: - YOU ARE TOTALLY OVERLOADING THE MICROPHONE AND AMPLIFIER.

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  • \$\begingroup\$ @aka thanks for you answer do you have any mic suggestion for this kind of measurements ? \$\endgroup\$ – Engine Sep 17 '14 at 9:54
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    \$\begingroup\$ @Engine I'm speculating on the likely SPLs - you'll have to do some research on this and see and find a microphone that can handle the levels but, you could try reducing the gain on the circuit as mentioned by JRE \$\endgroup\$ – Andy aka Sep 17 '14 at 9:56

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