Sensitive microphone

[brendonshaw]

I am working on a mini project for a school who want to detect sound levels in the classroom and been going around in circles for a few weeks. They want to measure sound levels but ranging from very low sound i.e. chair moving, door opening to levels of talking within the classroom.

I appreciate to get an accuracy levels a number of microphones located in different parts of the classroom would need to be used. But, just to be able to monitor sounds levels I would like to build a simple sound meter.

I have used many of the Arduino type microphones and built pre-amps using lm358, 386 and BC548 circuits but none of them provide the response I am looking for. Many of the circuits seem to be very noisy and not seeing the response which I am looking for.

Audio analogue electronics is not my area of expertise and looking for some ideas on suitable circuit and design.

 

[chuckey]

" Arduino type microphones ", Surely an Aduino is a microprocessor and has nothing to do with microphones. If you use an Electret type mic (Google "electret"), with a BC548 pre-amp (gain X 10) then any OP amp with a gain of 500X. You will get volts of good audio. To measure its amplitude, it has to be turned into DC and displayed on some device. If you just use a DVM on the output on AC volts, it will have a terrible frequency response. You can use a peak detector (diode + big capacitor) and use a DVM on DC volts to measure. Professionals would use a VUmeter or a PPM (peak programme meter). Use Google.
Frank

 

[audiomik]

For your application, have a look at the AD 637 IC from Analog.com which will convert your microphone pre-amplifier output signal directly into a 60dB range display.
Full information on how to do this is within the datasheet:
https://www.analog.com/en/special-linear-functions/rms-to-dc-converters/ad637/products/product.html
Think that AD do an evaluation kit including a pcb for this product which could also assist you.

This IC will easily cover the entire Audio frequency range unlike many DMMS as noted by Chuckey.

The 60dB range will be able to show low levels as well as higher ones of sound pressure within the same metering. Sound pressure is normally measured in this way and you might find it useful to look at A and C weighting of your microphone signal which relates to how the ear responds.
Again, Internet searching is your friend!

hope this assists
Mik

 

[Audioguru]

An LM358 opamp is too noisy and has a poor high frequency response and poor slew rate for audio. The LM386 is a low power inexpensive noisy POWER AMP, not a low level preamp.
A BC549 is a BC548 that is selected for low noise.