Sensitive microphone

Community Projects
1

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 been trying to use the MEMS Microphone Breakout & Electret Microphone Breakout and not getting any type of response, either the voltage is too low to measure on the arudino and the pre-amplifiers do not work and just seeing loads of noise on the signal line. I have ordered the lm386 OpAmp Breakout which I hope will amplifier the signal from the microphone boards.

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.

2

Sounds like you just need a more sensitive microphone.

http://www.analog.com/library/analogdia … ivity.html

http://www.sengpielaudio.com/calculator … factor.htm

3

Or identify where the noise might be comming from. Is it normal white noise, or could there be a switching powersupply messing up the supply to the amplifier. Or some other kind of external noise. Just thinking. Maybe more gain isn’t the solution, just lower noise level.

4

An oscilloscope can point you where the problem is: in the amp or in the ADC.

The arduino adc is 10 bits IIRC, and probably a bit noisy as well.

You could try to find a higher resolution/low noise ADC (>= 16 bit/40kHz or so)

Alternately you could try to build a linear-> logarithmic converter. Then you can use the 10 bit adc at low sampling speeds.

A suggestion is shown here: figure 1 on http://www.butleraudio.com/tubesvstrans1.html and probably any TL072 or LM358 opamp will do (500pf/1.5k not needed for these opamps).

btw what’s the problem with using cheap ass sound level meters (http://www.dx.com/p/portable-1-5-lcd-di … f22-120765) ?

5

as marcovansteen said, you need to look at the signal first to figure out what the real problem is… use a scope and then act on else as you mentioned you will be going around in circles.

6

marcovansteen:
An oscilloscope can point you where the problem is: in the amp or in the ADC.

The arduino adc is 10 bits IIRC, and probably a bit noisy as well.

You could try to find a higher resolution/low noise ADC (>= 16 bit/40kHz or so)

Alternately you could try to build a linear-> logarithmic converter. Then you can use the 10 bit adc at low sampling speeds.

A suggestion is shown here: figure 1 on http://www.butleraudio.com/tubesvstrans1.html and probably any TL072 or LM358 opamp will do (500pf/1.5k not needed for these opamps).

btw what’s the problem with using cheap ass sound level meters (http://www.dx.com/p/portable-1-5-lcd-di … f22-120765) ?

I think so that this [TL072 article is bit helpful for you, and you can easily understand the datasheet to work further](Introduction to TL072 - The Engineering Projects)