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[on:]

Hi, I’m about to start on what I hope will be a pretty simple project. I want to build an arduino based noise level sensor. I’m not interested in analyzing what the noise is, just whether it’s loud or quiet. (basically, I’m just trying to detect if a particularly loud item of machinery is functioning or not).

I’m trying to find out if I need an ‘envelope follower’ (and if I do, what is it?). There are several posts on this subject, but I’m still unclear on what it’s for. It sounds like it’s basically just a ‘sound averager’ that will give an output proportional to the ambient noise level, is this correct?

Can’t I just get a cheap electret microphone, connect it to a pre-amp (for example, this one…http://www.sparkfun.com/products/8872) and then wire the output of that pre-amp to one of the analogue input pins on an arduino and then use the analogueRead() function to decide how noisy it is?

Won’t this do what I need? Or is it more complicated than that?

Thanks

[Reply #1 on:]

Sure - then do the averaging in software to smooth the level some.

[Reply #2 on:]

Can’t I just get a cheap electret microphone, connect it to a pre-amp (for example, this one…http://www.sparkfun.com/products/8872) and then wire the output of that pre-amp to one of the analogue input pins on an arduino and then use the analogueRead() function to decide how noisy it is?

Won’t this do what I need? Or is it more complicated than that?

  It's a little more complicated then that. After the pre-amp you still have a audio AC voltage and the arduino analog input pins require that the voltage remain in a 0-5vdc range, no negative voltages allowed. What you must have after the preamp is a diode rectifier/filter circuit that will then give you a DC averaging value representitive of the AC signal. The other problem is that such a circuit will be basically a linear responding circuit and sound levels in the real world cover a hugh amplitude range and generally require a logrithmic amp to scale the range for useful measurement.

 So if your after a somewhat accurate 'loudness' measurement then you have further research to do. If you just want a circuit that gives a general "this sound is louder then that sound" kind of hardware/software function then just add a rectifier/filter and make sure the signal cannot go higher then 5vdc.

Lefty

[Reply #3 on:]

Thanks - I understand the need to keep the voltage within the 0-5volt range, but where could a negative voltage come from on the pre-amp circuit?

Good point about sound being logarithmic I forgot that. Cheers

[Reply #4 on:]

Thanks - I understand the need to keep the voltage within the 0-5volt range, but where could a negative voltage come from on the pre-amp circuit?

Is not audio a AC voltage with both a positive and negative voltage crossing the common reference? Unless you do something like add a DC offset voltage to a AC voltage there is always a positive and negitive voltage value at any give instance.
Good point about sound being logarithmic I forgot that. Cheers

[Reply #5 on:]

I remember from a few years back someone was trying to make a noise meter for a school canteen. The idea was similar to yours and so was the solution. However, he found after months of tinkering with levels that actual perceived loudness did not correlate with the signals from an envelope detector.

Just bear that in mind as it might not be such a simple project.