How to determine the loudness of a sound

[kerrick]

I need a sensor or sensor circuit that can determine the approximate loudness of a sound (which is in the range 30-80 dB). However, I cannot find any parts, schematics, or other helpful documentation on the internet. I have a microphone and AVR microcontrollers immediately available, so if anyone can recommend a solution involving solely those parts (and other common components such as resistors), it would be appreciated.
Thanks.

 

[vk6kro]

You need to purchase a sound level meter like this:

They are not expensive and very difficult to make and calibrate yourself, especially without already having a sound level meter.

http://www.frys.com/product/2965325?site=sr:SEARCH:MAIN_RSLT_PG
That is just an example. It covers 54dB to 126 dB in 7 ranges.

There was a similar request in this thread:

https://www.physicsforums.com/showthread.php?t=353426

It would be worth reading through that link.

There was a link in that thread to the following site if you really wanted to make a sound level meter yourself.
http://www.discovercircuits.com/V/vu-meter.htm

 

[Averagesupernova]

You can purchase microphones which are calibrated 'out of the box' for SPL use. I was on a design team for an SPL meter in which we purchased the mic, designed the rest of the circuit with a log amp, microprosessor and display and it was 'done'. We selected the parts based on the accuracy we needed, sent the thing off to a lab to be tested and when it came back within spec it was put on the market. No calibration at all. I wouldn't reinvent the wheel though, just buy one.

 

[kerrick]

vk6kro and Averagesupernova, thanks, I think I will be able to find what I need among your links.
I should perhaps explain more thoroughly what I'm trying to do: I have a sound that has distinct "beats" (more of a rise and fall in loudness than a sharp tapping), and I want to actuate a motor in response to the beat, similar to the concept of the iDog. So the circuit that detects loudness must sample many times a second and feed its output into the AVR for processing.

 

[vk6kro]

You may need to explain a bit more, but on that description, maybe you could amplify the signal from a microphone enough so that you could rectify and filter the output to get varying DC.

You could then use a comparator or an Opamp to determine if the output was above or below some reference level. This would give you a square wave out which should correspond to your beats.

You lost me after that. Do you want the rotation of the motor to match the frequency of the beats, or do you want to turn the motor on if the beats are present?

What are you actually doing and what frequencies are you talking about?

 

[Averagesupernova]

With a dynamic range of 40 db I don't see how you can do this without a log amp or switched gain amplifier. And depending on the response time required a switched gain amp may be out of the question.