Convert analog input from mic to decibels

Is there any way to build an Arduino sound level meter that outputs values in decibels rather than leds? Software or hardware?

yes, this is possible, but you will need to calibrate your microphone in some way (at least if you want the decibels for comparison to other data). you should look around and see if anyone has written an RMS function for arduino. it wouldnt bee too hard to implement elsewise. then its just 20*log(value). i have a fast log(sqrt()) i can dig up if necessary.

Lots of people have build "VU Meters". I've done a VU meter "lighting effect". It's not too difficult, but you would need a real SPL meter to calibrate your meter for SPL. You can probably use averaging instead of RMS (take a look at the "smoothing Example") or you can measure the peak (say the loudest sound within the last few-seconds) if you wish.

If you want a simpler-cheaper approach, there is the [u]LM3916[/u], which is a VU meter driver chip that doesn't require any programming. EDIT - Sorry, that's a string of LEDs... You'll need a 7-segment displays or an LCD display.

Real SLP meters usually have A-Weighting or C-Weighting which involves some frequency filtering and timing/averaging to simulate what we hear as "loudness", rather than just measuring a sound wave.

Yes, you can calculate dB for several ratios voltage uses 20*log (voltage2/voltage1) power uses10*log(power2/power1) absolute power referenced to a milliwatt of audio power dBm hitting the microphone. 10*log(power/1mW)

Which do you care about? Absolute power in the air waves hitting a mic area?

thank you all, it was the log equation that I was missing, now to pick some identical mic circuits, I had settled on a sample of a phidget 1133 but they won't be available again until October. I really appreciate your help. Next challenge, a believable and affordable UV sensor.....

Just gotta love arduinoites!