This frequency meter project squeezes a lot of performance out of the ATtiny2313 microcontroller. That chip does all of the work, measuring the frequency on the input pin as well as multiplexing ...

Such measurements are designated dBA (see NOISE LEVEL). The sound level meter, however, does not take into account the predominant frequency region of the noise, or other factors which may cause ...

[Wolf] sent in this site a while back, and I thought that this frequency meter how-to looked pretty useful. It’s based on an Atmel AT89C52 and a gutted multi-meter. The circuit isn’t too bad.