This is an 8-channel spectrum analyzer using ATtiny84A and an electret microphone. The 20x gain stage of the differential ADC is used so a separate amplifier is not necessary.
The code uses a DFT algorithm by Łukasz Podkalicki. Original code was based on his blog post, but after I contacted him he kindly shared a newer version of the code which I don't believe he had published previously. I adapted the newest code to the ATtiny84A with 8 channels and tweaked the thresholds to work with an electret microphone.
The code for an earlier experiment with the ATtiny13A is also included. The 13A doesn't include a gain stage on it's ADC so a LM386 audio amplifier was used to amplify the audio signal before passing it into the 13's ADC.
I have also designed a PCB for the ATtiny84A version but I haven't had it manufactured yet.