Sorry to bring up a 'dead' thread but i just resurrected my n900 system board with alcohol and oven-baking.

Looking at making an mp3 player that doesn't need pulseaudio for speaker protection. Goal is filter out low frequences before decoding.

I performed some experiments to determine lowest usable musical frequencies

sox -r 22050 -n output.wav synth 30 sine 150-300

This gave a 30 second sinewave sweep from 150hz to 300hz. Played straight to speakers (no speaker protection), the output becomes audible around 200hz.

I leave this here for those may want to test and confirm this on their own. Drop me a note if you think the cutoff for a mp3 player should be something besides 200hz.