Filtering signal from piezo microphone

Hi!

following situation:
I have a bosch-style Piezo Knocksensor hooked up to my wankel/rotary car engine.
I use it for tuning my engine, while listening to the noise trough my headphones. (amplified)
So far, so good.

Next step is a knock detection powered by an arduino connected to that signal.
The frequency when knocking/detonation appears is from 3,2 kHz to 3.8 kHz.

My question is, how to filter this frequency range. And after filtering how to analyze the sampled signal.

best regards
Bastian

A digital bandpass filter?

There are tools to help write these, for instance: http://www-users.cs.york.ac.uk/~fisher/mkfilter/trad.html