I made a more final version, and it goes blazing fast, I don't have the time to see the pure FFT transfrom happen on a PDA, definately way less than a second ! The time to first open my recorded, one second wave-file from disk, checking if it's the right format, putting the last 4096 (mono) bytes of it into an array, etc.. cost me another 3 seconds. But that's OK. 4 seconds total calculation time is great ! On the other hand (just some wishful thinking now), this makes me wonder: if I could capture the wave-bytes directly from memory while they are being received/recorded, then it would be possible to make a real-time tachometer of a continuous sound. Let's say with one indication of the RPM per second or so. But i guess the recorder.dll doesn't allow me to capture parts of this stream of bytes directly. Agraham, while we're discussing this, would you mind if I would mention you in the credit list of my app ? You sure deserved it ! In fact, you can see the start of it here, and even download a copy: http://users.telenet.be/heliport/ Don't expect too much, I'm just a hobbyist.