velophonic

/* Knock Sensor sound

   This sketch reads a piezo element to detect a knocking sound.
   It reads an analog pin and compares the result to a set threshold.
   If the result is greater than the threshold, it writes
   "knock" to the serial port, and toggles the LED on pin 13.

   This example plays every .WAV file it finds on the SD card when piezo is "knocked"

 */