A digitization of “knocks” taken using ATmega8 built-in ADC at 450 Hz sampling rate.  The signal was picked up directly from a piezoelectric buzzer (shunted with a 680 kΩ resistor) used as a microphone.  Note the relatively large signal falling times, resulting from the high internal resistance of the buzzer not allowing for the ADC-internal “store and hold” capacitor to discarge in timely manner.
Own work
Ivan Shmakov (talk)