A digitization of “knocks” taken using ATmega8 built-in <a href="https://en.wikipedia.org/wiki/Analog-to-digital_converter" class="extiw" title="w:Analog-to-digital converter">ADC</a> at 450 Hz sampling rate.  The signal was picked up with a piezoelectric buzzer (shunted with a 680 kΩ resistor) used as a microphone, and passed through an LM358-based unity gain <a href="https://en.wikipedia.org/wiki/Operational_amplifier" class="extiw" title="w:Operational amplifier">amplifier</a> for <a href="https://en.wikipedia.org/wiki/Impedance_matching" class="extiw" title="w:Impedance matching">impedance matching</a> reasons.
<a href="//commons.wikimedia.org/wiki/User:Ivan_Shmakov" title="User:Ivan Shmakov">Ivan Shmakov</a> (<a href="//commons.wikimedia.org/wiki/User_talk:Ivan_Shmakov" title="User talk:Ivan Shmakov">talk</a>)
<span class="int-own-work" lang="en">Own work</span>