This small (14mm diameter) piezo element is nicely enclosed so you can attach easily. Piezo elements convert vibration to voltage or voltage to vibration. That means you can use this as a buzzer for making beeps, tones and alerts AND you can use it as a sensor, to detect fast movements like knocks. You can also use it under a drum pad to make a drum/crash sensor

Technical Details:

Resonant Frequency: 8kHz
Max Rated Current: 3.0mA
Rated Voltage: 9V
Operating Temperature: -20ºC ~ 70ºC
Maximum Input Voltage: 40V

Dimensions:

External Diameter: 13mm / 0.51″
Centre Circumference: 2mm / 0.07″
Thickness: 3mm / 0.1″
Weight: 0.1g