Piezo elements come in handy when you need to detect vibration or a knock. You can use these for tap or knock sensors pretty easily by reading the voltage on the output. They can also be used for a very small audio transducer such as a buzzer.
|Datasheets||Piezoelectric Sound Components|
|Capacitance @ Frequency||10000 pF @ 1 kHz|
|Size||0.79" Dia (20 mm)|