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.
Frequency 6.3 kHz
Impedance 1 kOhm
Capacitance @ Frequency 10000 pF @ 1 kHz
Size 0.79″ Dia (20 mm)