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.
Capacitance at 1KHz(pF)25000±30%
Metal plate diameter (mm)φ35.0±0.2
Ceramic plate diameter (mm)φ25.0±0.5
Total thickness (mm)0.56±0.06
Metal plate thickness (mm)0.29±0.04