Package Include: 10pcs 20mm Piezo Transducer 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. Specification: Resonant Frequency (KHz) 6.5±0.5 Resonant impedance (Ω) ≤400 Capacitance at 1KHz (pF) 15000±30% Metal plate diameter (mm) (D) (mm) φ20.0±0.2 Ceramic plate diameter (mm) (d) (mm) φ15.0±0.4 Total thickness (mm) (T) (mm) 0.42±0.05 Metal plate thickness (mm) (t) (mm) 0.2±0.02 Metal material Brass (H65) RoHS Yes MSL(Moisture Sensitivity Level) 5a(≤30℃/60%RH 24H Floor life)