A 27mm piezoelectric Transducer sensor or element is a device that uses the piezoelectric effect, it is basically a device which converts voltage to vibration and vibration to voltage. It normally comes in handy to measure knock (knock sensor) or vibration (vibration sensor). 

Specifications Of 27mm Piezoelectric Transducer Sensor :

1. Outside Plate Diameter: 27mm (1.06 inches)
2. Element Diameter: 20mm (.79 inches)
3. Plate Thickness: ~0.5mm (.02 inches)
4. Lead Length: ~50mm (~2 inches)
5. Plate Material: Brass
6. Resonant Frequency: 4.6KHz +/- 0.5kHz
7. Resonant Impedance: 300 Ohm Max.
8. Capacitance: 20.0nF +/- 30% @ 1kHz

Package Contents:
1x (1 piece) 27mm Piezoelectric Disk Buzzer Sensor