Electret Microphone Insert (Condenser Mic Insert). dimensions: 10 Dia x 7mm long. Impedance 300 ohms. Freq response 50Hz to 12KHz. Works from 1.5-10VDC. Small battery will power this unit such as SG13S. 2 Solder Contacts on the back. Change 1K resistor to lower value for lower battery voltage.