- microphone (radio shack 270-092 or similar).
This simple circuit is an effective sound-level meter. R1 controls the gain of the 741 OP-AMP, hence the sensitivity of the circuit. The meter can be a panel meter of a multimeter set to read current. The circuit was tested with a piezo buzzer that emitted a 6.5KHz tone at a sound pressure of 90dB. When the buzzer was 2" from the microphone and R1 was set for maximum gain, the meter indicated 1mA.