I.C. VU Meter
This project is a very sensitive integrated circuit (IC) volume unit (VU) meter. Unlike most VU meters used to measure sound levels, this one has high sensitivity obtained by use of the IC.
Always start measuring the sound with the Control set at minimum. Increase the Control for a desired level of Meter pointer deflection.
This circuit is so sensitive that, if the control is increased too far, the circuit may oscillate and cause the Meter to be pinned without any sound received by the Speaker.
The Speaker is used as the microphone. Recall that when used like this, the Speaker is very similar to a dynamic microphone. That is, the Speaker coil moves back and forth in a strong permanent magnet field, causing a voltage to be induced in the coil.
The Meter circuit is a full-wave rectifier circuit that causes both half cycles of the AC to be passed on to the Meter with proper polarity to cause an upward pointer deflection.
The 10uF Capacitor helps to smooth out the rectified voltage so that the Meter will respond more smoothly to the changing signal levels.
You will want to measure and compare noises around the house. Also, when used outside, you will be surprised how much the Meter action causes you to notice sounds that are normally tuned out by your subconscious mind.