This is an “Audio VU Meter” or “Sound Level Meter”, it is a general-purpose bar-graph Audio VU meter designed for fun projects. All you need is to hook up one wire to the output of the audio amplifier’s speaker pin along with GND and see the magic. The response of the circuit is very fast and it provides beautiful visual representation from audio input signal.
A simplified schematic is provided to give the general idea of the operation. The signal is applied to a series of 20 comparators, each of them is biased to a different comparison level by the resistor string. In the circuit diagram, the resistor string is connected to the 100K potentiometer which provides reference voltage 1.9V to 12V.
Audio VU/Sound Level Meter with LM339 – [Link]