This is a decibel meter circuit. This circuit is used to measure sound pressure level (SPL) from about 60 to 70 dB. In this circuit, transistor stage and one LM324 op-amp section are used to amplify the sound that is picked up by an 8 ohm speaker. As voltage comparators, the circuit uses The remaining 3 sections of the LM324 quad op-amp. 3 indicator LEDs are driven by the LM324. Here is the circuit:
decibel meter1 circuit schematic
This circuit uses 3 LEDs as indicators. Each LED represents about a 3dB change in sound level, so that when all 3 LEDs are on, the sound level is about 4 times greater than the level needed to light one LED. the 500K pot can be used to adjust the sensitivity of the circuit. One LED is used as a reference sound level. The other two LEDs is used to indicate about a 2X and 4X increase in volume.