Speaker Sound Level Meter Circuit Schematic

We can measure the sound level of a speaker using this circuit. When the switch is in OFF close position and the microphone on low level sound, the resistance value on microphone high, and voltage output of the divider circuit is high. If the sound level on the microphone is high, the resistance value of microphone is low, and voltage output of the divider output is low. After passed through the IC and bridge diode, the voltage output can be measured using AVO meter. Here is the schematic :

Speaker Sound Level Meter Circuit Diagram

Estimated price : $5

Components list :
Resistor R1 : 10k ohm/0.25W
Resistor R2,R3 : 22k ohm/0.25W
Resistor R4,R5 : 100k ohm/0.25W
C1 : 1 uF/63V
C2 : 100 uF/25V
C3 : 220 uF/25V
Diode D1-D4 : BAT46 100V/150mA
IC op-amp : CA3140
Socket/terminal J1-J2 : CA3140
Switch S1 : Toggle switch on/off
Power supply : 9V
Meter : AVO meter
Microphone : Condenser microphone