Sound Pressure Level Meter

This electronic circuit project is to setup home-cinema set adjusting all the loudspeaker outputs to the same level when heard from the listening position.In practice this device is a simple (though linear and precise) ac millivoltmeter, using an existing multimeter set to 50 or 100µA fsd with the probes connected to J1 and J2 to read the results.

The precision of the measure is entirely depending on the frequency response of the microphone used but, fortunately, for the main purpose of this circuit an absolutely flat response is not required. Therefore, a cheap miniature electret microphone can be used.
Parts List:
R1 - 10K 1/4W Resistor
R2,R3 - 22K 1/4W Resistors
R4 - 100K 1/4W Resistor
R5 - 100R 1/4W Resistor
C1 - 1µF 63V Polyester or Electrolytic Capacitor
C2 - 100µF 25V Electrolytic Capacitor
C3 - 220µF 25V Electrolytic Capacitor
D1-D4 - BAT46 100V 150mA Schottky-barrier Diodes
IC1 - CA3140 Op-Amp IC
MIC - Miniature electret microphone (See Notes)
J1,J2 - 4mm Output sockets
SW1 - SPST Toggle or Slider Switch
B1 - 9V PP3 Battery
Clip for PP3 Battery
The amplifiers driving the loudspeakers must be fed, one at a time, with a sine wave in the 400Hz - 1KHz range, but different values can also be chosen. For this purpose you can use a simple signal generator circuit like one of those available on this site, namely: 1KHz Sine wave Generator or, better still, Spot-frequency Sine wave Generator.
As an alternative, the input sine wave can be provided by a CD test track, a cassette-tape or a personal computer. Please be careful and set the volume control very low, to avoid loudspeakers' damage. Switch-on the Sound Pressure Level Meter and increase the volume of the amplifier in order to obtain an approximate center-scale reading. Repeat the same steps with all channels.
Source