Sound Pressure Level Meter

Suitable for home-cinema setup
9 Volt operated portable unit

This project was requested by some correspondents wishing to setup their 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.

Use:

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.

Parts:

• 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

Notes:

• If external electret (battery powered) or dynamic microphone types are used, R1 must be omitted.
• Using a good quality microphone, this circuit can be very useful in setting-up parametric equalizers or tone controls in HiFi chains etc.