Sounds are picked up by MIC1 and fed to the input of the first op amp. The signal is then fed to the input of second op amp Ulb, where it is boosted again by a factor of between 1 and 33, depending upon the setting of range switch Sl. With the range switch set in the A position, R6 is 1 K!:J and R7 is 33 K!:J, so that stage has a gain of 33.
Sound-level-meter - schematic

In the B position, the gain is 10 !:J; in the Cposition, the gain is 22 !:J; and in the D position the gain is 1 !:J. As the signal voltage fed to the input of U2 at pin 5 varies, one of ten LEDs will light to correspond with the input-voltage level. At the input"s lowest operating level, U2 produces an output at pin 1, causing LEDl to light. The highest input level presented to the input of U2, about 1.2 V, causes LED10 to turn on.

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