The NE604`s signal-strength indicator section is used, based on an internal logarithmic converter. This enables a lineardecibel scale so that the moving-coil meter (shown in the diagram) can be replaced by a digital instrument. The signal source is assumed to be an electret microphone that converts ambient noise into an electrical signal. Because this type of microphone normally contains a buffer stage, R7, R8, and C13 have been included to provide the supply voltage for this stage. The NE604 delivers an output current (at pin 5) of 0 to 50, which causes a potential difference across R2 + R3 of 0 to 5 V.
Sound-Level Meter Circuit Diagram
The input and output signal range is equivalent to a sound range of 70 dB. lb compensate for the effects of temperature changes, the required resistance of 100 KOhmhm is formed by two resistors (R2 and R3) and a diode (Dl). Any ripple remaining on the output voltage is removed by R4/C9/C10 before the output is buffered by IC2. The indicating instrument, here a moving-coil meter, is connected to the output (pin 6) of IC2 via a series resistance, R^ + .
The preset is adjusted to give full-scale deflection (FSD) for an output voltage of 4 V. Calibrating the meter is a little tricky, unless you have access to an already calibrated instrument. Otherwise, if you know the efficiency of your loudspeaker, that is, how many decibels for 1W at 1 m, you can use that as reference. The scale of the meter can then be marked with the (approximate) value. In any case, the meter deflection must at all times be seen as an indication, not as an absolute value: it was not thought to be worthwhile to add a filter to the schema to enable absolute measurements to be made.
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