Sound Level Meter

8 008 TEST & MEASUREMENT Although the NE604 is, strictly speaking, intended primarily for h.f. applications, it may be used for a number of other purpos- es. One of these is a sound level meter for audio applications as presented here. Use is made of the IC"s signal-strength indicator that is based on an internal logarithmic converter. This enables us to obtain a linear decibel scale, so that the moving-coil meter shown in the diagram may be replaced by a digital instrument. The signal source is assumed to be an electret microphone that converts ambient noise into an electriocal signal. Since 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 0-50 AA, which causes a potential difference across R2+R3 of 0-5 V. The range over which the relation between input and oujtput signal is logarithmic is, however, slightly smaller: about 0-4 V. This is equi...