This is an instructive project aimed at learning something of measurement. Figure 15.1 shows a circuit with a diode, resistor and capacitor. Applying 0 dB tone to the input terminals, and connecting a voltmeter to the output, will give a reading of between 0.75 Vdc and 0.8 Vdc. An oscilloscope will read the same. The programme may confuse a digital voltmeter (DVM), but the oscilloscope should follow the variations in level; experiment with different timebase settings. Note how during periods of silence the voltage falls to zero.
This simple circuit can be turned into a useful piece of kit by building into a box with a moving coil meter connected to the output terminals.