A speaker can be used in reverse to create a microphone. In the case the incoming sound wave leads to a mechanical deflection of the cone and voice-coil. According to Faraday’s law, a time-varying current will be induced because the coil is moving in through a magnetic field (produced by the permanent magnet). Although any speaker could be used for a microphone, most speakers are unnecessarily large for this purpose, except in simple intercom applications where it is common to use the same component to perform both the speaker and microphone functions.
Another type of microphone is the “condenser” microphone, which exploits electrostatic forces instead of magnetic induction. The “cone” in this case is a thin metallic membrane that forms one side of a parallel-plate capacitor (condenser is an old-fashioned term for capacitor). An incoming sound wave causes the membrane to vibrate and hence the capacitance changes. If the capacitor is charged through an external pull-up resistor as shown in Figure 1-2a, the time-varying capacitance will induce a time-varying current through the resistor and hence an AC voltage.