Gap

In dynamic transducers such as most loudspeakers and dynamic microphones, the gap is a narrow circular trough in a magnet assembly in which the voice coil resides. The voice coil is attached to the cone of the speaker or mic diaphragm. In the case of a loudspeaker the voice coil becomes energized with electricity from an amplifier, which creates a magnetic field of varying polarity, which causes it to move in and out of the gap, thereby moving the speaker. In the case of a dynamic microphone the action is the opposite: acoustic energy moves the diaphragm, which causes the voice coil to move in and out of the magnetic gap, which generates an electrical signal that can be amplified.