A moving coil driver is the most common driver type for hi-fi and PA applications
and consists of a magnet and a voice coil.
- The magnetic field builds up in the air gap (green). Pole plate and pole
piece (grey) are extending the magnet (black).
- The voice coil moves inside the air gap (red).
When current flows through the voice coil (vertical red dots), the magnetic
field (green, directed horizontally) generates a force that moves in a right
angle to the current and the magnetic field, i.e. up or down.
This force causes the excursion (blue arrows) of the diaphragm (black).
The mostly thin piezoelectric crystal is equipped with electrodes (electrically
conductive areas). The crystal's small excursion is then transferred to the
diaphragm (the blue curved line in the picture). Piezoelectric crystals have
a relatively high impedance and, therefore, effect an amplifier like a capacitor
causing many amplifiers to oscillate in case no other loudspeakers are connected.
At low frequencies the impedance is high, drawing no power while at high frequencies
(1 to 10 kHz) the crystal has a low impedance, producing sound; as a result,
the piezoelectric transducer doesn't need a crossover.