The Tweeter, Basics

Dr.-Ing. Peter Strassacker

The dome tweeter: a moving coil driver

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. Prinzipbild Hochton-Lautsprecher

- 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).

Piezo tweeter

Piezo drivers are very popular in low cost applications, especially in automotive and PA systems. The piezoelectric effect: when a piezoelectric crystal is deformed, electric current develops and vice versa: electric current causes deformation of the piezoelectric crystal. Since a relatively high current over a short distance is needed, the piezoelectric crystal has to be quite thin and often has two layers. Piezoelectric transducers are also used for ultrasonic immersion and special drives (micro drives).

Prinzipbild Hochton-Lautsprecher 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.

Ribbon tweeter

ribbon tweeter The ribbon tweeter - like a moving coil driver - also uses a magnetic field (green arrows) and a conductor (grey), though not in form of a coil but a ribbon. The ribbon is both, a conductor and a diaphragm. Since the ribbon has a very low resistance, connection to an amplifier or crossover is done via a transformer (on the right side of the picture) attached to the ribbon.