|Radio and Electronics (DED Philippinen, 66 p.)|
Actually this type is working just the opposite way of the electrodynamic microphone. A coil fixed to a diaphragm is suspended within the field of a permanent magnet. If a changing current is passed through the coil, there will arise a force which will tend to move the coil to and for. As the diaphragm is connected to the coil it will be moved to and for according to the frequency of the current, and by doing so it will produce sound of this frequency. Most of the earphones and loudspeakers used nowadays work on this principle.
ELECTROSTATIC PRINCIPLE electric charges exert forces on each other.
If one plate of a capacitor is fixed on the housing of the loudspeaker and the other plate is fixed to the diaphragm, the diaphragm can be moved, by the forces of the electric charges brought on the plates by an ac-current. This kind of loudspeaker is found very rarely.
PIEZO ELECTRIC PRINCIPLE
Special crystals are not only able to produce voltage if they are under pressure, but they are able to produce forces if there is a voltage connected to their edges.
This principle is not very common up to now in connection with radios, but it is more and more used to produce special sounds for example in computers.