This kind of loudspeaker is created by applying the inverse principal to the one employed in electrodynamic microphones. A coil, upon which a membrane has been set up, is placed within a circular magnet generating the acoustic wave from the electric signal applied to the coil. This situation is illustrated in the following diagram:
Naturally the latter is a very simplified illustration, but adequate for analyzing the loudspeaker's characteristics. When an electric signal is applied at the far ends of a conductor, a current consisting of a flow of electrons passes through it. However, seeing that the electrons are blocked in their positions by the magnetic field generated by the magnet, in order for the electrons to flow through the conductor it is in fact the latter that is forced to move. So, the entire coil moves up and down following the polarity applied at its far ends, in other words, depending on the applied electric signal. If, for instance, we were to apply a sinusoidal signal with a certain amplitude, we'd see that the positive half wave would push the coil (and the membrane thereon) upwards, whereas during the negative half wave the coil (and the membrane) would be pushed downwards. These movements of the membrane generate compressions and dilations of the air and this generates sound.
The membrane is fixed over the coil by means of an elastic suspension system, as shown in the following figure:
The suspension system must be created with utmost care because it is responsible for the perfect centering of the coil inside the air gap[10 ] and of the correct attenuation of the oscillations. For this reason the suspension is made up of an undulated heavy material which is capable of slowing down the oscillations generated by the electric signal.
[10 ] The principal magnet has a central hole in which a metal cylinder is placed. The circular aperture which remains between the magnet and the metal is called air gap. The coil is placed within this air gap.