Electrodynamic Loudspeaker

a loudspeaker in which the interaction between the magnetic field of a permanent magnet and the current in a moving coil that is connected to a source of electric oscillations is used to convert the electric oscillations at audio frequencies to mechanical vibrations.

The coil, which is positioned in the air gap of the magnet, a diaphragm that is rigidly attached to the coil, and the magnetic system constitute the speaker unit (see Figure 1). The interaction of the current and the magnetic field produces mechanical vibrations of the diaphragm that are accompanied by the radiation of sound waves either directly, as in a direct-radiator loudspeaker, or through a horn, as in a horn loudspeaker. To ensure operational reliability and high-quality sound, the speaker unit is housed in a wooden, plastic, or metal cabinet.

Figure 1. Diagram of a direct-radiator electrodynamic loudspeaker: (M) magnet, (MD) moving diaphragm, (VC) voice coil

The electrodynamic loudspeakers are used in radio receivers, record players, and tape recorders. Loudspeaker power ratings, which vary with the intended use of the loudspeaker, range from 0.05 watt to 100 watts. The efficiency of a direct-radiator loudspeaker is usually no greater than 1–3 percent. Electrodynamic loudspeakers may reproduce sound within a relatively narrow frequency range, such as 300 hertz (Hz) to 5,000 Hz, or within a wide range, such as 40 Hz to 15,000 Hz. The complexity of broadband speaker units, however, makes them difficult to manufacture, and electrodynamic loudspeakers often use systems that consist of several speaker units, each of which reproduces sound over a given portion of a frequency range.