A class of energy transducers capable of receiving alternating current and generating acoustic waves resembling very closely the characteristics of that current. The movement of an element (diaphragm) is accomplished by magnetic attraction, electrostatic attraction, or the piezoelectric effect (the expansion or contraction of certain crystalline substances in response to electric charges). Earphone systems include the driver element with its diaphragm and arrangements for magnetic flux, or electrostatic or direct electric charge, plus a casing, one or more acoustic cavities and ports, acoustic damping and insulation, and some arrangement for coupling the driver to the human ear. The wiring connecting the precedent amplifier to the driver, which may be incorporated into the earphone system, may in modern systems be a complicated circuit which feeds, for each of two stereophonic channels, some part of the current to each ear. The time delays and energy ratios at the two earphones (each of which may contain two drivers) can be appropriately adjusted so that the listener is given the illusion that the sound sources are not “within the head” but are externalized appropriately in all three planes of space. The various types of earphones are described below. See also Transducer.
The magnetic type is a permanently magnetized diaphragm that is moved in and out by an electromagnet energized by alternating current. Dynamic earphones are actually small dynamic loudspeakers. In some, a small coil fed by the sound source is bonded to the membranous diaphragm. Alternating current in the coil thus drives the diaphragm toward and away from a permanent magnet in the rear of the casing. In another configuration, the coil is relatively large and is attached to the diaphragm only at its edge. A miniature magnetic earphone is a small unit of which the output port fits snugly into a plastic olive in the ear canal. These are widely used with small radios, with the more powerful hearing aids, by television commentators, in business transcription devices, and in many other communications situations. See also Hearing aid.
Efficiency is increased if the mass of the diaphragm is reduced to a minimum. The electrostatic earphone utilizes a thin (2.5–12 micrometers) metallized plastic film, on which a large constant electrostatic charge is placed by an auxiliary unit. The motion of the diaphragm is controlled by the audio signal impressed on perforated wire mesh plates on either side. The assembly is mounted in a relatively large cavity and coupled to the head by a circumaural cushion. Such earphones are light and comfortable and have excellent frequency-response and transient-distortion characteristics.
The diaphragm of a dynamic-electrostatic (or orthodynamic) earphone is a permanently polarized electret of fluorocarbon. Consequently the need for an added source of polarization voltage (a drawback inherent to electrostatic earphones) is eliminated. See also Electret transducer.
Certain crystalline substances expand and contract when alternating voltage is applied. In some piezoelectric earphones, a crystal element is coupled mechanically to the center of a small (about 2.5 cm diameter) cone. Such earphones can be lightweight and cheap and may be acceptable for speech communication. See also Piezoelectricity.
Any single-channel recording of a real acoustic event will, when played back to an earphone in one ear, sound “in the head.” Even stereo recordings from two channels played back to earphones on two ears, while furnishing the illusion of movement, still are not externally localized by the listener. Externalization is improved if some of the signal from the left channel is time-delayed and applied also to the right earphone (the same, of course, for the right channel to the left earphone), and it is also improved if a realistic ratio is achieved between the acoustic energy density ratio from the “direct” versus the “indirect” sound sources (as from reflective walls). Stereophonic earphone systems have been built which incorporate these time delays and ratios, and which furthermore feed the signal from the right channel of an artificial-head stereophonic recording to the left earphone (and the left channel to the right earphone) through frequency filters which simulate the differential acoustic effects at various frequencies of the head and external ears in the original recording situation. Thus the eardrums under earphones are presented with the exact acoustic conditions generated by a loudspeaker or other sound source in an actual room, and such earphone systems very materially advance the important psychoacoustic feature of externalization of sound and of acoustic realism generally. See also Loudspeaker; Microphone; Sound-reproducing systems; Stereophonic sound.
