22.6

*Applications of Electromagnetic Induction to the Reproduction of Sound

Electromagnetic induction plays an important role in the technology used for the reproduction of sound. Virtually all electric guitars, for example, use electromagnetic pickups in which an induced emf is generated in a coil of wire by a vibrating string. Most guitars have at least two pickups located below each string and some, as Figure 22-17 illustrates, have three. Each pickup is sensitive to different harmonics produced by the vibrating string. The string is made from a magnetizable metal, and the pickup consists of a coil of wire within which a permanent magnet is located. The permanent magnet produces a magnetic field that penetrates the guitar string, causing it to become magnetized with north and south poles. When the magnetized string is plucked, it oscillates, thereby changing the magnetic flux that passes through the coil. The changing flux induces an emf in the coil, and the polarity of this emf alternates with the vibratory motion of the string. A string vibrating at 440 Hz, for example, induces a 440-Hz ac emf in the coil. This signal, after being amplified, is sent to the speakers, which produce a 440-Hz sound wave (concert A).

Figure 22-17 When the string of an electric guitar vibrates, an emf is induced in the coil of the pickup. The two ends of the coil are connected to the input of an amplifier.

The physics of the electric guitar pickup.

The playback head of a tape deck uses a moving tape to generate an emf in a coil of wire. Figure 22-18 shows a section of magnetized tape in which a series of “tape magnets” have been created in the magnetic layer of the tape during the recording process (see Section 21.9). The tape moves beneath the playback head, which consists of a coil of wire wrapped around an iron core. The iron core has the approximate shape of a horseshoe with a small gap between the two ends. The drawing shows an instant when a tape magnet is under the gap. Some of the field lines of the tape magnet are routed through the highly magnetizable iron core, and hence through the coil, as they proceed from the north pole to the south pole. Consequently, the flux through the coil changes as the tape moves past the gap. The change in flux leads to an ac emf, which is amplified and sent to the speakers, where the original sound is reproduced.

Figure 22-18 The playback head of a tape deck. As each tape magnet goes by the gap, some magnetic field lines pass through the core and coil. The changing flux in the coil creates an induced emf. The gap width has been exaggerated.

The physics of the playback head of a tape deck.

There are a number of types of microphones, and Figure 22-19 illustrates the one known as a moving coil microphone. When a sound wave strikes the diaphragm of the microphone, the diaphragm vibrates back and forth, and the attached coil moves along with it. Nearby is a stationary magnet. As the coil alternately approaches and recedes from the magnet, the flux through the coil changes. Consequently, an ac emf is induced in the coil. This electrical signal is sent to an amplifier and then to the speakers. In the type of microphone called a moving magnet microphone, the magnet is attached to the diaphragm and moves relative to a stationary coil.

Figure 22-19 A moving coil microphone.

The physics of a moving coil and a moving magnet microphone.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.