Wave Guide, Acoustic

Wave Guide, Acoustic 

a channel along which acoustic energy (sound) is transmitted. Acoustic wave guides are channels with sharply defined boundaries in the form of walls whose properties differ markedly from the properties of the inner and outer mediums (water pipes, ventilation ducts, and so on) or channels created by a pronounced difference between the properties of the inner and outer mediums them-selves (bars, strings, and so on); in all such cases, as a rule, the energy flow to the outer medium is insignificant and can be neglected.

Acoustic wave guides also develop in continuous inhomogeneous mediums when there is a smooth transition, rather than pronounced boundaries, between the properties of the mediums inside and outside the channel. Such wave guides are observed in the atmosphere and in the ocean in the form of layers with different inner and outer temperatures. In these cases there is a noticeable energy flow through the“walls,” but it is nonetheless small, so that the major portion of the energy is propagated along the acoustic wave guide.

An example of an acoustic wave guide with sharply defined boundaries is a pipe having perfectly rigid walls, through which acoustic energy cannot pass. If the cross-sectional dimensions of the pipe are small compared to the wavelength of the sound being propagated along it (for example, in ships’ speaking tubes), the propagation of the sound in the pipe can be represented by a one-dimensional plane wave. When the pipe’s cross-sectional dimensions are comparable to or considerably larger than the wavelength, the phenomenon is more complicated. In the case of nonrigid walls (an air or water line in the form of a rubber tube), despite the energy drain through the boundaries, the general character of the wave propagation is similar to the preceding example. The acoustic wave guide phenomena in an elastic, solid medium are complicated by the existence of two types of waves— compression and shear.

Acoustic wave guides play an important role in the atmo-sphere and the ocean; the sound propagation is similar in many ways to the propagation of electromagnetic waves in atmospheric wave guides. The effect of the sea bottom and surface makes it possible in many cases to regard the ocean as an acoustic wave guide. In the ocean and atmosphere the temperature and density variations of the water (in the ocean and seas with depth) and air (in the atmosphere with altitude) lead to the formation of natural wave guides. Sound vibrations can propagate in such channels for distances on the order of hundreds and thousands of kilometers. In particular, the very long-range sound propagation in the ocean is attributed to the existence of a deep-water channel.

REFERENCES

Brekhovskikh, L. M. Volny v sloistykh sredakh. Moscow, 1957. Chapters 5,6.
Brekhovskikh, L. M.“Rasprostranenie zvukovykh i infrazvukovykh voln v prirodnykh volnovodakh na bol’shie rasstoianiia.” Uspekhilfiucheskikh nauk, 1960, vol. 70, no. 2, pp. 351-60.

L. M. LIAMSHEV