Dissipation of Acoustic Waves in Barium Monochalcogenides

Abstract

The term acoustic refers to a periodic pressure wave. The term includes waves in the audio frequency range as well as those above audio frequency range (ultrasonic and hypersonic) and below the audio frequency range. Acoustic waves are characterized by their speed and absorption. Acoustic absorption is a measure of the energy removed from the acoustic waves by conversion to heat as the wave propagates through a given thickness of material; it has unit dB/cm (or Np/cm). Absorption is a material property, in contrast to attenuation, which includes energy loss due to scattering and reflection as well as and depends upon sample size and experimental configuration. The elastic and inelastic properties of solids are suitable for the study of acoustic dissipation which account for the direct conversion of acoustic energy into thermal energy. In measurement of the attenuation of acoustic waves in solids using pulse echo method, the attenuation is usually found to be greater than the absorption due to intrinsic dissipation. Acoustic energy is removed from the propagating acoustic wave, but is not immediately converted into heat. The most important cause of the attenuation is the scattering of acoustics wave from imperfections. In terms of phonon description of acoustic waves, this is a two-phonon process, in which incoming and outgoing phonons have different wave vectors. The perturbation at the scattering centre may be due to a mass difference of an impurity atom from the normal mass or to a change in interatomic forces. In polycrystalline solids, sound is scattered from the boundaries between the microcrystal grains. The grain boundaries act as scattering centers due to the discontinuity of the elastic constants, and the amount of loss depends on the grain size and on the wavelength of the acoustic wave. When the wavelength of the acoustic wave is small compared to the grain size, the loss is independent of frequency and inversely proportional to the mean grain diameter. When the wavelength is large compared to the grain size, the loss is proportional to the fourth power of the frequency (Rayleigh scattering) and to the third power of grain diameter. Another source of nondissipative loss is diffraction of acoustic field from the transducer. This is an important loss mechanism in megahertz frequency range, but at higher frequencies it is negligible. At higher frequencies, a loss mechanism occurs due to lack of flatness and parallelism of the end faces of the specimen. At higher frequencies (10 GHz), the wavelength of the sound in a solid is of the order of optical wavelength in visible range and hence, the surface of the specimen should be polished with optical quality. Source: Acoustic Waves, Book edited by: Don W. Dissanayake, ISBN 978-953-307-111-4, pp. 466, September 2010, Sciyo, Croatia, downloaded from SCIYO.COM