Introduction to Acoustics

Physical
6. Physical
Acou
Acoustics
An overview of the fundamental concepts needed
for an understanding of physical acoustics is
provided. Basic derivations of the acoustic wave
equation are presented for both fluids and solids.
Fundamental wave concepts are discussed with
an emphasis on the acoustic case. Discussions
of different experiments and apparatus provide
examples of how physical acoustics can be applied
and of its diversity. Nonlinear acoustics is also
described.
6.1 Theoretical Overview............................ 209
6.1.1 Basic Wave Concepts.................. 209
6.1.2 Properties of Waves ................... 210
6.1.3 Wave Propagation in Fluids ........ 215
6.1.4 Wave Propagation in Solids ........ 217
6.1.5 Attenuation.............................. 218
6.2 Applications of Physical Acoustics.......... 219
6.2.1 Crystalline Elastic Constants ........ 219
6.2.2 Resonant Ultrasound
Spectroscopy (RUS)..................... 220
Physical acoustics involves the use of acoustic techniques
in the study of physical phenomena as well as
the use of other experimental techniques (optical, electronic,
etc.) to study acoustic phenomena (including the
study of mechanical vibration and wave propagation in
solids, liquids, and gasses). The subject is so broad that
a single chapter cannot cover the entire subject. For example,
recently the 25th volume of a series of books
entitled Physical Acoustics was published [6.1]. Mason
[6.2] began the series in 1964. The intermediate volumes
are not repetitious, but deal with different aspects
of physical acoustics. Even though all of physical acoustics
cannot be covered in this chapter, some examples
will illustrate the role played by physical acoustics in
the development of physics.
Since much of physics involves the use and study of
waves, it is useful to begin by mentioning some different
types of waves and their properties. The most basic definition
of a wave is a disturbance that propagates through
6.2.3 Measurement Of Attenuation
(Classical Approach) ................... 221
6.2.4 Acoustic Levitation .................... 222
6.2.5 Sonoluminescence .................... 222
6.2.6 Thermoacoustic Engines
(Refrigerators and Prime Movers) 223
6.2.7 Acoustic Detection of Land Mines 224
6.2.8 Medical Ultrasonography............ 224
6.3 Apparatus ........................................... 226
6.3.1 Examples of Apparatus .............. 226
6.3.2 Piezoelectricity and Transduction 226
6.3.3 Schlieren Imaging ..................... 228
6.3.4 Goniometer System ................... 230
6.3.5 Capacitive Receiver.................... 231
6.4 Surface Acoustic Waves......................... 231
6.5 Nonlinear Acoustics ............................. 234
6.5.1 Nonlinearity of Fluids ................ 234
6.5.2 Nonlinearity of Solids ................ 235
6.5.3 Comparison of Fluids and Solids.. 236
References .................................................. 237
a medium. A simple analogy can be made with a stack
of dominoes that are lined up and knocked over. As the
first domino falls into the second, it is knocked over into
the third, which is knocked over into the next one, and so
on. In this way, the disturbance travels down the entire
chain of dominoes (which we may think of as particles in
a medium) even though no particular domino has moved
very far. Thus, we may consider the motion of an individual
domino, or the motion of the disturbance which is
traveling down the entire chain of dominoes. This suggests
that we define two concepts, the average particle
velocity of the individual dominoes and the wave velocity
(of the disturbance) down the chain of dominoes.
Acoustic waves behave in a similar manner. In physical
acoustics it is necessary to distinguish between particle
velocity and wave (or phase) velocity.
There are two basic types of waves: longitudinal
waves, and transverse waves. These waves are defined
according to the direction of the particle motion in the
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Part B 6