Introduction to Acoustics

terms. In this equation γ is the ratio of specific heats and
c 2 o
= γ po
ρo
1
�
1 + ∂ξ
� . (6.71)
γ −1
∂a
One can also generalize this equation in a form which
applies �to all � fluids. By expanding the equation of state
γ
ρ
p = po in powers of the condensation s = ρo
ρ−ρo , ρo
one obtains
p = po + As + B
2! s2 + ... and
c 2 = c 2 � � � �
B
o 1 + s + ... . (6.72)
A
This makes it possible to obtain the nonlinear wave
equation in the form [6.24]:
∂2ξ =
∂t2 �
1 + ∂ξ
∂a
c2 o
� B
2+ A
∂2ξ . (6.73)
∂a2 In this form one can recognize that the quantity 2 + B/A
for fluids plays the same role as γ + 1 for ideal gases.
Values of B/A for fluids given in Table 6.3 indicate that
nonlinearity of fluids, even in the absence of bubbles of
air, cannot always be ignored. The nonlinearity of fluids
is discussed in greater detail in Chap. 8.
6.5.2 Nonlinearity of Solids
The propagation of a wave in a nonlinear solid is described
by first introducing third-order elastic constants.
When extending the stress–strain relationship (which essentially
is a force-based approach) it becomes difficult
to keep a consistent approximation among the various
nonlinear terms. However, If one instead uses an energy
approach, a consistent approximation is automatically
maintained for all the terms of higher order.
Beginning with the elastic potential energy, one can
define both the second-order constants (those determining
the wave velocity in the linear approximation)
and the third-order elastic constants simultaneously. The
Table 6.4 K2 and K3 for the principal directions in a cubic crystal
Direction K2 K3
[100] C11 C111
[110]
[111]
C11 +C12 +2C44 2
C11 +2C12 +4C44 3
Table 6.3 Values of B/A
C 111 +3C 112 +12C 166
4
1
9 (C111 + 6C112 + 12C144 + 24C166 + 2C123 + 16C456)
Physical Acoustics 6.5 Nonlinear Acoustics 235
Substance T ( ◦ C) B/A
Distilled water 0 4.2
20 5.0
40 5.4
60 5.7
80 6.1
100 6.1
Sea water (3.5%) 20 5.25
Methanol 20 9.6
Ethanol 0 10.4
20 10.5
40 10.6
N-propanol 20 10.7
N-butanol 20 10.7
Acetone 20 9.2
Beneze 20 9/0
Chlorobenzene 30 9.3
Liquid nitrogen b.p. 6.6
Benzyl alcohol 30 10.2
Diethylamine 30 10.3
Ethylene glycol 30 9.7
Ethyl formate 30 9.8
Heptane 30 10.0
Hexane 30 9.9
Methyl acetate 30 9.7
Mercury 30 7.8
Sodium 110 2.7
Potassium 100 2.9
Tin 240 4.4
Monatomic gas 20 0.67
Diatomic gas 20 0.40
elastic potential energy is
φ (η) = 1 �
2!
ijkl
+ 1
3!
Cijklηijηkl
�
Cijklmnηijηklηmn + ... , (6.74)
ijklmn
Part B 6.5