Small-amplitude sound is a linear disturbance of the medium, which means
that we can write, for instance,
and keep only terms linear in
which is the deviation from the
average mass density
.
We will also assume that
itself is small, which still allows us
to treat sound in drifting fluids by going to a frame moving with the fluid
(this works as long as the sound's wavelength is smaller than the size of
the drift). The effect of gravity is included to leading order by letting
the undisturbed density
vary with altitude.
Neglecting viscosity, the Navier-Stokes equation reduces
to the linearized Euler equation
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(10) |
![]() |
(11) |
![]() |
(12) |
From equations (5.10), (5.11), and (5.12), it follows
that r, p, and v obey the wave
equation. Let us derive, for instance, the equation for r.
From (5.10) and (5.12) we get
![]() |
(13) |
![]() |
(14) |
This is the wave equation. As is well known, it has plane wave solutions of
the form with
What should one use for the bulk modulus B: the adiabatic value
BS or the isothermal value BT?
Intuition suggests that one should use BS
for low frequencies (long wavelenghts) and BT
for high frequencies (short wavelenghts), with the crossover at
or
where D is the thermal diffusivity (more about this later).
For audible sound in air,
is between 40 Hz and 20kHz
(decreasing with age), while
Hz. Thus in practice
one should use BS even for ultrasound, and one finds
The derivatives of
are the same as those of
and the same is true for p.
We use this fact to write the equations more compactly.