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A week ago I recommended the introduction into the
WKB (Wentzel, Kramers, Brillouin) [or Liouville-Green]
approximation in section 9.1 of the book "Physics of
Waves" by W. C. Elmore and M. A. Heald (Dover,
New York, 1969).
While I still think that their treatment is recommendable
for non-mathematicians (like me), I now find that some
details are not perfect.
For those who own the book:
After Equation (9.1.12) they wrote:
"Substitution of (9.1.10) and (9.1.12) into (9.1.6) gives ..."
(9.1.10), however, is no longer valid if (9.1.12) is adopted.
Equation (9.1.13) must be replaced by an equation
involving derivatives of the function epsilon(x).
A function similar to the right-hand side of (9.1.13),
however, can be obtained in a simpler way, by
requiring that the neglected second derivative A''
in (9.1.6) be small compared to the second term in
(9.1.6), omega^2 * A / c^2. That new function (which
is required to be << 1) is twice the right-hand side
A second point:
After Equation (9.1.13), Elmore and Heald wrote:
"... the term containing the second derivative c'' will
generally be negligible compared with that
In the case of the often-used cochlear model
c(x) = c(0) * e^[-x/(2d)],
where d is about 5 mm, the above statement is wrong.
For a "stiffness-dominated" basilar-membrane
impedance (featuring a real wave number k and so
a real phase velocity c = omega / k ), I find the
following WKB criterion:
? 2 c'' * c - c'^2 ? << 4 omega^2 ,
or equivalent inequalities involving the local wave
number k or the local wavelength lambda.
Dr. phil. nat.,
r. PSI and ETH Zurich,
Phone: 0041 56 441 77 72.
Mobile: 0041 79 754 30 32.
E-mail: reinifrosch@xxxxxxxxxx .