Birkhäuser Verlag, Basel–Boston–Berlin, x + 202 pp., $49.95, ISBN 978-3-76437705-2 “If the area of a membrane be given, there must evidently be some form of boundary for which the pitch (of the principal tone) is the gravest possible, and this form can be no other than the circle,” wrote Lord Rayleigh in his book “The Theory of Sound” ([R], vol. 1, §210.) A membrane is a planar domain Ω, and it...