Acoustic critical depth and asymptotic absorption of dissipative fluids

Abstract

The thickness of a dissipative fluid (or equivalent fluid in the case of a porous material) for which its acoustic absorption coefficient reaches an asymptotic value is reviewed. This thickness is called critical depth or penetration depth. It is approximately equal to two or three times the relaxation depth at which the amplitude of a sound pressure wave has been attenuated by a factor 1/e in the dissipative fluid. A simple expression allowing to estimate this critical depth on the only knowledge of the static airflow resistivity is proposed. Its validity is also discussed.