Civil Engineering Reference
In-Depth Information
2
Acoustic impedance at normal
incidence of fluids. Substitution
of a fluid layer for a porous layer
2.1
Introduction
The concept of acoustic impedance is very useful in the field of sound absorption. In this
chapter, the impedance at normal incidence of one or several layers of fluid is calculated.
The laws of Delany and Bazley (1970) are presented and used to replace a layer of porous
material by a layer of equivalent fluid. The surface impedance at normal incidence for a
layer of porous material backed by a rigid wall with and without an air gap is calculated.
The main properties of both the reflection coefficient and the absorption coefficient are
also discussed in this chapter.
2.2
Plane waves in unbounded fluids
2.2.1 Travelling waves
As indicated in the previous chapter, a simple displacement potential solution of the
linear wave equation (1.56) in a compressible lossless fluid is
A
ρω
2
ϕ(x,t)
=
exp[
j(ωt
−
kx)
]
(2.1)
In this equation,
ω
is the angular frequency and
k
the wave number, given by
ω(ρ/K)
1
/
2
k
=
(2.2)
