## ABSTRACT

In order to understand some of the concepts presented in this chapter it is necessary to define the following terms:

Acoustic impedance is the ratio of the pressure to the volume velocity (product of particle velocity and duct cross-sectional area) [47, Section 1.12].

Specific acoustic impedance is defined as the ratio of the pressure to the particle velocity [47, Section 1.12] and is given by the product of the effective complex density in the media, ρeff, and the complex speed of sound ceff [45].

Characteristic acoustic impedance is the specific acoustic impedance in the free field and is given by z0 = ρ0c0.