ABSTRACT

Sound is a mechanical interaction between local pressure perturbation p and particle velocity v:

p ¼ Zv 8:1

The proportionality constant Z in the above expression is called the acoustic impedance and is directly related to the elastic characteristics of the medium. Because of the mechanical interaction, particle motion will induce a pressure variation and vice versa, creating a wave propagating in the direction of the motion and pressure variation, i.e. a longitudinal wave. For a homogeneous medium the acoustic impedance can be expressed as the product of mass density r (kg/m3) of the medium and sound propagation speed c (m/s):

Z ¼ rc 8:2

Consequently, the unit of Z is kg/(m2s), but in most articles it is referred to as MRayl. Table 8.1 lists for some (biological) materials their sound-related characteristics. The above expressions are rather independent of the sound frequency f (Hz = 1/s). Defining the wavelength l (m) as the spatial distance along the propagation direction between similar points in subsequential cycles, then it follows that:

l ¼ c=f 8:3

The wavelength is inversely proportional with the frequency; a higher sound frequency automatically implies a shorter wavelength if the sound propagation speed is the same.