Intensity and Pressure Amplitude

It is worth noting here, that for the ultrasound, as for any sound wave, the wavelength of sound in the medium is given by the relationship in Eq. 2.5. 

In our previous discussions (tuning fork in air) we pointed out that sound was a form of energy. The particles of the medium were set into vibratory motion and thereby possessed kinetic energy. This energy was derived from the wave itself Using this principle we can deduce the energy (and hence intensity) associated with our applied ultrasonic field. 

Consider the movement of a layer of the medium of area A and thickness dx, (i. e. volume A dx) under the action of the ultrasonic wave (Fig. 2.8). 

The energy for the whole wave E, may be obtained by summing all such elements (i. e. integrating Eq. 2.6) to give Eq. 2.7. 

If the sound energy passes through unit cross-sectional area (A = 1) with a velocity of c, then the volume swept out in unit time is c (since A = 1), and the energy flowing in unit time is given by E c. Since intensity (I) has been defined as the amount of energy flowing per unit area (A = 1) per unit time (Eq. 2.9) 

---