(a)[4]
Explain the principles behind the generation of ultrasound waves for use in medical diagnosis.
(b)[4]
The linear attenuation (absorption) coefficient for a parallel beam of ultrasound waves in air is $1.2\,\text{cm}^{-1}$. The parallel beam travels through a $3.5\,\text{cm}$ thick layer of air. Calculate the ratio, in dB, $\frac{\text{intensity of beam after passing through the layer of air}}{\text{intensity of beam entering the layer of air}}$.