(a)[2]
State what the specific acoustic impedance of a medium means.
(b(i))[3]
A bone sample has density $1.7 \times 10^{3}\,\text{kg m}^{-3}$. Find the wavelength, in $\text{mm}$, of ultrasound with frequency $9.0 \times 10^{5}\,\text{Hz}$ in the bone.
(b(ii))[4]
An ultrasound beam of intensity $I$ is incident normally on the boundary between two media with specific acoustic impedances $Z_{1}$ and $Z_{2}$, as shown in Fig. 10.2. The reflected ultrasound has intensity $I_{R}$. The ratio $\dfrac{I_{R}}{I}$ is given by $\dfrac{I_{R}}{I} = \dfrac{(Z_{1} - Z_{2})^{2}}{(Z_{1} + Z_{2})^{2}}$. Using the data for air, gel and soft tissue, explain quantitatively why gel is normally placed on the skin during medical ultrasound diagnosis.