By reference to ultrasound waves, state what is meant by acoustic impedance.
An ultrasound wave is incident on the boundary between two media. The acoustic impedances of the two media are 
and 
, as illustrated in the figure below.
Explain the importance of the difference between and for the transmission of ultrasound across the boundary.
Ultrasound frequencies as high as 
are used in medical diagnosis. State and explain one advantage of the use of high-frequency ultrasound compared with lower-frequency ultrasound.
) is small, most of the incident intensity is transmitted. When the difference between 
).
