State what is meant by the acoustic impedance 
of a medium.
Two media have acoustic impedances 
and 
. The intensity reflection coefficient 
for the boundary between the two media is given by
Describe the effect on the transmission of ultrasound through a boundary where there is a large difference between the acoustic impedances of the two media.
Data for the acoustic impedance and the absorption coefficient 
for fat and for muscle are shown in the table below.
The thickness 
of the layer of fat on an animal, as illustrated in the figure below, is to be investigated using ultrasound.
The intensity of the parallel ultrasound beam entering the surface S of the layer of fat is 
. The beam is reflected from the boundary between fat and muscle. The intensity of the reflected ultrasound detected at the surface S of the fat is 
. Calculate
the intensity reflection coefficient at the boundary between the fat and the muscle,
the thickness of the layer of fat.
.
