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State what is meant by the specific acoustic impedance of a medium.
The specific acoustic impedances 
of some media are given in the table below.
The density of a sample of bone is 
. Determine the wavelength, in 
, of ultrasound of frequency 
in the bone.
Ultrasound of intensity 
is incident normally on the boundary between two media of specific acoustic impedances 
and 
, as shown in the figure below.
The intensity of the ultrasound reflected from the boundary is 
.
The ratio 
is given by the expression
By making reference to the data for air, gel, and soft tissue, explain quantitatively why, during medical diagnosis using ultrasound, a gel is usually put on the skin.
.
, which shows that the difference between the acoustic impedances of gel and tissue is small; therefore, most of the ultrasound will be transmitted, almost no reflection.
