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State what is meant by the specific acoustic impedance of a medium.
A parallel beam of ultrasound of intensity 
is incident on the boundary between two media A and B, as illustrated in the figure below.
The two media A and B have specific acoustic impedances 
and 
respectively. The intensity of the beam transmitted through the boundary is 
.
State how the ratio
depends on the relative magnitudes of and .
The linear absorption (attenuation) coefficient 
of medium B is 
. Calculate the thickness of medium B required to reduce the intensity of the ultrasound beam to 
of its initial intensity in medium B.
.
