Section 2: Physics of Ultrasound

When the origins of the two interacting waves are not the same, it is a little
harder to picture the wave interaction, but the principles are the same. Up
until now, we have primarily looked at waves in the form of a 2D plot of wave
amplitude versus wave position. However, anyone that has dropped
something in a pool of water can picture the waves radiating out from the
source with a circular wave front. If two objects are dropped a short distance
apart into the pool of water, their waves will radiate out from their sources and
interact with each other. At every point where the waves interact, the
amplitude of the particle displacement is the combined sum of the amplitudes
of the particle displacement of the individual waves.
With an ultrasonic transducer, the waves propagate out from the transducer
face with a circular wave front. If it were possible to get the waves to
propagate out from a single point on the transducer face, the sound field
would appear as shown in the upper image to the right. Consider the light
areas to be areas of rarefaction and the dark areas to be areas of
compression.