Figure 4-7.-Transducer installed on a moving ship.
If the transducer is aimed perpendicular to the
direction of motion, the particles in the water will
echo the same note sent out because the transducer is
neither going toward the particles nor away from
them. (See figure 4-7.)
Now consider the echo from the submarine,
shown in figure 4-8. Again, the transducer is shown
stationary. When the submarine is neither going
toward nor away from the transducer, it must be either
stopped or crossing the sound beam at a right angle.
If it is in either condition, it reflects the same sound as
the particles in the water. Consequently, the
submarine echo has exactly the same pitch as the
reverberations from the particles.
Figure 4-9.-Comparison of echo frequency and reverberation
frequency when submarine moves toward transducer.
Suppose that the submarine is going toward the
transducer, as shown in figure 4-9. It is as though the
submarine is the train heading toward the car that is
blowing its horn at the crossing. The horn sounds
higher as the train approaches the car. In the same
manner, the sound beam sounds higher to the
submarine as it approaches the transducer.
Figure 4-8.-Transducer supported by helicopter. Doppler
effect is absent when submarine is stationary or moves at
right angles to sound beam.
Figure 4-10.-Comparison of echo frequency and
reverberation frequency when submarine moves away