When there is relative motion between the source
of a wave of energy and its receiver, the received
frequency differs from the transmitted frequency.
When the source of wave motion is moving towards
the receiver, more waves per second are received than
when the source remains stationary. The effect at the
receiver is an apparent decrease in wavelength and,
therefore, an increase in frequency. On the other
hand, when the source of wave motion is moving
away from the receiver, fewer waves per second are
encountered, which gives the effect of a longer
wavelength and an apparent decrease in frequency.
This change in wavelength is called the Doppler
The amount of change in wavelength
depends on the relative velocity between the receiver
and the source. Relative velocity is the resultant
speed between two objects when one or both are
You have heard the term Doppler effect many
times, but may not have known what the phenomenon
was. An example of this is what you hear at a railroad
crossing. As a train approaches, the pitch of the
whistle is high. As the train passes you, the pitch
seems to drop.
Then, as the train goes off in the
distance, the pitch of the whistle is low. The Doppler
effect causes the changes in the pitch.
Sound waves generated by the whistle were
compressed ahead of the train. As they came toward
you, they were heard as a high-pitched sound because
of the shorter distance between waves. When the
train went by, the sound waves were drawn out,
resulting in the lower pitch. Refer to figure 4-6 as you
read the following explanation of Doppler effect.
If you examine 1 second of the audio signal
radiated by the train whistle, you will see that the
signal is composed of many cycles of acoustical
energy. Each cycle occupies a definite period of time
and has a definite physical wavelength. (Because of
space limitations, only every 10th wave is illustrated
in view A of figure 4-6.) When the energy is
transmitted from a stationary source, the leading edge
will move out in space the distance of one wavelength
by the time the trailing edge leaves the source. The
cycle will then occupy its exact wavelength in space.
If that cycle is emitted while the source is moving, the
source will move a small distance while the complete
cycle is being radiated. The trailing edge of the cycle
radiated will be closer to the leading edge.
Figure 4-6, view B, shows the effect of relative
motion on a radiated audio signal. Notice the
wavelength of the sound from the stationary emitter,
as illustrated in condition (1) of view B.
In condition (2) of view B, the emitter is moving
towards the listener (closing). When the cycle is
compressed, it occupies less distance in space. Thus,
the wavelength of the audio signal has been
a n d t h e f r e q u e n c y h a s b e e n
proportionately increased (shifted). This apparent
increase in frequency is known as UP Doppler.
The opposite is true in condition (3) of view B.
The emitter is moving away from the listener
(opening). The wavelength occupies more distance in
space, and the frequency has been proportionately
decreased. This apparent decrease in frequency is
known as DOWN Doppler. The factors that
determine the amount of Doppler shift are the velocity
of the sound emitter, the velocity of the receiver, and
the angle between the direction of motion of the
receiver and the direction of motion of the sound
emitter. This angle, known as angle 8, is used in a
formula to determine the velocity of the emitted
signal at the receiver and the frequency of the Doppler
The Doppler shift works both ways. If you were
on the train and had listened to a car horn at the
crossing, the pitch of the horn would have changed.
The effect is the same because the relative motion is
The sonar equipment deals with three basic
sounds. One of these sounds is the sound actually
sent out by the equipment. The second sound is the
reverberations that return from all the particles in the
waterseaweed, fish, etc. The third sound is the
most important one, the echo from the submarine.
The sound sent into the water (the actual ping) is
seldom heard by the operator. Most of the equipment
is designed to blank out this signal so that it doesnt
distract the operator. This means there are only two
sounds to deal within the discussion of Doppler effect
Reverberations are echoes from all the small
particles in the water.
Consider just one of these
particles for a moment.
A sound wave from the
transducer hits the particle and bounces back, just as a
ball would if thrown against a wall. If the particle is
stationary, it will not change the pitch of the sound.
The sound will return from the particle with the same
pitch that it had when it went out.