Page 42 - Electronic Navigation Cyber Book

DOPPLER LOG

Examples of the Doppler phenomenon with sound:

The Doppler principle is the effect, which makes the tone of a fire engine
change as it passes the observer.

The fire engine is continuously emitting sound waves but if it is moving
towards the observer the wave fronts arrive closer together, which is
equivalent to a higher frequency.

As the fire engine starts to move away from the observer, the wave fronts
arrive less frequently at the observer and the tone is of a lower frequency.
As the train approaches a stationary listener, the pitch (frequency) of the
rumbling sound of the train is higher than when the train passes by, at
which time the pitch sounds the same as if the train were stationary.

As the train recedes from the listener, the pitch decreases.

Electromagnetic waves radiated by radar, as well as sound waves, obey
the Doppler principal, although electromagnetic waves travel at the
speed of light and audio waves travel at the speed of sound.
The Doppler effect is a frequency shift that results from relative motion
between a frequency source and a listener.

If both source and listener are not moving with respect to each other
(although both may be moving at the same speed in the same
direction), no Doppler shift will take place.

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