DOPPLER EFFECT 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 effect.” 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 moving. 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 decreased, 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 shift. 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 same. 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 water—seaweed, 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 doesn’t distract the operator. This means there are only two sounds to deal within the discussion of Doppler effect in sonar. 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. 4-6