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Sonar (sound navigation and ranging) is a device used in detecting the presence of underwater objects. Ultrasonic waves generated by a piezoelectric crystal generator are transmitted towards the target. These powerful bursts of ultrasonics are sent out into water at regular intervals from a ship and the time required for the ultrasonics to return to the ship after reflection is determined. By knowing the velocity of ultrasonics in water, the distance of the objects under water from the ship can be determined. The depth of the ocean at any place can also be determined in a similar way (Figure 7.5).

Description: FIG-7.14.tif

Figure 7.6 Doppler Effect

Reflection of Sound

When sound waves strike a surface, they return into the same medium. This phenomenon is called reflection. The reflection of sound waves is similar to that of light rays. The only difference is that sound waves being longer in wavelength, require bigger surfaces for reflection.
An echo is produced by the reflection of sound from an obstacle. If a person standing at a distance of about 25 m from a mountain or a big building claps his hands once, he hears the sound of clapping twice; the first, soon after the sound is produced and the second, a little later. The second sound heard is called an echo.
The sensation of sound persists in the human ear for about one-tenth of a second. This is known as persistence of hearing. In order to distinguish between the original sound and its echo, the time interval between them should be more than 0.1 s. If we take the speed of sound in air as 340 ms−1, the distance travelled by sound in one-tenth of a second would be 34 m. This means that we will be able to distinguish between the original and reflected sound (the echo), if we are at a distance of 17 m or more from the reflecting surface.
Due to multiple reflections of sound waves, multiple echoes are formed. For example, if a cracker is burst in a hilly region, the sound of explosion is heard as multiple echoes due to multiple reflections of sound between the hills.

Doppler Effect

We are familiar with the change in the pitch of the whistle of a fast moving train as it passes a station. The pitch of the whistle increases as the train approaches a listener on the platform and decreases when the train gets away from the listener (Figure 7.6). A similar effect is observed when an aeroplane flies past a stationary listener or a fast moving ambulance passes a stationary listener. The apparent change in the frequency due to relative motion between a source and an observer is known as The Doppler effect. This apparent change in frequency was first predicted by the Austrian physicist Christian Johan Doppler in 1842.

Description: FIG-7.15.tif
Figure 7.6 Doppler Effect


Applications of Doppler Effect
  1. Used in estimating the speed of distant stars, planets and other celestial bodies.
  2. Used in finding the velocities of aeroplanes and submarines.
  3. In police radar systems, it is used to measure the speed of automobiles.

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