Echo:

Echo: The sound waves received after being reflected from a high tower or mountains is called echo.

To hear echo, the minimum distance between the observer and reflector should be 17m (16.6m).

Persistence of ear (effect of sound on ear) is 1/10 sec.

Due to refraction, sound is heard at longer distances in nights than in day.

Resonance: if the frequency of imposed periodic force is equal to the natural frequency of a body, the body oscillates with very large amplitude. This phenomenon is called resonance.

Interference of sound: The modification or redistribution of energy at a point due to superposition of two (or, more) sound waves of same frequency is called interference of sound.

If two waves meet at a point in same phase, intercity of sound is maximum at that point. Such type of interference is called constructive interference. Similarly, if the two waves meet at a point in opposite phase, intensity of sound at that point is minimum. Such type of interference is called destructive interference.

Diffraction of sound: Wavelength of sound is of the order of 1 m. If an obstacle of that range appears in the path of sound, sound deviates at the edge of obstacle and propagates forward. This phenomenon is called diffraction of sound.

Doppler’s Effect:

If there is a relative motion between source of sound and observer, the apparent frequency of sound heard by the observer is different from the actual frequency of sound emitted by the source. This phenomenon is called Doppler’s effect.

When the distance between the source and observer decreases, the apparent frequency increases and vice-versa.

Mach number: It is defined as the ratio of speed of sound source to the speed of sound in the same medium under the same condition of temperature and pressure.

If mach number > 1, body is called supersonic.

If Mach number > 5, body is called hypersonic.

If Mach number < 1, the body (source) is said to be moving with subsonic speed.

Shock waves: A body moving with supersonic speed in air leaves behind it a conical region of disturbance which spreads continuously. Such a disturbance is called shock wave. This wave carries huge energy and may even make cracks in window panes or even damage a building.

Bow Waves: When a motor boat in a sea travels faster than sound, then waves just like shock- waves are produced on the surface of water. These waves are called bow waves.