Update your General Knowledge (Sound Wave)

Sound Wave

► Sound waves are longitudinal mechanical waves.

►According to their frequency range, longitudinal mechanical waves are divided into the following categories:

1.      Audible or Sound waves: The longitudinal mechanical wave which lie in the frequency range 20 Hz to 20000 Hz are called audible or sound waves. These waves are sensitive to human ears. These are generated by the vibrating bodies such as tuning fork, vocal cords etc.

2.      Infrasonic Waves: The longitudinal mechanical waves having frequencies less than 20 Hz are called Infrasonic. These waves are produced by sources of bigger size such as earth quakes, Volcano eruptions, ocean waves and by elephants and whales.

3.      Ultrasonic Waves: The longitudinal mechanical waves having frequencies greater than 20000 Hz are called Ultrasonic Waves. Human ears can not detect these waves. But certain creatures like dog, cat, bat, and mosquito can detect these waves. Bat not only detect by also produce ultrasonic waves.           

Applications of Ultrasonic Waves:

1.      For sending signals.

2.      For measuring depth of sea.

3.      For cleaning cloths, aeroplanes and machinery parts of clock.

4.      For removing lamp-shoot from the chimney of factories.

5.      In sterilizing of a liquid.

6.      In ultrasonography.

Speed of Sound:

►Speed of sound is different in different mediums. In a medium, speed of sound basically depends upon elasticity and density of medium.

►Speed of sound is maximum in solids and minimum in gases.

►When sound enters from one medium to another medium, its speed and wavelength changes but frequency remains unchanged.

►In a medium, the speed of sound is independent of frequency.

Speed of sound in different mediums

Medium	Speed of sound (In m/s)
Carbon dioxide	260
Air (0oC)	332
Air (20oC)	343
Steam (100oC)	405
Helium	965
Alcohol	1213
Hydrogen	1269
Mercury	1450
Water (20oC)	1482
Sea Water	1533
Copper	3560
Iron	5130
Glass	5640
Granite	6000
Aluminum	6420

Effect of pressure on speed of sound: The speed of sound is independent of pressure i.e. speed remains unchanged by the increase or decrease of pressure.

Effect of temperature on speed of sound: The speed of sound increases with the increase of temperature of the medium. The speed of sound in air increases by 0.61 m/s when the temperature increased by 1^oC.

Effect of humidity on speed of sound: The speed of sound is more in humid air then in dry air because the density of humid air is less than the density of dry air.

Characteristics of sound waves: Sound waves have following three characteristics. 
Intensity: Intensity of sound at any point of space is defined as amount of energy passing normally per unit area held around that point per unit time. SI unit of intensity is watt/m² .

Intensity of sound at a point is,

(i)                 inversely proportional to the squire of the distance of point from the source.

(ii)               Directly proportional to squire of amplitude of vibration, squire of frequency and density of the medium.

      Due to intensity, a sound appears loud or faint to the ear. Actually, the sensation of the sound perceived in ear is measured by another term called loudness which depends on intensity of sound and sensitiveness of the ear. Unit of loudness is bel. A practical unit of loudness is decibel (dB) which of equal to 1/10^th of bel. Another unit of loudness is phon.

2. Pitch: Pitch is that characteristic of sound which distinguishes a sharp sound from a grave (dull or flat) sound. Pitch depends upon frequency. Higher the frequency, higher will be the pitch and shriller will be the sound. Lower the frequency, lower will be the pitch and grave will be the sound.
3. Quality: Quality is that characteristic of sound which enables us to distinguish between sounds produced by two sources having the same intensity and pitch. The quality depends upon number, frequency and relative intensities of overtones.

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 (16.6 m).

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

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

Resonance: If the frequency of the imposed periodic force is equal to the natural frequency of a body, the body oscillates with a 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, intensity of sound is maximum at that point. Such type of interference is called constructive interference. Similarly, if the two point 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 objective 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 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 Effect.

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

Match 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 match number >1, body is supersonic.

► If match number >5, body is called hypersonic.

► If match 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 waves. These waves carries a huge energy and may even cracks in window panes or even damage a building.

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

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