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 waves which lie in the frequency range 20 Hz to 20000 Hz are called audible or sound waves. These waves are sensitive to human ear. 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, volcanic 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 ear can not detect these waves. But certain creatures like dog, cat, bat, mosquito can detect these waves. Bats not only detect but also produce ultrasonic.

Ultrasonic waves can be produced by Galton’s whistle or Hartman’s generator or by the high frequency vibrations of a quartz crystal under an alternating electric field (Piezo – electric effect) or by the vibrations of a ferromagnetic rod under an alternating magnetic field (Magnetostriction)

1. For sending signals.

2. For measuring the depth of sea.

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

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

5. In sterilizing of a liquid.

6. In Ultrasonography.

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

⇒ Speed of sound is S maximum in solids and H minimum in gases.

⇒ When sound enters from one medium to M another medium, its W speed and wavelength S changes but frequency remains unchanged.

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

⇒ The speed of sound in as is given as g v = γp/ρ where, γ = C_p/C_v – is the adiabatic exponent of gas, p and ρ being the pressure and density of gas.

For example: In case of hydrogn and oxygen, density of oxygen is 16 times that of hydrogen. As a result the velocity of sound in hydrogen is 4 times that in oxygen as other parameters are same.

⇒ Wavelength of sound in air is affected by air temperature.

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 is increased by 1°C.

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

Characteristics of Sound waves: Sound waves have the following three characteristics.

1. Intensity Intensity of sound at any point in 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,

1. inversely proportional to the square of the distance fiets of point from the source.

2. directly proportional to square of amplitude of vibration, square of frequency and density of the medium.

This gives I₁/I₂ = A₁²/A₂²‘ where, I₁ and I₂ are intensities of two waves of amplitudes A₁ and A₂ respectively.

Due to intensity, a sound appears loud or faint to the ear. Actually, the sensation of a 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 1/10th of bel. Another unit of loudness is phon.

2. Pitch Pitch is that characteristic of sound which distinguishes a sharp (or shrill) 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 17 m (16.6 m)

⇒ 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 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 waves meet at a point in opposite phase, intensity of sound at that point is minimum. Such type of interference is called destructive interference.

Stationary wave: When two progressive waves of same type (ie both longitudinal or both transverse) having the same amplitude and same time period/frequency/wavelength travelling with same speed along the same straight line in opposite directions, superimpose, a new set of waves are formed which are called stationary or standing waves.

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 source of sound 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.

⇒ Reflection of sound-when sound wave is reflected from a rigid boundary, compression returns as compression with no change in phase.

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