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We have already studied that sound is produced by vibrating bodies and they are longitudinal waves.

We can hear sounds of frequency ranging from 20 Hz to 20 KHz. This range of frequencies is called audible range.

Sound of frequencies less than 20 Hz is called infrasonic or subsonic, and of frequencies greater than 20 KHz is called ultrasonic.

Sound waves, being mechanical, require a material medium for their propagation.

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Velocity of Sound

The medium required for the propagation of sound must have the following properties:
  1. The medium must be elastic so that its particles have the tendency to return to their positions.
  2. Medium must have inertia so that its particles have the capacity to store energy.
The velocity of sound depends mainly on the elasticity (E) and the density (ρ) of the medium of propagation.
In general, Description: 68163.png, where E is the modulus of elasticity and ρ is the density.

Newton–Laplace formula to find the velocity of sound (in gases):
Description: 68175.png (where elasticity E is represented by γP for gases),
where P is the pressure of the medium, ρ the density of the medium and Description: 68185.png the ratio of specific heat at constant pressure to the specific heat at constant volume.


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Intensity and Loudness

The intensity of a wave is the average energy transported across unit area per second, the area being normal to the direction of propagation. The intensity

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   I = 2Description: 72165.jpg2a2f 2Description: 72167.jpgv
The human ear is capable of perceiving sounds of intensity in the range of 10−12 W m−2 to 1.0 W m−2. The relation between loudness and intensity is logarithmic in nature.
Description: 68223.png
where L is the intensity level, I is the intensity of the sound in the medium and I0 the standard intensity I0 = 10−12 W m−2.

Ultrasonic Waves

Sound of frequency higher than 20,000 Hz are called ultrasonic waves.

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Uses of Ultrasonic Waves

  1. The study of ultrasonic waves in a medium gives information about its physical properties such as elasticity, configuration of atoms, etc.
  2. They are used to manufacture alloys for mixing of immiscible liquids and to manufacture emulsion for photographic films.
  3. They are used in dry cleaning of clothes to remove grease and dirt.
  4. They can be used to detect cracks and flaws in metal mouldings and castings.
  5. They are used for welding, local heating and drilling small holes in hard materials such as glass and granite.
  6. They serve as insect repellants and for sterilization purposes.
  7. They are used to cure neuralgic and rheumatic pains.
  8. They are used in bloodless surgery, to break gallstones and stones in the kidney.

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