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Sound waves are mechanical longitudinal waves and require a medium for their propagation.

Sound cannot propagate through vacuum.
When it propagates, speed and wavelength changes but the frequency remain constant.
It is of three types
Infrasonic waves – (0 to 20 Hz)
Audible waves – (20 to 20,000 Hz)
Ultrasonic waves – (>20,000 Hz)

Properties of Sound Wave


  • The bouncing back of sound when it strikes a hard surface, is called reflection of sound.
  • The laws of reflection of light are also obeyed during reflection of sound.
  • The working of megaphone, soundboards, and ear trumpets are based on the reflection of sound.
  • The repetition of sound due to reflection of sound waves, is called an echo.

  • The minimum distance from a sound-reflecting surface to hear an echo is nearly 17 m.

  • The persistence of hearing on the human ear is 1/10th of a second.

  • Soundproof rooms are made of two layers of walls having a vacuum between them. 


  • It arises due to multiple reflections of sound.
  • While designing an auditorium for speech or musical concerts, one has to take proper care for the absorption and reflection of sound.
  • Time taken by reverberant sound to decrease its intensity by a factor of 106 is called reverberation time.


When a sound wave moves from one mechanical medium to another mechanical medium, it shows deviation from the original path of the incident wave. The phenomenon is called refraction. It is due to the difference in speed of sound in media.


When sound waves originated by a vibrating source, they spread in the medium and if the medium is homogeneous, this leads to a bending of sound waves around the edges. Which is known as diffraction.
The sound waves diffracted broadly, therefore, one can easily hear the voice of another person.

Doppler’s Effect

The apparent change in the frequency of source due to relative motion between the source and the observer is called Doppler’s effect.

Applications of Doppler’s Effect

1. The measurement of Doppler shift (based on Doppler’s effect) has been        used by police to check over speeding of vehicles.
2. By Astrophysicists to measure the velocities of planets and stars.
3. At airports to guide the aircraft.
4. To study the heart and blood flow in different parts of the body.


  • SONAR stands for Sound Navigation and Ranging. It is used to measure the depth of a sea, to locate the enemy submarines and shipwrecks.
  • The transmitter of a sonar produces pulses of ultrasonic sound waves of the frequency of about 50000 Hz. The reflected sound waves are received by the receiver.


We are able to hear with the help of an extremely sensitive organ of our body called the ear.

There are three parts of the human ear
  • The outer ear is called pinna. It collects the sound from the surroundings.
  • The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear.
  • In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve and the brain interprets them as sound.




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