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CBSE Class 9 Physics Notes for Chapter 5 Sound

Sound is produced by vibrating objects. When an object vibrates, the particles around the object start vibrating. This disturbance travels in the medium and sound reaches the ear. This is called a sound wave. A wave is a disturbance that moves through a medium when the particles are set in motion.

Sound needs a medium to travel as it is a mechanical wave. It travels as successive compressions and rarefactions in the medium.

Sound waves are a longitudinal wave, which means that the particles move parallel to the direction of propagation.


Time period (T): The time taken by the wave to complete one oscillation is called time period.

Frequency ():  The number of oscillations per unit time is called the frequency. ν=  1/T

Wavelength (λ): The distance between two consecutive compressions or two rarefactions is called the wavelength.

Speed (v) = νλ


The incident sound and the reflected sound make equal angles with the normal to the reflecting surface and the three lie in the same plane.

An echo is the reflected sound that can be heard a little later than the original sound. To hear a distinct echo, the time interval between the original sound and the reflected sound must be 0.1 second and the minimum distance of the obstacle from the sound source must be 17.2 metres.

Reverberation: The repeated reflection which results in persistence of sound is called reverberation.

Intensity: the amount of sound energy passing each second through a unit area is called the intensity of sound.

The audible range of hearing for humans is 20 Hz- 20 kHz.

Infrasonic: The sound waves with frequencies below the audible range are called infrasonic.

Ultrasonic: The sound waves with frequencies above the audible range are called ultrasonic.


  • To clean parts located in places that are hard to reach.
  • To detect cracks and flaws in metal
  • For medical purposes such as echocardiography or obtaining images of internal organs.
  • SONAR: It stands for Sound Navigation and Ranging. It is a device used to measure distance, direction and speed of underwater objects using ultrasonic waves. If the time interval between transmission and reception of ultrasound signal is t, the speed of sound through water is v then the total distance travelled 2d= v x This method is called echo ranging.

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