CBSE Class 9 Science Chapter 12 Revision Notes

Chapter 12: Sound Revision Notes

Sound Generation

  • Sound is generated by the vibrating of things.
  • The quick to and fro motion of an item is known as vibration.
  • All noises originate from vibrating things. Noise is created by irregular, chaotic vibrations. Music may be created through regular, regulated vibration. All sound is made up of pure frequency combinations. When a stretched rubber band is pulled, it vibrates and makes a sound.


  • The particles in the medium vibrate when an item vibrates. When a particle comes into touch with a vibrating object, it is initially shifted out of its equilibrium position.
  • The vibrational disturbance moves across the medium, but the particles themselves do not move ahead.
  • A wave is a disturbance that travels across a medium due to the vibration of the medium’s particles. As a result, sound is seen as a wave.
  • The transmission of sound waves necessitates the use of a medium.
  • Mechanical waves are the same as sound waves.
  • When a vibrating item goes ahead, it compresses and pushes the air in front of it, creating a high-pressure zone known as compression (C).
  • When a vibrating item goes backward, it creates a low-pressure zone known as rarefaction (R).
  • The particles in these waves travel back and forth parallel to the disturbance’s propagation path. Longitudinal waves are the name for these types of waves.
  • Another type of wave is known as transverse waves. The particles oscillate up and down perpendicular to the propagation of the disturbance direction in these waves.
  • In a medium, sound propagates as a succession of compressions (C) and uncommon factions (R).
  • The higher component of the curve, known as the crest, represents compressions, which are places of high pressure and density where the particles are packed.
  • Rarefactions are low-pressure, low-density areas where particles are dispersed, and they are represented by the trough section of the curve.


Sound wave frequency

  • The frequency of a sound wave is defined as the number of oscillations per unit time. The letter v is used to symbolise it (Greek letter nu). The SI unit for it is** hertz (Hz)**
  • The time it takes for a sound wave to complete one oscillation is known as its time period.


  • The height of the crest or tough is the amplitude of a sound wave.
  • The letter A is used to signify it.
  • The SI unit for density and pressure is the same.
  • The wavelength is the distance between two waves’ “crests” that are close together.
  • The amplitude refers to the height of the crests.

**Pitch and Volume **

  • The frequency of vibration determines the pitch of sound (shrillness or flatness).
  • The sound has a high pitch if the frequency is high, and a low pitch if the frequency is low.

Speed of Sound

  • Solids have a faster sound speed than liquids, while gases have the slowest sound speed.
  • The temperature of the medium also affects the speed of sound.
  • The speed of sound increases as the temperature of the medium rises.


  • The principles of reflection govern how sound is reflected at the surface of a solid or liquid.
  • The angle of incidence and reflection are the same.
  • At the point of incidence, the incident ray, reflected ray, and normal are all in the same plane.

Reflection of soundSource:


  • We hear the same sound whether we yell or clap near a reflecting surface, such as a tall structure or a mountain. The sound we hear is known as **echo. **
  • It occurs as a result of sound reflection. The time delay between the original sound and the echo must be at least 0.1 second to hear an echo clearly.
  • Because sound travels at 344 m/s in air, the distance travelled in 0.I s = 344 m/s x 0.1 s = 34.4 m
  • To hear an echo clearly, the reflecting surface should be at least half this distance, or 17.2 metres.


  • Due to repeated or numerous reflections of sound from many reflecting surfaces, echoes may be heard more than once. This generates reverberation, or the persistence of sound.
  • To lessen reverberation in large halls or auditoriums, sound absorbent materials such as compressed fibre boards, rough plaster, or drapes are used on the roofs and walls.

Uses of Sound Reflection

(i) Megaphones, horns, and musical instruments such as trumpets are designed to transmit sound in a specific direction via many reflections rather than spreading out in all directions.

ii) Doctors use a stethoscope to listen to noises coming from the human body. Multiple reflections carry the sound of the heartbeat to the doctor’s ears.

iii) In most cinemas and auditoriums, the ceilings are curved so that sound reaches all portions of the room after repeated reflections. A curved sound board is sometimes installed behind the stage so that sound travels evenly around the auditorium after repeated reflections.

Hearing Capacity

  • The audible range of sound is defined as the range of frequencies that humans can hear between 20 Hz and 2000 Hz.
  • **Infrasonic sound **is defined as sound with a frequency less than 20 Hz.
  • Ultrasonic sound is defined as sound with a frequency greater than 2000 Hz.

Ultrasonic sound applications

  • Ultrasonic sound is used to clean electronic components, identify flaws in metal blocks, and shatter tiny stones generated in the kidneys into fine grains.
  • It is also utilised in ultra sound scanners to obtain pictures of interior organs of the human body.


  • It is a gadget that measures the distance, direction, and speed of submerged objects using ultrasonic waves.
  • Knowing the speed of sound in water and the time between transmission and reception of ultrasound may be used to compute the object’s distance.


Structure of the human earSource:

  • The sound waves go via the ear canal to the eardrum, which is a thin membrane. Vibrations are felt in the eardrum.
  • The vibrations are magnified by the hammer, anvil, and stirrup bones of the middle ear.
  • The sound waves are subsequently transmitted to the inner ear by the middle ear.
  • The impulses are subsequently translated into sound by the brain.



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