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Class 8 Sound Notes for CBSE Based on NCERT Pattern

Important topics covered in class 8 sound notes:

  • What is sound?
  • Production of sound
  • Propagation of sound
  • Human ear
  • Amplitude
  • Time Period
  • Frequency of a Vibration
  • Loudness and Pitch
  • Audible and Inaudible Sounds
  • Noise and Music
  • Noise Pollution
  • Harms of Noise Pollution
  • Measures to control noise pollution

What is sound?

What is sound

  • Sound is the form of energy which causes sensation of hearing to our ears.
  • Sound helps the living organism to communicate with each other.
  • Vibrating body produces sound.
  • After a bell being hit by the gong, the bell starts vibrating thus produces sound.
  • Sound doesn’t travel through a vacuum.
  • These waves are longitudinal waves.



  • The to and fro or back and forth motion of a body is said to be vibration.
  • Different musical instruments produce different types of sound due to vibration of its parts.

Production of sound by humans

voice box or larynx

  • In humans sound is produced by larynx also called voice box.
  • It is situated in the upper part of the windpipe.
  • Our vocal cord vibrates when lung force air through the slit.
  • Muscles which are attached to the vocal cord enable us to make the vocal cords tight or loose as per our need.
  • The voices of men, women and children are different from each other because vocal cords in men are longer (about 20 mm) but they are shorter in women and children.

Propagation of sound:

propagation of sound

  • Sound needs a material medium to travel.
  • Through vacuum, sound cannot travel. We can’t hear sound in space.
  • Sound can also travel through solid and liquid.
  • Sound propagation happens in all directions in a medium.

Human Ear

Human ear

Human ear consists of three parts i.e. outer ear, inner ear, and middle ear.

External ear

  • Outer part of the ear looks like funnel.
  • It is also called Pinna.
  • It collects the sound from surrounding and transfers to middle ear.

Middle ear

  • The middle ear is composed of a stretched membrane which is called eardrum.
  • Eardrum appears as the stretched rubber sheet.
  • The vibration of eardrum helps in passing the vibration to inner ear.
  • Middle ear also consists of three small bones- hammer, anvil, and stirrup.

Inner ear

  • The internal ear is consists of cochlea and semicircular canals.
  • From outside, cochlea appears like a snail.
  • Vibrations derived from middle ear reach the cochlea.
  • Signals from cochlea reach the brain. The brain interprets message and makes us listen to it.
  • Semicircular canals have no role in sense of hearing, rather they maintain the balance of the body.

Amplitude, Time Period and Frequency of a sound

  • The to and fro motion of any object produces sound and this motion is said to be as vibration.
  • Sound propagates from one place to other in the form of waves.
  • The phenomenon or disturbance in which energy is transferred from one place to other is said to be wave.
  • So, sound is also considered as wave.



  • Maximum displacement of a wave on either side from mean position is said to be amplitude.
  • We can conclude that it shows how far the vibrating object moves from the mean position.
  • It is denoted by ‘A’.
  • The SI unit of Amplitude is metre.

Time period

time period

  • The total time taken by the particle of medium to complete one oscillation is said to be time period.
  • We denote it by ‘T’.
  • The SI unit of time is second.
  • Time period = Time / Number of oscillation or vibration



  • The number of complete oscillation done by an object is said to be frequency of the sound.
  • Frequency = \(\frac{Number\: of \:Oscillation}{Time\:taken}\)
  • The unit used for frequency is hertz (Hz).
  • Oscillation of x Hz is called as the x oscillation in a second.
  • For same amplitude, the frequency may vary.



  • The loudness of sound is softness which depends on the amplitude of vibration.
  • Loudness is directly proportional to the square of the amplitude of vibration.
  • The louder sound has a higher amplitude, while lower sound has lower amplitude.
  • We express loudness in terms of decibel.
  • Loudness of some common sounds is in the following table.

Loudness ∝ (Amplitude)2

Pitch or Shrillness

  • Pitch is the sensation (Brain interpretation) of the frequency of emitted sound.
  • Pitch depends on the frequency of vibration of sound.
  • Sound with greater frequency is shriller and has higher pitch whereas the lower frequency sound has a lower pitch.

Audible and inaudible sound

  • Sounds with frequency range 20 Hz to 20,000 HZ is said to be audible sound.
  • Also, Sound with frequency below 20 Hz or above 20,000 Hz can’t be listened by humans and hence comes in inaudible range.
  • Humans can’t hear the sound of inaudible range.
  • Some animals like dogs, flies, cats can even listen the sound having frequency above 20,000 Hz.

Noise and Music

Noise and Music

  • The sound which appears unpleasant to listen is said to be noise.
  • Sounds produced by vehicles, train etc. are some examples of noise.
  • The sound which appears pleasant to listen is called music.
  • Sound which comes out from musical instruments appears pleasant to listen and hence is music.

Noise pollution:

  • The presence of excessive, loud, unwanted or unpleasant sound to our ears in the environment is said to be noise pollution.
  • Sound produced from crackers, loudspeakers, explosions etc. cause noise pollution.

Effects of noise pollution

  • Due to noise pollution, many type of disease/health related problems take place.
  • Lack of sleep (insomnia), hypertension (high blood pressure), loss of hearing etc. are some common effects seen in humans as the cause of noise pollution.
  • A person who is exposed to loud sound continuously may get temporary or permanent hearing loss.
  • Class 8 sound notes will simplify your exam.


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