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13. Sound

This chapter talks about Sound and its importance. Sound plays an important role in our lives as it helps us to communicate with one another.

13.1  Vibration

The back & forth motion of an object is called Vibration. A vibrating object produces sound. In some cases, the vibrations are easily visible to us, but in most cases, their amplitude is so small that we cannot see them, however, we can feel them.

When we pluck the string of an instrument, like the sitar, the sound that we hear is not only that of the string, but the whole instrument is forced to vibrate & it is the sound of the vibration of instrument that we hear.

13.2     Sound Produced by Humans

In humans, the sound is produced by the voice box or the larynx. The voice box is situated at the upper end of the windpipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When lungs force air through the slit, the vocal cords vibrate, producing sound. The muscles attached to the vocal cords can make the cords tight or loose. Whether the vocal cords are tight & thin or loose & thick decides the quality or type of voice.

The vocal cords can be felt as a bump in the throat when we try to swallow.

The vocal cords in men are about 20 mm long, in women about 15 mm & very short in children. This explain why the voices in men, women & children are different.

Sound needs a medium for Propagation

Sound needs a medium to Travel


13.3     Vacuum

When air has been removed completely from a vessel, it is called as Vacuum. Sound can not travel through a vacuum as vacuum being an empty space does not provide any medium for sound to travel.

13.4     Solids and Liquids as medium for sound to travel

Sound can travel through solids as well as liquids.

13.4.1       Solid as medium

Sound can travel through solids. Example – when we use a toy telephone.

13.4.2       Liquid as medium

Sound can travel through liquids is proved by this activity – when we ring a bell in the bucket full of water, we can hear the sound of bell on the water surface.

13.5     Human Ear

We hear Sound through Our Ears. The shape of the outer part of the ear is like a funnel. When sound enters it, it travels down a canal, at the end of which a thin membrane is stretched tightly. This thin membrane is called as Eardrum.

13.5.1       How do we hear?

The Eardrum is like a stretched rubber sheet

  • Sound vibrations make the eardrum vibrate.
  • The eardrum sends vibrations to the inner ear.
  • From the inner ear, the signal goes to the brain & he hears the sound.

13.6     Amplitude, Time Period & Frequency of a Vibration

To & Fro motion of an object is called a vibration or also an oscillatory Motion.

13.6.1       Frequency and Amplitude

The number of oscillations per second is called the Frequency of Oscillation. Frequency is expressed in Hetrz & is expressed as Hz.

A frequency of 1 Hz means 1 Oscillation per second.

Properties of Sound

We can recognize different sounds without seeing the objects producing them. The factors that make different sounds different from each other are –

  1. Amplitude
  2. Frequency

Frequency determines the shrillness or Pitch of a sound

Amplitude defines the Loudness of sound


  •  When amplitude is small, the sound produced is feeble, When amplitude is large, the sound produced is loud.
  • When frequency of vibration is lower, the sound has is higher, sound has a higher pitch & the sound is a shrill.

Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. Example: if the amplitude becomes twice, the loudness increases by a factor of 4. The loudness is expressed in a unit called decibel (dB)

What is the difference between the voice of a bird & the roar of a lion?

  • The bird makes a high pitched sound as the frequency of vibration is higher, but the amplitude is small, so the sound produced is feeble.
  • Whereas, the roar of a lion is very loud as the amplitude is large, but the frequency of vibration is lower, so lion’s roar is low-pitched.

13.6.2       Audible & Inaudible sounds

We can not hear the sound of all vibrating bodies as the sounds of frequencies less than 20 vibrations per second (20 Hz) can not be detected by the human ear.          Inaudible Sounds

The sounds of frequencies less than 20 vibrations per second (20 Hz), that can not be heard by human ear are called as Inaudible sounds. In the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 k Hz) are also not audible to human ear.

Inaudible sounds are < 20 Hz    &        > 20 kHz          Audible Sounds

For human ear, the sounds ranging in the frequencies 20 Hz to 20,000 Hz are called as Audible Sounds.

13.6.3       NOISE

The unpleasant sounds are called NOISE

13.6.4       MUSIC

The sound produced by different musical instruments, which is pleasant to the ears is called as Music.

13.7     Noise Pollution

Presence of excessive or unwanted sounds in the environment is called Noise Pollution. Major causes of noise pollution are –

  1. Sounds of vehicles
  2. Explosions including busting of crackers
  3. Sounds of Machines, loudspeakers
  4. Running TV & Radio at high volumes
  5. Sound coming from Kitchen appliances, cooler, AC etc.

13.7.1       Harmful effects of Noise Pollution

Presence of excessive noise in the surroundings can cause many health related problems like lack of sleep, high B.P., anxiety etc. Also, a person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.

13.7.2       Measures to limit Noise Pollution

To control noise, we need to control sources of noise. For this,

  1. Silencing devices must be installed in air craft engines, transport vehicles, industrial machines & home appliances.
  2. Noisy operations must be conducted away from residential areas.
  3. Use of automobile horns should be minimized.
  4. TV & music systems should be run at low volumes.
  5. Trees to be planted along the roads & around buildings to cut down on the sounds reaching the residents, thus reducing the harmful effects of noise pollution.