Class 8 Science Chapter 13 Notes- Sound Notes

In this post, we will discuss Class 8 Science Chapter 13 notes pdf Sound Notes. Class 8th science ch 13 notes help students in revision. In sound notes class 8, we will learn about sound, How is a sound produced? How does it travel from one place to another?, How do we hear the sound? and Why are some sounds louder than others?. You can also download the Sound notes class 8 pdf for further study. NCERT notes for the Class 8 Science Chapter 13 are designed by our subject expert team.

In this Class 8th Science Ch 13 Notes pdf, we will learn about Sound.

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Class 8 Science Chapter 13 notes Overview

Sound plays an important role in our lives. It helps us to communicate with one another. We hear a variety of sounds in our surroundings. in this chapter we will learn about sound.


Sound is those vibrations that travel either through the air or another medium and at the same time can be heard when they reach a person’s or animal’s ear.

  • Musical Sound:- The sound which leads to a pleasing effect on the ear.
  • Noise:- The sounds which cause a jarring or unpleasant effect.
  • Vibration:- fro or back and forth motion of an object is called vibration.

Types of Sound:

  1. Audible Sound:- Those vibrations whose frequency lies between 20 Hertz to 20,000 Hertz (20 kHz) can be easily heard by the human ear.
  2. Inaudible Sound:- Sounds that have frequencies either above 20,000 Hertz or below 20 Hertz cannot be heard by the normal human ear.

Sound Produced by Humans

Class 8 Science Chapter 13 Notes  Voicebox in humans
Voicebox in humans
  • In humans, the sound is produced by the voice box or the larynx. It is 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 the lungs force air through the slit, the vocal cords vibrate, producing sound.
  • Muscles attached to the vocal cords can make the cords tight or loose.
  • When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.
  • The vocal cords in males are of the length of 20 mm and in females have 15mm long vocal cords. Where as Children have very short-length vocal cords. Hence, the voice quality and their type are always different in women, men and children.

Sound Needs a Medium for Propagation

Actually, sound needs a medium to travel. When the air has been removed completely from a vessel, it is said that there is a vacuum in the vessel. The sound cannot travel through a vacuum.

  • Sound needs some vibration of molecules to travel i.e Sound needs a medium for propagation.
  • Solids, liquids and gases all have molecules present in them which allow the propagation of sound.
  • These molecules or particles in solids, liquids and gases are packed in varied ways.
  • Solids have tightly packed particles in them and hence they allow fast propagation of sound through them as the vibrations can be carried easily from one particle to another.
  • Liquids have slightly loosely packed particles and hence it takes a little time for sound to travel in water or through a liquid.
  • Gases have completely loosely packed particles and hence sound takes the most time in travelling through the air.

We Hear Sound through Our Ears

 Human ear
Human ear
  • 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 there is a thin stretched membrane. It is called the eardrum.
  • The eardrum performs an important function. The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate.
  • The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear.

We must NEVER put a sharp, pointed or hard thing into our ear. It can damage the eardrum. The damaged eardrum can impair hearing.

Amplitude, Time Period and Frequency of a Vibration

oscillatory motion:- When an object travels in a to and fro motion object is known as vibration. This motion is also called oscillatory motion.

Frequency of oscillation:- The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz. Its symbol is Hz. 1 Hz = 1 oscillation per second, 20 Hz = 20 oscillations per second

Time Period:- The time taken for one complete oscillation of a sound wave is called the time period of the sound wave.

Amplitude:- The amplitude of a sound wave can be defined as the maximum displacement of the particles from their mean position due to the vibrations.

The loudness of a sound

  • The loudness of a sound depends upon the amplitude of the sound.
  • The higher the amplitude, the higher the displacement of the particles and the higher the loudness of the sound.
  • When the amplitude is small, the sound produced is feeble.
  • The loudness of the sound is directly proportional to the square of its amplitude.
  • The SI unit for measuring the loudness of a sound in decibels (dB).

Pitch of the sound

  • The frequency determines the shrillness or pitch of a sound.
  • If the frequency of vibration is higher we say that the sound is shrill and has a higher pitch.
  • If the frequency of vibration is lower, we say that the sound has a lower pitch.
  • For example, a drum vibrates with a low frequency. Therefore, it produces a low-pitched sound. On the other hand, a whistle has a high frequency and therefore, produces a sound of higher pitch.
  • A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud while the sound of the bird is quite feeble.
  • Every person has a different sound quality.

Noise and Music

We hear different types of sounds around us. Some sounds are pleasant to the ear, whereas some are not.

  • Noise can be regarded as an unpleasant sound. Noise has no particular periodic pattern of repetition. Therefore, noise is regarded as a mixture of sound waves with irregular frequencies.
  • Music is a pleasant sound that has a clear pitch. Musical sound can be created by arranging and combining different sounds in a particular order. The frequencies of a musical sound are harmonious in nature.

Noise Pollution

Noise Pollution:- Presence of excessive or unwanted sounds in the environment is called noise pollution.

  • Major causes of noise pollution are the sounds of vehicles, explosions including the bursting of crackers, machines, loudspeakers etc.
  • Human beings can bear sounds ranging up to 85 decibels only. Above that, any noise can damage our hearing power.
  • Generally, any sound that has a frequency of more than 30 dB is considered noise.

What are the Harms of Noise Pollution?

  • Noise Pollution may cause many health-related problems.
  • Lack of sleep, hypertension (high blood- pressure), anxiety and many more health disorders may be caused by noise pollution.
  • A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.

Measures to Limit Noise Pollution

  • Silencing devices must be installed in aircraft engines, transport vehicles, industrial machines and home appliances.
  • All noisy operations must be conducted away from any residential area.
  • Trees must be planted along the roads and around buildings to cut down on the sounds reaching the residents, thus reducing the harmful effects of noise pollution.

What we learn Class 8 Science Chapter 13 Notes Sound Notes

In Class 8th Science Ch 13 Notes pdf / Sound Notes

  • Sound is produced by vibrating objects.
  • In human beings, the vibration of the vocal cords produces sound.
  • Sound travels through a medium (gas, liquid or solid). It cannot travel in a vacuum.
  • The eardrum senses the vibrations of sound, It sends the signals to the brain. This process is called hearing.
  • The number of oscillations or vibrations per second is called the frequency of oscillation.
  • The frequency is expressed in hertz (Hz)
  • The larger the amplitude of vibration, the louder the sound.
  • The higher the frequency of vibration, the higher the pitch, and shriller the sound.
  • Unpleasant sounds are called noise.
  • Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings.
  • Attempts should be made to minimise noise pollution.
  • Plantation on the roadside and elsewhere can reduce noise pollution.

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