Study Module & Revision Notes – Chapter 13: Sound

1. Introduction to Sound

We hear many sounds around us – people talking, birds chirping, vehicles moving on the road, a bell ringing, music from a radio and so on. All these are different types of sound. Sound helps us to communicate and to understand our surroundings.

In NCERT Class 8 Science, this chapter explains how sound is produced, how it travels, and how we hear it. It also tells us about noise, music and noise pollution along with ways to reduce it.

2. Production of Sound – Vibrations

Vibration means a to-and-fro or back-and-forth motion of an object about its mean position. Whenever an object vibrates, sound is produced.

Some simple examples from daily life:

• When you pluck the string of a guitar, the string starts vibrating and you hear sound.
• When you hit the membrane of a drum, it vibrates and produces sound.
• When a school bell is struck, the metal bell vibrates and a ringing sound is heard.
• Our vocal cords in the voice box vibrate when we speak and produce sound.

If the vibrations are stopped, the sound also stops. Thus, vibrations are necessary for the production of sound.

3. Propagation of Sound – Need for a Medium

The process by which sound travels from the source to the listener is called propagation of sound. Sound cannot travel in vacuum; it needs a material medium.

Sound can travel through:

• Solids – for example, if you put your ear on a railway track, you can hear a train from far away.
• Liquids – sound can travel through water, as in underwater communication in submarines.
• Gases – sound travels in air from the source to our ears.

4. Sound Waves and Their Basic Characteristics

When an object vibrates, it sets the particles of the medium (air, water, etc.) into back-and-forth motion. This disturbance travels in the form of sound waves and reaches our ears.

Some basic terms related to sound are:

• Amplitude: The maximum displacement of particles from their mean position during vibration. It is related to the energy of the wave.
• Frequency: The number of complete vibrations or oscillations made by a vibrating body in one second. Its unit is hertz (Hz).
• Time Period: The time taken to complete one full vibration. It is the inverse of frequency.
• Speed of Sound: Distance travelled by sound in one second in a given medium.

5. Loudness and Pitch of Sound

Though we hear many sounds, they are not all the same. Some are loud, some are soft, some are shrill and some are deep.

Loudness:

• Loudness tells us how strong or soft a sound is.
• It depends on the amplitude of vibration.
• Greater amplitude → louder sound. Smaller amplitude → softer sound.
• Loudness is measured in decibel (dB).

Pitch (Shrillness of Sound):

• Pitch tells us whether a sound is shrill or deep.
• It depends on the frequency of vibration.
• Higher frequency → higher pitch (for example, voice of a child).
• Lower frequency → lower pitch (for example, voice of an adult man).

6. Audible and Inaudible Sounds

The human ear can hear only a certain range of frequencies. Sounds within this range are called audible sounds.

• The normal audible range for a human being is roughly from 20 Hz to 20,000 Hz.
• Sounds with frequency less than 20 Hz are called infrasonic sounds.
• Sounds with frequency greater than 20,000 Hz are called ultrasonic sounds.

Many animals like dogs, bats and dolphins can hear ultrasonic sounds, but human beings cannot.

7. Musical Sound and Noise

Not all sounds are pleasant to hear. Some are soothing and melodious, while others are harsh and irritating.

Music:

• Sound that is pleasant, regular and has a pattern is called music.
• Examples: sound from musical instruments, songs sung in rhythm, a properly tuned flute or violin.

Noise:

• Sound that is unpleasant, irregular and disturbing is called noise.
• Examples: honking of many vehicles at a busy road, loudspeakers at very high volume, loud shouting, bursting of crackers.

8. Noise Pollution

When there is excessive or unwanted sound in the environment that causes discomfort to human beings and animals, it is called noise pollution.

Major causes of noise pollution:

• Heavy traffic on roads – continuous honking by vehicles.
• Use of loudspeakers during functions, rallies and celebrations.
• Running of large machines in factories and construction sites.
• Loud music in homes, markets and entertainment places.
• Crackers during festivals and celebrations.

Harmful effects of noise pollution:

• Headache, stress and lack of concentration.
• Disturbed sleep, irritability and tiredness.
• Temporary or permanent hearing loss if exposed for long time.
• Disturbance to students during study and to patients in hospitals.

Methods to control noise pollution:

• Avoid unnecessary use of horns; use them only when needed.
• Keep volume of TV, music systems and mobile phones low.
• Use soundproofing or silencers in industries and vehicles.
• Plant more trees along roads and around buildings – trees absorb sound.
• Strict laws to limit the use of loudspeakers and bursting of crackers.
• Create silence zones around schools, hospitals and residential areas.

9. Structure and Working of the Human Ear

Our ears are sensitive organs which help us hear sound. The human ear has three main parts: outer ear, middle ear and inner ear.

1. Outer Ear:

• Consists of the visible part called the pinna and the ear canal.
• Pinna collects sound waves and sends them into the ear canal.
• Sound waves travel through the canal and strike the eardrum.

2. Eardrum (Tympanum):

• It is a thin, stretched membrane at the end of the ear canal.
• When sound waves fall on it, the eardrum starts vibrating.

3. Middle Ear:

• Contains three tiny bones – hammer, anvil and stirrup.
• These bones increase (amplify) the vibrations received from the eardrum.

4. Inner Ear:

• Contains the cochlea filled with fluid and many nerve cells.
• Vibrations are converted into electrical signals (nerve impulses).
• These signals are carried by the auditory nerve to the brain.

The brain interprets these signals and we experience the sensation of hearing.

10. Caring for Our Ears

Since our ears are delicate organs, we must protect them to avoid damage and hearing problems.

• Do not listen to very loud music using earphones or headphones.
• Avoid staying for long in noisy places like loud functions or traffic areas.
• Never insert sharp objects like pins or matchsticks into the ear.
• Visit a doctor if there is pain, discharge or discomfort in the ear.

11. Quick Recap – Exam-Oriented Summary

Before your test, revise the following important points from NCERT-based notes:

• Sound is produced by vibrations of objects.
• Sound requires a material medium to travel; it cannot travel through vacuum.
• Amplitude → loudness of sound; Frequency → pitch of sound.
• Human audible range: 20 Hz to 20,000 Hz.
• Musical sound is pleasant; noise is unpleasant and irregular.
• Excessive noise causes noise pollution, which is harmful to health.
• Human ear has outer, middle and inner parts; eardrum vibrations finally reach the brain as signals.
• We must protect our ears and help reduce noise pollution in our environment.