Chapter SOUND - class 8 (under 30 mins)

Описание: Mediums of Sound Travel

Sound can travel through:

Solids

Liquids

Gases

When an object vibrates, it pushes nearby particles. Those particles push the next particles, and this chain continues. Without particles, the sound cannot move forward.

Sound Does NOT Travel in Vacuum

Vacuum is a space with no particles.
Since sound needs particles to travel, it cannot travel in vacuum.

This is why astronauts cannot hear each other in space unless using special radio equipment.

Speed of Sound

Sound travels at different speeds in different mediums:

Fastest in Solids (particles are tightly packed)

Slower in Liquids

Slowest in Gases

Example:

When you place your ear on a railway track, you hear an approaching train earlier through the solid track than through the air.

3. Sound Waves

When sound travels, it does not carry matter with it. It transfers energy from one place to another in the form of waves.

Longitudinal Waves

Sound waves in air are longitudinal.
This means particles of the medium move back and forth in the same direction as the wave travels.

Compressions and Rarefactions

As the sound wave travels:

Compression: particles are pushed together → high pressure region

Rarefaction: particles spread apart → low pressure region

A continuous pattern of compressions and rarefactions forms a sound wave.

4. Characteristics of Sound

Sound has properties that affect how it is heard.

A. Amplitude

Amplitude is the maximum displacement of a particle from its rest position.

It decides how loud a sound is.

Relationship with Loudness

Large amplitude → louder sound

Small amplitude → softer sound

Example:

Speaking loudly increases amplitude.

Whispering lowers amplitude.

B. Frequency

Frequency is the number of vibrations per second.

Measured in Hertz (Hz).

Relationship with Pitch

High frequency → high pitch (sharp sound)

Low frequency → low pitch (deep sound)

Examples:

A mosquito sound is high pitch.

A drum beat is low pitch.

Women often have higher pitch than men.

C. Time Period

The time taken to complete one vibration.

Formula:

Time Period
=
1
Frequency
Time Period=
Frequency
1
​


If frequency increases, the time period decreases.

D. Loudness

Loudness tells us how strong or weak a sound is.

Measured in Decibels (dB).

Example loudness levels:

Whisper → 30 dB

Normal speaking → 60 dB

Heavy traffic → 80–90 dB

Jet engine → 120 dB

Loud sounds can damage hearing if exposed for long.

5. Audible and Inaudible Sounds

Humans can hear only a small range of sound frequencies.

Human Hearing Range

Humans can hear sounds between:

20 Hz and 20,000 Hz
Infrasonic Sounds

Below 20 Hz

Too low for humans to hear

Used by elephants and whales to communicate over long distances

Ultrasonic Sounds

Above 20,000 Hz

Too high for humans to hear

Used by bats for navigation

Used in medical imaging (ultrasound)

6. Human Voice Production

Sound in humans is created by the voice box (larynx).

Vocal Cords

Inside the larynx are two stretched cords called vocal cords. When air from the lungs passes through them, they vibrate and produce sound.

Differences in Vocal Cords

Men have thicker, longer vocal cords → lower pitch

Women have thinner vocal cords → higher pitch

Children have very thin cords → very high pitch

By tightening or relaxing these cords, we change our pitch while speaking or singing.

7. How We Hear Sound

The hearing process involves several steps:

1. Outer Ear

The pinna collects sound waves.

The waves enter the ear canal.

2. Eardrum

The sound waves hit the eardrum (a thin membrane).

It starts vibrating.

3. Middle Ear

Contains three tiny bones:

Hammer

Anvil

Stirrup

These bones amplify the vibrations and pass them to the inner ear.

4. Inner Ear

Contains the cochlea (a fluid-filled structure).

Vibrations create waves in the fluid.

Hair cells convert vibrations into electrical signals.

5. Brain

The auditory nerve carries signals to the brain.

The brain interprets these signals as sound.

8. Noise and Music
Music

Pleasant, rhythmic, organized, and soothing.

Created by instruments or singing.

Noise

Unpleasant, irregular, and disturbing.

Examples: traffic, construction, loudspeakers.

Noise Pollution

When noise crosses safe levels, it becomes noise pollution.

Sources of Noise Pollution

Honking and traffic

Loudspeakers and DJs

Construction machinery

Industrial machines

Household appliances like mixers

Effects of Noise Pollution

Headaches

Stress and irritation

Difficulty concentrating

Lack of sleep

Risk of hearing loss

Reduced learning ability in children

Ways to Control Noise Pollution

Avoid unnecessary honking

Use soundproofing in noisy areas

Maintain vehicles and machines

Use ear protection in loud places

Reduce speaker volume

Plant trees (trees absorb sound)

Follow restricted hours for loud music