1. How Sound is Produced

Sound plays an important role in our life. It helps us to communicate with one another. We hear a variety of sounds in our surroundings.

Sound is produced by a vibrating body. Touch your throat while you speak. Do you feel any vibrations? The to and fro or back and forth motion of an object is called vibration. When a tightly stretched band is plucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound.

2. Sound Needs a Medium for Propagation

When you call up your friend who is standing at a distance, your friend is able to hear your voice. How does the sound travel to her? Sound needs a medium to travel. It can travel through solids, liquids and gases.

• Solids: Sound travels fastest in solids. You can hear a train coming by putting your ear to the track (not recommended for safety!).
• Liquids: Whales and dolphins communicate under water. Divers can hear sounds under water.
• Gases (Air): This is the most common medium. We hear each other through air.

3. 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 there is a thin stretched membrane. It is called the eardrum.

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.

4. Amplitude, Time Period and Frequency

We have learnt that the to and fro motion of an object is known as vibration. This motion is also called oscillatory motion.

Key Terms

• Frequency: The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz (Hz). A frequency of 1 Hz is one oscillation per second.
• Amplitude: The maximum displacement of a vibrating body from its mean position. It determines the loudness of sound.
• Time Period: The time taken to complete one oscillation. Time Period = 1 / Frequency.

Loudness and Pitch

The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble. Loudness is expressed in a unit called decibel (dB).

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 (e.g., whistle). If the frequency of vibration is lower, we say that the sound has a lower pitch (e.g., drum).

5. Audible and Inaudible Sounds

We know that we need a vibrating body for the production of sound. Can we hear the sound of all vibrating bodies? No.

• Inaudible Sound: Sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible. On the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible to the human ear.
• Audible Range: For human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.

Some animals like dogs can hear sounds of frequencies higher than 20,000 Hz. The police use high frequency whistles which dogs can hear but humans cannot.

6. Noise and Music

We hear different types of sounds around us.

• Noise: Unpleasant sounds are called noise. Example: Sound produced in a classroom if all the students speak together, sounds from construction sites, horns of bus and trucks.
• Music: Musical sound is one which is pleasing to the ear. Sound produced by a harmonium is musical sound. The string of a sitar also gives out a musical sound.

7. Noise Pollution

Presence of excessive or unwanted sounds in the environment is called noise pollution. Major causes of noise pollution are vehicles, explosions including bursting of crackers, machines, loudspeakers etc.

Harmful Effects

Lack of sleep, hypertension (high blood pressure) and anxiety. A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.

Measures to Limit Noise Pollution

To control noise, we must control the sources of noise. Silencing devices must be installed in air craft engines, transport vehicles and industrial machines. Trees must be planted along the roads and around buildings to cut down on the sounds reaching the residents.

A form of energy that produces the sensation of hearing.

Sound is produced by vibrating bodies.

The rapid to and fro (or back and forth) motion of an object.

Sound needs a material medium (solid, liquid, or gas) to travel.

Sound cannot travel through a vacuum.

Sound travels fastest in solids, slower in liquids, and slowest in gases.

The speed of sound in air is approximately 340 m/s at room temperature.

The voice box in humans responsible for producing sound.

Two cords stretched across the larynx that vibrate to produce sound.

A thin stretched membrane at the end of the ear canal that vibrates when sound hits it.

The number of oscillations per second. Measured in Hertz (Hz).

The maximum displacement of a vibrating body from its mean position.

The time taken to complete one full oscillation.

The characteristic of sound that depends on its amplitude.

The characteristic of sound that depends on its frequency (shrillness).

For humans, sound frequencies between 20 Hz and 20,000 Hz (20 kHz).

Sounds with frequency less than 20 Hz.

Sounds with frequency higher than 20,000 Hz.

Unpleasant and unwanted sound.

Pleasant and melodious sound.

The unit used to measure the loudness of sound.

Sound becomes physically painful and harmful above 80 dB.

Light travels much faster than sound (Reason for seeing lightning before hearing thunder).

Reflected sound heard distinct from the original sound.

A technique using ultrasonic waves to measure depth of ocean and locate underwater objects.

Part of the inner ear that converts vibrations into nerve signals.

The visible outer part of the ear that collects sound waves.

The SI unit of frequency.

Generally has lower frequency (deeper voice) due to longer vocal cords.

Generally has higher frequency (shriller voice) due to shorter vocal cords.

The repeated back and forth movement of an object.

Presence of excessive or unwanted sound in the environment.

Loudness of normal breathing.

Loudness of a soft whisper.

Loudness of normal conversation.

High blood pressure, a potential health effect of noise pollution.

Lack of sleep caused by noise.

Devices fitted in vehicles and machines to reduce noise.

Planting trees along roads helps absorb sound and reduce noise pollution.

Whales communicate underwater using low-frequency sounds that travel long distances.

Bats use ultrasonic waves (echolocation) to navigate and find prey.

A wind instrument where sound is produced by vibrating air columns.

A string instrument where sound is produced by vibrating strings.

A percussion instrument where sound is produced by a vibrating membrane.

Frequency (f) and Time Period (T) are inversely related: f = 1/T.

Loudness is proportional to the square of the amplitude of vibration.

A device used by people with hearing loss to amplify sound.

Eardrum is extremely sensitive and delicate.

We cannot hear the vibrations of a pendulum if its frequency is below 20 Hz.

Speed of sound: Solids > Liquids > Gases.