Lesson: Sound
Topic 1: Sound Propagation and Human Beings

# Question: 1

How is sound produced?

## Solution:

A sound is produced due to a vibration. When the vibrations travel through air or another medium and reach our ears, we hear it as sound.

# Question: 2

How do we hear sound?

## Solution:

The outer part of the ear is shaped like a funnel to collect incoming sounds. When a sound is produced, the air vibrates and these vibrations enter the ear and travel to the middle ear through a passage called the ear canal.

The middle ear consists of a thin, tightly stretched membrane called the ear drum which transmits the vibrations coming through the outer ear to the inner ear

The inner ear consists of cochlea containing hair-like nerve cells which translate the vibrations coming from the outer ear via the ear canal and ear drum, into electrical impulses which are sent to the brain. The brain interprets these electrical impulses and we become aware of the sounds.

# Question: 3

Explain the functioning of larynx.

## Solution:

Human beings produced sounds from their voice box - also known as the larynx - which is located at the upper end of the windpipe.

It has two vocal cords which are stretched across, leaving a narrow slit between them for the passage of air. These vocal cords are also called vocal folds.

During normal breathing, the vocal chords are held apart and no sound is produced.

When we want to speak or make a sound, we send air from our lungs towards the voice box. This air causes the vocal chords to vibrate. When the vocal chords vibrate, they open and close alternately, trapping the air and then releasing it.  Each release sends a little puff of air which produces the sound.

# Question: 4

Explain the working of eardrum.

## Solution:

The eardrum is a thin, tightly stretched membrane which transmits the vibrations coming through the outer ear to the inner ear.

# Question: 5

List any three musical instruments which produce sound by vibrating strings.

## Solution:

The three musical instruments which produce sound by vibrating strings are:

Violin

Guitar

Sitar

# Question: 6

Can sound travel on the Moon?

## Solution:

No, sound cannot travel on the moon because there is no atmosphere on the moon and sound cannot travel through vacuum. Sound needs a medium to propagate.

# Question: 7

Why can’t sound travel through vacuum?

## Solution:

Vacuum does not contain any particles of matter. Since sound is propagated by the vibration of particles, it cannot travel through vacuum.

# Question: 8

What is oscillatory motion?

## Solution:

The to and fro motion of an object about its mean position is called vibration or oscillatory motion. Examples of oscillatory motion are a child on a swing or vibration of a guitar string.

# Question: 9

What are the vibrating parts of the following musical instruments?

Veena, Tabla, Drum, Violin, Guitar, Sitar, Dholak, Flute, Mridangam

## Solution:

The vibrating parts of the given instruments are:

Veena $–$ Stretched string

Tabla $–$ Stretched membrane

Drum $–$ Stretched membrane

Guitar $–$ Stretched string

Sitar $–$ Stretched string

Dholak $–$ Stretched membrane

Flute $–$ Air column

Mridangam $–$ Stretched membrane

# Question: 10

Why should we not put a pin or a pencil in our ears?

## Solution:

Putting objects into the ear pushes earwax further back into the ear canal, where it's more likely to become impacted. Sharp objects can also damage the eardrum and it is thus not advisable to put sharp objects in our ears.

# Question: 11

During thunderstorms, why does the flash of light reach us first while the sound of thunder is heard a little later?

## Solution:

Light travels with a greater speed compared to the speed of sound. Hence, we see a flash of lightning before we hear the sound of thunder.

# Question: 12

Through which of the following can sound travel?

Wood, air, water, diamond, steel, plastic, ice, hydrogen, and oxygen

## Solution:

Sound can travel through all the mediums listed, whether they are solids, liquids or gases. Sound does not travel through a vacuum.

Topic 2: Sound characteristics and Noise

# Question: 1

Why do women have shriller voices than men?

## Solution:

A woman’s voice is generally shriller because females have shorter vocal folds, which produce higher frequency sounds.

# Question: 2

Define amplitude of vibration.

## Solution:

The maximum displacement of a vibrating object from its mean position is called its amplitude.

# Question: 3

Define time period of an oscillation.

## Solution:

The time taken by a vibrating object to complete one vibration cycle or oscillation is called the time period of the vibration or time period of oscillation.

# Question: 4

Define frequency of oscillations.

## Solution:

The number of oscillations made by a vibrating object in one second is called its frequency.

# Question: 5

Define pitch.

## Solution:

The shrillness of a sound is called its pitch. It depends on the frequency of vibration.

# Question: 6

Why is the sound produced by a bee is quite different from the roar of a lion?

## Solution:

Sound produced by a bee and a lion are different because they have different pitch and loudness. The sound produced by a bee is feeble (low amplitude) and high pitched (high frequency) whereas the roar of a lion is loud (high amplitude) but low pitched (low frequency).

# Question: 7

In what ways does loudness of sound, depend on its amplitude? If the amplitude is increased by three times, by how much does the loudness increase?

## Solution:

Loudness is proportional to the square of the amplitude. This means that if amplitude is doubled, loudness increases four times. If amplitude is increased three times loudness will increase by nine times.

# Question: 8

What is the audible range of frequencies for the human ear?

## Solution:

Sounds lying in the frequency range of 20 Hz to 20,000 Hz are considered audible to human beings. Any sound lying outside this frequency range is inaudible to us. Sounds with frequency less than 20 Hz are called infrasonic sounds while sounds with a frequency greater than 20,000 Hz are called ultrasonic.

# Question: 9

Distinguish between infrasonic and ultrasonic.

## Solution:

Infrasonic is any sound that has a frequency below the human capability of hearing, i.e. below 20Hz. Ultrasonic sound is, conversely, a sound with frequency that is above the human hearing range, or over 20,000Hz.

# Question: 10

What is the difference between noise and music?

## Solution:

The difference between noise and music is perceived differently by different people. Unpleasant sounds are called noise while pleasing sounds are called music. But, if a musical sound becomes too loud, it can be turn out to be noise.

# Question: 11

List a few sources of noise pollution in your surroundings.

## Solution:

Sources of noise pollution are:

(a)               Construction work

(c)               Fire crackers

(d)               Sound from loudspeakers

(e)               Television at very high volumes

(f)                Music concerts

# Question: 12

Explain in what ways noise pollution is harmful to humans.

## Solution:

Noise pollution can have a very bad effect on our health. Prolonged exposure to loud noises can damage the ear drums, causing hearing impairment or hearing loss.

More than just hearing damage, the stress of noise pollution can lead to sleepless nights, anxiety, and high blood pressure and even increased chances of heart trouble.

# Question: 13

Your friend is going to buy a house. S/he has been offered one close to the market and another away from the market. Which house would you suggest your friend should buy?

## Solution:

She should buy the house which is away from the market in order to avoid exposure to excessive noise and noise pollution and live in a comparatively peaceful place.

# Question: 14

What do you understand by the term noise pollution? How can we minimise noise pollution?

## Solution:

The presence of noise in the environment is called noise pollution.

Noise pollution can be reduced by:

Use of strong and effective silencers in vehicles

Trees plantation on the roadside and around buildings in residential areas

Locating Industries away from residential areas

Reducing the use of car horns

TV and music systems at low volumes

# Question: 15

How does loudness of sound vary with its amplitude?

## Solution:

Loudness of sound is directly proportional to the square of the amplitude of the vibration producing the sound. In other words, if the amplitude doubles, the loudness increases by a factor of four.

# Question: 16

How is pitch related to frequency?

## Solution:

The pitch of a sound is proportional to its frequency. As the frequency of vibration increases, the pitch of the sound also increases. A sound is said to be high pitched if its frequency of vibration is high, and is low pitched if it has a small frequency of vibration.

# Question: 17

A loudspeaker emits a sound of constant pitch. Over a period of time, this sound becomes louder and louder. How does the amplitude or frequency change during this time?

## Solution:

Loudness of the sound depends upon the amplitude whereas pitch depends upon the frequency.

Since the sound is getting louder and louder, it means there is a change in the amplitude while the frequency remains constant.

# Question: 18

When we strike a steel glass lightly with a spoon, we hear a faint sound, but if hit the glass harder, the sound gets louder. Why?

## Solution:

When we strike the glass lightly, the amplitude of vibrations is small and hence the sound produced is also feeble. But when hit harder, the amplitude is large, which produces a louder sound.