Living Science Physics Class 8 ICSE Solution Chapter 7 Sound

ICSE Living Science Physics for Class 8 Solutions Chapter 7 Sound

Check Your Progress

A. Fill in the blanks.
Question 1.

1. The time taken by a wave to complete one oscillation is called its ……….
2. Loudness of a sound decreases with …………. in its amplitude.
3. A shrill voice has …………… frequency.
4. The ………….. distinguishes the sounds of a guitar and a bongo with the same pitch and loudness.

Answer:

1. The time taken by a wave to complete one oscillation is called its time period
2. Loudness of a sound decreases with decrease in its amplitude.
3. A shrill voice has larger frequency.
4. The timbre distinguishes the sounds of a guitar and a bongo with the same pitch and loudness.

B. Answer the following.

Question 1.
Name three factors on which loudness of a sound depend.
Answer:
Loudness of a sound depends on the following factors :

1. Amplitude of the sound wave.
2. Distance between the source of sound and the listener.
3. Area of the vibrating body.

Question 2.
Which of the two will have a higher pitch – the sound of a whistle or that of a drum?
Answer:
The sound of a whistle will have a higher pitch that of a drum.

Question 3.
How is pitch different from the frequency?
Answer:
Frequency of a sound wave is the number of compressions or rare-factions passing a point in one second. However, pitch is the sensation of frequency of the sound wave on our ears.

Question 4.
What do you understand by quality of sound or timbre?
Answer:
The quality of sound or timbre distinguishes two notes of the same pitch (or frequency) and loudness (or amplitude) produced by different bodies.

Think and answer

Question 1.
Why does the pitch of the sound increases when a nail is hammered into a wall ?
Answer:
As a nail is hammered into a wall, its length keeps on decreasing. We know that the pitch of a sound is inversely proportional to the length of the vibrating body. So, the pitch of the sound increases as the length of the nail hammered into the wall decreases.

A. Tick the most appropriate answer.

Question 1.
Loudness of a sound depends on the
a. the wavelength of sound.
b. volume of air around the vibrating body.
c. time.
d. none of these
Answer:
d. none of these

Question 2.
The pitch of a sound increases with increase in its
a. frequency.
b. loudness.
c. amplitude.
d. time period.
Answer:
a. frequency.

Question 3.
The pitch of a flute can be increased by
a. increasing the length of its vibrating air column.
b. decreasing the length of its vibrating air column.
c. increasing the length of its vibrating string.
d. decreasing the length of its vibrating string.
Answer:
b. decreasing the length of its vibrating air column.

Question 4.
A sound produced by guitar and violin with the same pitch and loudness is distinguished by its
a. frequency.
b. amplitude.
c. timbre.
d. time period.
answer:
c. timbre.

Question 5.
Which of the following properties of sound is responsible for its loudness?
a. frequency
b. pitch
c. amplitude
d. quality
Answer:
c. amplitude

Question 6.
Musical sounds are caused due to –
a. regular vibrations.
b. irregular vibrations.
c. damped vibrations.
d. none of these
Answer:
a. regular vibrations.

Question 7.
The sound that never changes in pitch, volume, speed or timbre is known as –
a. monologue.
b. monolingual.
c. monophony.
d. monotone.
Answer:
d. monotone.

B. Fill in the blanks.

1. Loudness of a sound ………… with increase in the distance between the listener and the source.
2. The characteristic of a sound which distinguishes a shrill sound from a hoarse one is called ………….
3. A longer string will produce a sound of a …………. pitch.
4. Drum produces sound due to vibration of its …………
5. Unpleasant sound that is very loud is called …………
6. Humans can pick up sounds in the range of to ………….
7. Sound of about …………. decibels or more causes many health hazards.
8. Constant exposure to sounds above 80 dB can cause ………….

Answer:

1. Loudness of a sound decreases with increase in the distance between the listener and the source.
2. The characteristic of a sound which distinguishes a shrill sound from a hoarse one is called pitch.
3. A longer string will produce a sound of a lower pitch.
4. Drum produces sound due to vibration of its stretched membrane.
5. Unpleasant sound that is very loud is called noise.
6. Humans can pick up sounds in the range of to 10 dB to 180 dB.
7. Sound of about 80 dB decibels or more causes many health hazards.
8. Constant exposure to sounds above 80 dB can cause health problems.

C. Write true or false. Correct the false statements.

Question 1.
A sound with higher amplitude is a soft sound whereas a sound with lower amplitude is loud.
Answer:
False.
Correct : A sound with higher amplitude is a loud sound whereas a sound with lower amplitude is a soft sound.

Question 2.
The pitch of a sound of string instruments decreases with the decrease in the thickness of the vibrating string.
Answer:
False.
Correct : The pitch of a sound of string instruments decreases with increase in the thickness of the vibrating string.

Question 3.
Pitch of a sound depends on its frequency.
Answer:
True.

Question 4.
The pitch of a stringed instrument depends on the tension in the string.
Answer:
True.

Question 5.
A flute produces sound due to vibration of air column inside it.
Answer:
True.

Question 6.
Music has a disturbing effect on health and behaviour of a person.
Answer:
False.
Correct : Noise has a disturbing effect on health and behaviour of a person.

Question 7.
Sounds below 50 dB cause noise pollution arid are harmful.
Answer:
False.
Correct : Sounds above 80 dB can cause noise pollution and are harmful.

Question 8.
Percussion instruments produce sound due to the vibration of thin metal strips.
Answer:
False.
Correct : Percussion instruments produce sound by a vibrating stretched membrane or by vibration of their whole bodies.

D. Match the columns.

Answer:

E. Define the following

1. Amplitude
2. Frequency
3. Time period
4. Loudness
5. Pitch
6. Timbre
7.  Monotone
8. Music
9.  Noise

Answer:

1. Amplitude : The maximum displacement of a wave on either side of its mean position is known as its amplitude.
2. Frequency : Frequency of a wave is the number of oscillations completed by it in one second.
3. Time period : Time period of a wave is the time taken by it to complete one oscillation.
4. 4. Loudness : Loudness is a characteristic of sound which distinguishes a feeble sound from a loud one of the same frequency.
5. Pitch : The characteristic of sound that differentiates a shrill sound from a hoarse sound is called pitch.
6. Timbre : Timbre distinguishes two notes of the same pitch (or frequency) and loudness (or amplitude) produced by different bodies.
7. Monotone : A sound which never changes or goes up or down is called a monotone. A monotone sound never changes in pitch, loudness, timbre or in speed.
8. Music : A sound that has a pleasing sensation on our ears and is produced due to regular vibrations is called music.
9. Noise : A sound unpleasant to our ears and is produced by irregular vibrations is called noise.

F. Answer the following in short.

Question 1.
Name three characteristics of sound.
Answer:
Three characteristics of sound are – loudness or intensity, pitch and quality or timbre of sound.

Question 2.
Define the characteristic of a sound that distinguishes a shrill sound from a hoarse sound.
Answer:
The characteristic of sound that distinguishes a shrill sound from a hoarse sound is called pitch.

Question 3.
How are loudness and amplitude related?
Answer:
Loudness of sound is directly proportional to its amplitude. Higher the amplitude, louder is the sound.

Question 4.
Write two differences between loudness and pitch.
Answer:

Question 5.
How is it possible to distinguish between sounds produced by different instruments tuned to the same frequency?
Answer:
The quality of sound or timbre is used to distinguish between sounds produced by different instruments tuned to the same frequency.

Question 6.
Name the different types of musical instruments.
Answer:
The different types of musical instruments are wind instruments, string instruments and percussion instruments.

Question 7.
What do you understand by monotone?
Answer:
A sound which never changes or goes up or down is called a monotone. A monotone sound never changes in pitch, loudness, timbre or in speed.

Question 8.
State the unit of loudness. Mention the effect of different loudness levels on human ears.
Answer:
The unit of loudness is decibel (dB). Even though human ears can pick up sound from 10 dB to 180 dB, different loudness levels have different effect on human ears. A sound between 50 dB and 60 dB is considered normal. Sounds up to 80 dB can be tolerated. Sounds above 80 dB can have harmful effects.

Question 9.
Mention two effects produced on humans by sounds of 90 dB or more.
Answer:
The effect produced on humans by sounds of 90 dB or more are
a. feeling of annoyance, aggression, anxiety and stress.
b. headache, feeling of fatigue and sleep disturbances.

G Answer the following in detail.

Question 1.
Define amplitude, time period and frequency with respect to a sound wave. State the relationship between time period and frequency of a wave.
Answer:
The maximum displacement of a wave on either side of its mean position is known as its amplitude. Frequency of a wave is the number of oscillations completed by it in one second. Time period of a wave is the time taken by it to complete one oscillation. The time period and frequency of a sound wave are related by the expression
f = 1/T

Question 2.
State the factors affecting the loudness of sound. How do these factors affect the loudness of sound?
Answer:
The factors affecting the loudness of a sound are mentioned below.
a. Amplitude of the sound wave : Loudness of a sound is directly proportional to its amplitude. Higher the amplitude of sound wave, higher is its loudness.
b. Distance between the source of sound and the listener : Loudness of a sound is inversely proportional to the distance between the source of sound and the listener. A sound of the same frequency heard at a small distance will be louder than when it is heard from a greater distance.
c. Area of the vibrating body : Loudness increases with an increase in the area of the vibrating body.

Question 3.
What do you understand by the terms pitch, timbre and loudness of a sound?
Answer:
Pitch : The characteristic of sound that differentiates a shrill sound from a hoarse sound is called pitch.
Timbre : Timbre distinguishes two notes of the same pitch (or frequency) and loudness (or amplitude) produced by different bodies.
Loudness : Loudness is a characteristic of sound which distinguishes a feeble sound from a loud one of the same frequency.

Question 4.
How can you prove experimentally that the pitch of a string instrument depends on the length of the vibrating string?
Answer:
The following activity shows that the pitch of a string instrument depends on the length of the vibrating string.
Materials required : A rectangular cardboard box (covered), a pencil and a rubber band.
Procedure : Stretch the rubber band over the cardboard box. Place the pencil under the band in such a way that it divides the band into two unequal parts. Pluck the longer portion of the rubber band and then its shorter portion.
Observation : You will observe that the shorter portion of the rubber band produces shriller sound (with a higher pitch) than the longer portion.
Conclusion : The pitch of sound decreases with the increase in the length of the vibrating string.

Question 5.
Prove that the pitch of a wind instrument depends on the length of the vibrating air column.
Answer:
The following activity shows that the pitch of a wind instrument depends on the length of the vibrating column.
Materials required : Four test tubes, a beaker containing water and a test tube holder.
Procedure : Place the test tubes in the test tube holder and label them as A, B, C and D from left to right. Fill the test tubes with water such that the level of water rises as you go from A to D. This means that the length of air column decreases as we move from A to D. Blow air in the test tube a by putting your lips over its mouth. Now, repeat the procedure with the test tubes B, C and D.
Observation : The sound produced by test tubes becomes shriller as we move from A to D. In other words, the pitch of the sound produced by the test tubes when air is blown over their mouths increases as we move from A to D.
Conclusion : The pitch of the vibrating air column increases with the decrease in the length of air column.

Question 6.
Classify percussion instruments on the basis of the pitch produced by them.
Answer:
Percussion instruments are classified on the basis of pitch produced by them into two groups : pitched and unpitched instruments.

Question 7.
Differentiate between music and noise. Why is it advisable to listen to music at a moderate level?
Answer:
Musical sound:

1. Musical sound has a pleasing effect on the ears.
2. It is produced by regular vibrations-in a material
3. Amplitude of vibration and frequency do not change suddenly.
4. The waveform is regular.

Noise :

1. Noise has an irritating and unpleasant effect on the ears.
2. It is produced by irregular vibrations in a material.
3. Amplitude of vibration and frequency change suddenly.
4. The waveform is irregular.

It is advisable to hear music at moderate level because sounds above 80 dB can cause many health hazards, some of which are listed below.

1. Feeling of annoyance, aggression, anxiety and stress
2. High blood pressure heart problems
3. Headache, feeling of fatigue, sleep disturbances
4. Reduced work efficiency
5. Loss of hearing

H. The waveforms of some sounds are given below.

Answer the questions that followed.

1. Identify the sounds with same loudness.
2. Identify the sounds with same pitch.
3. Identify the most shrill sound.
4. Identify the most soft sound.
Answer:

Think and answer

Question 1.
If a key of a piano is first struck gently and then much harder, how will the loudness and pitch of the sound change?
Answer:
In a piano, musical sound is produced by pressing different keys. When a key is pressed, it causes a hammer to strike a string of a particular length and thickness and produces a sound of a particular frequency. Each key produces a sound of a particular frequency. So, pressing a key gently or harder doesn’t change the pitch of the sound. However, hitting a key harder causes the hammer attached to it strike the string more harder, hence, increasing the loudness of the sound.

Question 2.
A person hears two sounds – one of 60 dB, 100 Hz and other of 45 dB, 700 Hz. Which sound will the person perceive to be louder?
Answer:
Loudness is measured in the units of decibels. So, a sound of higher decibel is louder than a sound of lower decibel. This implies that the sound of 60 dB, 100 Hz is louder than the sound of 45 dB, 700 Hz.

Question 3.
If a stretched string is cut into half, how will the pitch of the sound produced by the vibrating string change?
Answer:
Pitch of a sound produced by a vibrating string is inversely proportional to the length of the string. If a string is cut into half then the pitch produced by the vibrating string increases.

Question 4.
Why is it advised not to use earphones at loud volumes?
Answer:
It is advised not to use earphones at loud volumes because sounds above 80 dB can cause many health hazards, some of which are listed below.

1. Feeling of annoyances aggression, anxiety and stress.
2. High blood pressure heart problems.
3. Headache, feeling of fatigue, sleep disturbances.
4. Reduced work efficiency.
5. Loss of hearing.

Question 5.
Loudness of a sound depends on its amplitude. Give reason.
Answer:
Loudness of a sound depends on its amplitude. Higher the amplitude, louder is the sound. As the amplitude of a wave is directly related to the energy it carries, we can say that greater the energy carried by a wave, louder is the sound produced.

Question 6.
No two musical instruments can have the same timbre. Give reason.
Answer:
No two instruments can have the same timbre. This happens because a sound wave also has a unique pattern of secondary waves called harmonics and no two musical instruments can have the same harmonics.

Question 7.
A monotone is unable to deliver the message imparted by the orator. Give reason.
Answer:
If an orator speaks in a monotone, he won’t be changing the pitch,;volume, rhythm, timbre and speed of his voice, For instance, he won’t make his voice assertive while asking a question, stress words to add emphasis, pause to add suspense, vary his voice to express different emotions. So, he won’t be able to deliver his message.

Living Science Physics Class 8 ICSE Solutions