# Selina Concise Chapter 7 Sound ICSE Solutions Class 8 Physics

## ### A. Objective Questions

1. Write true or false for each statement
(a) When sound propagates in air, it does not carry energy with it.
False.

(b) In a longitudinal wave, compression and rarefaction are formed.
True

(c) The distance from one compression to nearest rarefaction is called wavelength.
False

(d) The frequency is measured in second.
False

(e) The quality of a sound depends on the amplitude of wave.
False

(f) The pitch of sound depends on frequency.
True

(g) Decibel is the unit of pitch of a sound.
False

2. Fill in the blanks
(a) The time period of a wave is 2s. Its frequency is ____.
(b) The pitch of a stringed instrument is increased by _____ tension in string.
(c) The pitch of a flute is decreased by ______ length of air column.
(d) Smaller the membrane, _____ is the pitch.
(e) If a drum is beaten hard, its loudness ______.
(f) A tuning fork produces sound of _____ frequency.
(a) The time period of a wave is 2s. Its frequency is 0.5 s-1.
(b) The pitch of a stringed instrument is increased by increasing tension in string.
(c) The pitch of a flute is decreased by increasing length of air column.
(d) Smaller the membrane, higher is the pitch.
(e) If a drum is beaten hard, its loudness increases.
(f) A tuning fork produces sound of single frequency.

3. Match the following:
 Column A Column B (a) Amplitude (i) frequency (b) frequency (ii) Amplitude (c) Loudness (iii) maximum displacement on either side (d) Pitch (iv) presence of other frequencies (e) Wave form (v) 1/time period

 Column A Column B (a) A stone at a height (iii) Maximum displacement (b) A moving hall (v) 1/time period (c) Energy (ii) amplitude (d) Power (i) frequency (e) watt (iv) presence of other frequencies

4. Select the correct alternative

(a) Sound can not travel in
1. solid
2. liquid
3. gas
4. vacuum
4. vacuum

(b) When sound travels in form of a wave
1. the particles of medium move from the source to the listener
2. the particles of medium remains stationary
3. the particles of medium start vibrating up and down
4. the particles of medium transfer energy without leaving their mean positions.
4. the particles of medium transfer energy without leaving their mean positions.

(c) The safe limit of loudness of audible sound is
1. 0 to 80 dB
2. above 80 dB
3. 120 dB
4. above 120 dB
1. 0 to 80 dB

(d) The unit of loudness is
1. cm
2. second
3. hertz
4. decibel
1. cm

(e) In a piano, pitch is decreased by
1. using thicker string
2. increasing tension
3. reducing length of string
4. striking it hard
1. using thicker string

1. How does sound travel in air ?

A periodic disturbance in the medium (Air) is created by the vibration of sound and the particles of the medium vibrate about their mean position and transfer of energy in the form of sound waves takes place, i.e. in the longitudinal waves.

2. What is longitudinal wave ?

Longitudinal wave: The wave in which the particles of the medium vibrate about their mean positions in the direction of propagation of sound is called longitudinal wave. Such a wave can be produced in solids, liquids as well as gases.

3. Explain the mechanism of formation of a longitudinal wave when source vibrates in air.

Propagation of Sound in Air: When a source of sound vibrates, it creates a periodic disturbance in the medium near it (i.e., the condition of medium changes). The disturbance then travels in the medium in form of waves. This can be understood by the following example.
Example: Take a vertical metal strip with its lower end fixed. Push its upper end to one side and then release it. As it vibrates, i.e. moves alternately to the right and left, sound is produced. Figure shows the steady (or mean position) of the metal strip and normal condition of air layers near the strip.

As the strip moves to the right from a to b in Figure it pushes the particles of air layer in front of it. So the particles of air in this layer come closer to each other i.e., air in that layer gets compressed (or compression is formed at C). The particles of this layer while moving towards right, pushes and compresses the layer next to it, which then compresses the next layer and so on. Thus the disturbance moves forward in form of compression. The particles of the medium do not move with the compression.

As the metal strip starts returning from b to a in Figure after pushing the particles near the strip, the compression C moves forward and the particles of air near the strip return back to their normal positions due to the elasticity of the medium.

When the strip moves to the left from a to c in Figure it pulls the layer of air near it towards left and thus produces a space of very low pressure on its right side. The air layers on the right side of the strip expands in this region thus forming the rarefied layers. ‘This region of low pressure is called a rarefaction R. By the time the strip returns from c to its mean position a in Figure the rarefaction R
moves forward and air layers near the strip return back to their normal position due to the elasticity of the medium. In this manner, as strip moves to the right and left repeatedly, the compression and
rarefaction regions are produced one after the other which carry the disturbance along it with, definite speed depending on the nature of the medium.
One complete to and fro motion of the strip forms one compression and one rarefaction which together constitute one wave. This wave in which the particles of the medium vibrate about their mean positions, in the direction of propagation of sound, is called the longitudinal wave. Thus sound travels in air form of longitudinal waves. Actually the longitudinal waves can be produced in solids, in liquids as well as in gases. Thus, due to propagation of wave in a medium, the particles of the medium vibrate about their mean positions (without leaving their positions) and they transfer the energy with a constant speed from one place of medium to the other place.

4. Define the following terms :

(a) Amplitude
(b) Frequency
(c) Time period.

(a) Amplitude (A): The maximum displacement of a wave on either side of its mean position is called Amplitude A = XY = is amplitude.
(b) Frequency (f) or nu (𝜈): Number of oscillations made by a wave in one second is known as its frequency.
(c) Time Period (T): Time taken to complete one vibration is called Time Period, i.e. from A to B

5. Obtain relationship between the time period and frequency.

Relationship between the time period (T) and Frequency (ƒ): By definition time period is the time taken to complete 1 vibration
Or
In time T, number of vibration= 1
∴ In 1 second, the frequency or number of vibrations = 1/T
ƒ = 1/T Or T = 1/ƒ

6. Name three characteristics of a musical sound.

Characteristics of Sound:
Sounds can be distinguished from one another by the following three different characteristics:

1. loudness
2. pitch or shrillness, and
3. quality or timbre.

7. Name the quantity from below which determines the loudness of a sound wave :
(a) Wavelength
(b) Frequency, and
(c) Amplitude.

Quantity which determines loudness of a sound wave is (c) amplitude.

8. How is loudness related to the amplitude of wave ?

Greater the amplitude, greater is the loudness.

If we strike the drum gently, a faint sound is heard. But if we strike it hard, a loud sound
is heard.

9. If the amplitude of a wave is doubled, what will be the effect on its loudness ?

The loudness of sound is directly proportional to the square of amplitude of wave.
Loudness ∝ (amplitude)2

When amplitude is doubled loudness becomes (2)2 = 4 times

10. How does the wave pattern of a loud note differ from a soft note ? Draw a diagram.

The amplitude of soft note (faint) has smaller amplitude.
The amplitude of loud sound has more amplitude as shown in figure.

11. Name the unit in which the loudness of sound is expressed.

Unit of loudness is decible(dB).

12. Why is the loudness of sound heard by a plucked wire increased when mounted on a sound board ?

A wire mounted on a sound board is plucked, the surface area of vibrating air increases and sends forth greater amount of energy, So the amplitude of vibration is large and louder is the sound.

13. State three factors on which loudness of sound heard by a listener depends.

Three factors for loudness of Sound:

• Surface area of sounding body. i.e. is directly proportional to surface area of vibrating body.
• On the distance of source of sound, i.e. decreases with distance.
• On Amplitude of Sound i.e. increases with amplitude.

14. What determines the pitch of a sound ?

Frequency i.e. number of vibrations per second determines the pitch. Higher frequency, higher pitch means shrill sound. A low pitch has flat sound.

15. Name the characteristic of sound related to its frequency.

Characteristic of sound related to its frequency is Pitch.

16. Name and define the characteristic which enables one to distinguish two sounds of same loudness, but of different frequencies, given by the same instrument.

Two sounds of same loudness (amplitude) of different frequencies given by same instrument can be distinguished by the characteristics called Pitch.

17. Draw a diagram to show the wave pattern of high pitch note and a low pitch note, but of the same loudness.

Pattern of low pitch and high pitch note:

18. How is it possible to detect the filling of a bucket under a water tap by hearing the sound standing at a distance ?

The Sound becomes shriller and shriller as the water in the bucket rises, the length of air column decreases. So the frequency of the sound produced increases. Thus by hearing the sound from a distance, one can get the idea of water level in the bucket.

19. The frequencies of notes given by flute, guitar and trumpet are respectively 400 Hz, 200 Hz and 500 Hz. Which one of these has the highest pitch ?

The instrument with highest frequency has highest pitch.
Hence, trumpet frequency 5 00 Hz has highest pitch.

20. Figure shows two jars A and B containing water up to different heights. Which will produce sound of higher pitch when air is blown on them ?

Jar B which has less air column above water will produce sound of higher pitch. Less air column produces increased frequency.

21. Two identical guitars are played by two persons to give notes of the same pitch. Will they differ in quantity ? Give reason for your answer.

No, They have same wave form.

22. Two musical notes of the same pitch and same loudness are played on two different instruments. Their wave patterns are as shown in figure.

How do they differ in
(a) loudness,
(b) pitch and
(c) quality

(a) Loudness of each is same as amplitude is same.
(b) Pitch of each is same.
(c) Quality is different as the wave form is different

23. Which characteristics of sound makes it possible to recognize a person by his voice without seeing him ?

Characteristic is timber or quality which makes it possible to recognise a person by his voice without seeing him.

24. State the factors that determine
(a) the pitch of a note.
(b) the loudness of the sound heard.
(c) the quality of the note.

(a) Frequency

(b) Amplitude

(c) Waveform

25. Name the characteristic of the sound affected due to a change in its (a) amplitude (b) wave form (c) frequency.

(a) Loudness
(b) Quality
(c) Pitch.

26. Figure shows four waves A, B, C, and D.

Name the wave which shows
(a) a note from a musical instrument,
(b) a soft note
(c) a shrill note.

(a) a note from a musical sound is shown by (D)

(b) a soft note is shown by (A)

(c) a shrill note is shown by (C)

Question 27. How is the pitch of sound in a guitar changed if (a) thin wire is used, (b) wire under less
tension is used ?

(a) Thin wire used ___ Higher pitch is obtained.
(b) Wire under less tension is used ___ a low pitch is obtained.

### C. Numerical

1. Two waves of the same pitch have amplitudes in the ratio 1: 3 What will be the ratio
of their (i) loudness, (ii) pitch ?

Loudness a2
(amplitude)2

2. Two waves have frequencies 256 Hz and 512 Hz, but same amplitude. Compare their
(i) loudness, and (ii) pitch;