Sound

Objective Questions

Question 1

Write true or false for each statement:

(a) When sound propagates in air, it does not carry energy with it.

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

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

(d) The frequency is measured in second.

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

(f) The pitch of sound depends on frequency.

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

Answer

(a) False
Correct Statement — When sound propagates in air, it carries energy with it.

(b) True

(c) False
Correct Statement — The distance between two consecutive compressions or between two consecutive rarefactions is equal to one wavelength.

(d) False
Correct Statement — The frequency is measured in hertz.

(e) False
Correct Statement — The quality of a sound depends on the wave form.

(f) True

(g) False
Correct Statement — Decibel is the unit of loudness of a sound.

Question 2

Fill in the blanks:

(a) The time period of a wave is 2 s. 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.

Answer

(a) 0.5 hertz

(b) increasing

(c) increasing

(d) higher

(e) increases

(f) single

Question 3

Match the following:

Answer

Question 4a

Select the correct alternative:

Sound cannot travel in:

1. solid
2. liquid
3. gas
4. vacuum

Answer

vacuum

Reason — Sound needs a medium for its propagation. It cannot travel in vacuum.

Question 4b

Select the correct alternative:

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 remain stationary
3. the particles of medium start vibrating up and down
4. the particles of medium transfer energy without leaving their mean positions.

Answer

the particles of medium transfer energy without leaving their mean positions.

Reason — Sound travels in air in form of longitudinal waves. The particles of the medium vibrate about their mean positions and transfer energy with a constant speed from one place of medium to another place during propagation of wave.

Question 4c

Select the correct alternative:

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

Answer

0 to 80 dB

Reason — The safe limit of level of sound for hearing is from 0 to 80 dB.

Question 4d

Select the correct alternative:

The unit of loudness is:

1. cm
2. second
3. hertz
4. decibel

Answer

decibel

Reason — The unit of loudness of sound is decibel.

Question 4e

Select the correct alternative:

In a piano, pitch is decreased by:

1. using thicker string
2. increasing tension
3. reducing length of string
4. striking it hard

Answer

using thicker string

Reason — In stringed instruments like piano, a note of lower pitch can be obtained by vibrating the string under low tension or by vibrating a thicker string.

Short/Long Answer Questions

Question 1

How does sound travel in air?

Answer

Sound travels in air in form of longitudinal waves. When a body vibrates, it creates a periodic disturbance in air. The disturbance then travels in air in the form of waves.

Question 2

What is a longitudinal wave?

Answer

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.

Question 3

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

Answer

When a source of sound vibrates, it creates a periodic disturbance in the medium near it. The disturbance then travels in the medium in the form of waves. This can be understood by the following example —

Take a thin metal strip. Keeping it vertical, fix it's lower end. Push it's upper end to one side and then release it. As it vibrates (i.e., moves alternatively to the right and left) sound is heard.

When the strip advances to the right from a to b, it pushes the particles of air in layers in front of it. So the particles of air in these layers gets closer to each other i.e., air of these layers gets compressed.

The particles of these layers while moving forward, push and compress the layers next to them, which then compress the next layers and so on. Thus, the disturbance moves forward in form of compression. The particles of the medium get displaced, but they do not move along with the compression.

As the metal strip starts returning from b to a as shown in figure, after pushing the particles in front, the particles of air near the strip starts returning back to their mean positions due to the elasticity of the medium.

When the strip moves to the left from a to c, it pushes back the layers of air near it towards it's left and thus produces a space of very low pressure on it's right side. The air layers on the right side of the strip expand in this region thus forming the rarefied air layers. This region of low pressure is called the rarefaction R.

When the strip returns from c to it's normal position a, it pushes the rarefaction R forward and the air layers near the strip again pass through their mean positions due to the elasticity of the medium.

In this manner, as the strip moves to the right and left repeatedly, the compressions and rarefactions regions are produced one after the other which carry the disturbance with it with a definite speed depending on the nature of the medium. Gradually due to friction, the strip losses its energy to the medium and the disturbance dies out.

One complete to and fro motion of the strip forms one compression and one rarefaction which together constitute one longitudinal wave. This is how sound waves propagate through the formation of a longitudinal wave when source vibrates in air.

Question 4

Define the following terms:

(a) Amplitude

(b) Frequency

(c) Time period

Answer

(a) Amplitude (a) — The maximum displacement of a particle of medium on either side of its mean position, is called the amplitude of wave.

(b) Frequency (f) — The number of vibrations produced by a particle of the medium in one second is called the frequency of the wave.

(c) Time period (T) — The time taken by a particle of medium to complete its one vibration is called the time period of the wave.

Question 5

Obtain relationship between the time period and frequency.

Answer

If T is the time period of a wave, then by definition

In time T, the number of waves = 1

∴ In 1 second, number of waves (or frequency) =

$f = \dfrac{\text{1}}{\text{T}}$

Thus,

$\text{frequency} = \dfrac{\text{1}}{\text{Time period}}$

or

$\text{time period} = \dfrac{\text{1}}{\text{frequency}}$

Question 6

Name the three characteristics of a musical sound.

Answer

The three characteristics of a musical sound are:

1. Loudness
2. Pitch (or shrillness) and
3. Quality (or timbre or wave form).

Question 7

Which of the following determines the loudness of a sound wave?

(a) Wavelength

(b) Frequency or

(c) Amplitude.

Answer

Amplitude determines the loudness of a sound wave.

Question 8

How is loudness related to the amplitude of a wave?

Answer

Loudness is directly proportional to the square of amplitude of wave.

Loudness ∝ (amplitude)²

Hence, greater the amplitude of vibrations, louder is the sound produced.

Question 9

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

Answer

The loudness (L) of a sound is directly proportional to the square of amplitude of wave.

Loudness ∝ (amplitude)²

L = (2)² = 4 times.

So, if the amplitude (a) of a wave is doubled, loudness will be four times.

Question 10

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

Answer

Loudness is the characteristic by virtue of which a loud sound can be distinguished from a faint one, both having the same pitch and quality.

The figure below shows how a wave pattern of a loud note differ from the soft note.

Question 11

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

Answer

The unit in which loudness of sound is expressed is decibel (dB).

Question 12

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

Answer

Larger the surface area of the vibrating body, louder is the sound heard as a large vibrating area sends forth a greater amount of energy. By mounting a wire on a sound board, the surface area increases which in turn increases the loudness of the sound produced.

Question 13

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

Answer

The three factors on which loudness of sound heard by a listener depends are:

1. Amplitude of wave.
2. Distance of source of the sound.
3. Surface area of the vibrating body.

Question 14

What determines the pitch of a sound?

Answer

Frequency determines the pitch of a sound. Higher the frequency, higher is the pitch and sound produced is shrill. Lower the frequency, lower is the pitch and sound produced is flat.

Question 15

Name the characteristic of sound related to its frequency.

Answer

The characteristic of sound related to its frequency is pitch.

Question 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.

Answer

Pitch of sound enables to distinguish two sounds of same loudness, but of different frequencies, given by the same instrument.
Pitch is the characteristic of sound which distinguishes a shrill sound from a flat sound. Pitch of a sound depends on the frequency of the vibrating body.

Question 17

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

Answer

Wave patterns of high pitch note and low pitch note of the same loudness are shown in the diagram below:

Question 18

Standing at a distance, how is it possible to detect the filling of a bucket under a water tap by hearing the sound?

Answer

As the bucket fills up with water, the length of air column decreases, so the frequency of sound produced increases. Pitch or shrillness of sound will increase with increase in frequency of sound. So with increase in shrillness of sound we can detect the filling of a bucket under water tap.

Question 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?

Answer

Trumpet has the highest pitch because it has the highest frequency i.e. 500 Hz.

Pitch of a sound depends on its frequency. More the frequency of the note, higher is its pitch.

Question 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 in them?

Answer

Jar B will produce sound of higher pitch when air is blown in it because jar B has less air column above water. Frequency of sound produced increases with decrease in the length of air column. Pitch or shrillness of sound increases with increase in frequency of sound.

Question 21

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

Answer

When two identical guitars are played by two persons to give notes of the same loudness and pitch then they will not differ in quality as the identical guitars will produce identical wave forms.

Quality of a musical instrument depends on the number of subsidiary notes and their relative amplitudes present in it along with the principal note.

As same instruments produce same principal and subsidiary notes. Hence, the quality of identical guitars will be same.

Question 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?

Answer

(a) Loudness will be same for both the instruments because they have same amplitude.

(b) Pitch will be same for both the instruments as they have same frequencies.

(c) Quality will be different as they have different wave forms as seen in the diagram of the given wave patterns.

Question 23

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

Answer

Timbre or quality of sound makes it possible to recognize a person by his voice without seeing him. The vibrations produced by the vocal cord of each person have a characteristic wave form which is different for different persons.

Question 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.

Answer

(a) Frequency determines the pitch of a note.

(b) Amplitude determines the loudness of the sound heard.

(c) Wave form determines the quality of the note.

Question 25

Name the characteristic of the sound affected due to a change in its:

(a) amplitude

(b) wave form

(c) frequency

Answer

(a) Loudness.

(b) Quality.

(c) Pitch.

Question 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.

Answer

(a) Note from a musical instrument is shown by figure (d) because the sound produced by the musical instruments comprises of different frequencies and amplitudes resulting in a mixed type of wave form.

(b) Soft note is shown by figure (a) because this wave has low amplitude.

(c) Shrill note is shown by figure (c) because this wave has high frequency.

Question 27

How is the pitch of a sound in a guitar changed if:

(a) a thin wire is used

(b) a wire under less tension is used?

Answer

(a) Pitch of sound will be higher if a thin wire is used in a guitar.
Thin wires in stringed musical instruments produce sound with higher frequency and hence higher pitch.

(b) Pitch of sound will be low if a wire under less tension is used in guitar.
In stringed musical instruments, wires under less tension produce sound with lower frequency and hence lower pitch.

Numericals

Question 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?

Answer

(i) Loudness (L) ∝ [amplitude (a)]²

Ratio of loudness =

$\dfrac{\text{L}_1}{\text{L}_2} = \dfrac{(\text{a}_1)^2}{(\text{a}_2)^2} = \dfrac{(1)^2}{(3)^2} = \dfrac{1}{9}$

So L₁ : L₂ = 1 : 9.

(ii) Frequency remains the same if the pitch remains unchanged.

$\dfrac{\text{pitch of 1st wave}}{\text{pitch of 2nd wave}} = \dfrac{\text{1}}{\text{1}} = 1:1$

So pitch will be 1 : 1.

Question 2

Two waves have frequencies 256 Hz and 512 Hz, but same amplitude. Compare their:

(i) loudness, and

(ii) pitch.

Answer

(i) Since amplitude of two waves are same so loudness of both the waves will be same.

$\dfrac{\text{amplitude of 1st wave}}{\text{amplitude of 2nd wave}} = \dfrac{\text{1}}{\text{1}} = 1:1$

So, loudness of first wave : loudness of second wave = 1:1.

(ii) Frequency determines the pitch of the sound.

$\dfrac{\text{frequency of 1st wave}}{\text{frequency of 2nd wave}} = \dfrac{\text{256}}{\text{512}} = \dfrac{\text{1}}{\text{2}} = 1:2$

So pitch of first wave : pitch of second wave = 1 : 2.

Question 3

Two waves have the same pitch but their amplitudes are in the ratio of 1 : 2. Draw a figure to show the two waves.

Answer

In figure (a) the amplitude of wave A is 2 m and in figure (b) the amplitude of wave B is 4 m. But both the waves have same frequency (pitch).

Ratio of amplitude of wave A to wave B = 2 : 4 = 1 : 2.

Question 4

Two waves of the same amplitude have frequencies 256 Hz and 512 Hz respectively. Represent the two waves in the graphical form.

Answer

The two waves with frequencies 256 Hz and 512 Hz are represented in graphical form below :