Aarohi mishra 8th red

Chapter 7: Sound
17-22 minutes

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.

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.

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.

The unit of loudness is:

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

Answer

decibel

Reason — The unit of loudness of sound is decibel.

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.

By reducing the amplitude of a sound wave, its:

1. pitch decreases
2. loudness decreases
3. loudness increases
4. pitch increases

Answer

loudness decreases

Reason — Loudness is proportional to the square of the

amplitude i.e.,

Loudness ∝ (amplitude)2

So, when a body vibrates with a lesser amplitude, it sends

forth a smaller amount of energy.

Hence, the energy received by the ear drum is also small, so

the loudness of the sound decreases.

Pitch of a note depends on :

1. amplitude
2. loudness
3. frequency
4. waveform

Answer
frequency

Reason — The pitch of a sound depends on the frequency

of the vibrating body. A sound of high frequency is said to
have a high pitch, while a sound of low frequency is said to
have a low pitch.

Assertion (A) : To make a shrill sound in a flute, holes are

opened.

Reason (R) : The holes are opened to increase the length of

the vibrating air column.

1. Both A and R are true and R is the correct explanation of

A
2. Both A and R are true and R is not the correct
explanation of A
3. Assertion is true but reason is false
4. Assertion is false but reason is true

Answer

Assertion is true but reason is false

Explanation

Assertion (A) is true because to increase the pitch (or make

the sound shrill) in a flute, the holes are opened so as to
reduce the length of the vibrating air column.

Reason (R) is false because in a flute, holes are opened to

reduce the length of the vibrating air column and vice versa.

Assertion (A) : Loudness and quality of sound are

independent of each other.

Reason (R) : Loudness depends on amplitude while quality

of sound depends on the waveform of sound.

1. Both A and R are true and R is the correct explanation of

A
2. Both A and R are true and R is not the correct
explanation of A
3. Assertion is true but reason is false
4. Assertion is false but reason is true

Answer

Both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because loudness and quality (or

timbre) of sound are independent properties and changing
one does not necessarily affect the other.
Reason (R) is true because loudness depends on the
amplitude of the sound wave such that greater amplitude
means louder sound and quality depends on the waveform
or the shape of the sound wave, which is influenced by the
mixture of frequencies.

So here reason correctly explains why the loudness and the

quality of sound are independent of each other.

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) The time period of a wave is 2 s. Its frequency is 0.5

hertz.

(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.

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.

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

Answer

Column A Column B
(a) Amplitude (iii) maximum displacement on either side
(b) Frequency (v) 1/Time period
(c) Loudness (ii) Amplitude
(d) Pitch (i) frequency
(e) Wave form (iv) presence of other frequencies

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.

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.

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.

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) =

1
f=
T
Thus,

1
frequency =
Time period
or

1
time period =
frequency
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).

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.

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.

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.

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

Answer

The unit in which loudness of sound is expressed is decibel

(dB).

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.

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.

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.

Name the characteristic of sound related to its frequency.

Answer

The characteristic of sound related to its frequency is pitch.

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.

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.

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.

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.

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.

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.

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.

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:

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.

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.

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.

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.

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.

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.
What happens when a lighted candle is placed in front of a
loudspeaker making a loud and steady noise. Give reason.

Answer

When a lighted candle is placed in front of a loudspeaker

producing a loud and steady noise, the flame of the candle
flickers.
A loudspeaker produces sound waves, which are
longitudinal pressure waves in the air. These waves consist
of alternating regions of compression and rarefaction that
move outward from the speaker, causing the air near the
loudspeaker to vibrate rapidly back and forth. The resulting
air currents disturb the flame, making it flicker in step with
the pressure variations of the sound waves.

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 =

L1 (a1 )2 (1)2 1
= = =
L2 (a2 )2 (3)2 9

So L1 : L2 = 1 : 9.

(ii) Frequency remains the same if the pitch remains

unchanged.

pitch of 1st wave 1

= = 1: 1
pitch of 2nd wave 1
So pitch will be 1 : 1.

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.

amplitude of 1st wave 1

= = 1: 1
amplitude of 2nd wave 1
So, loudness of first wave : loudness of second wave =
1:1.

(ii) Frequency determines the pitch of the sound.

frequency of 1st wave 256 1

= = = 1: 2
frequency of 2nd wave 512 2
So pitch of first wave : pitch of second wave = 1 : 2.

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.

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 :

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