# WBBSE Class 9 Physical Science Solutions Chapter 7 Sound

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 7 Sound offer valuable context and analysis.

## WBBSE Class 9 Physical Science Chapter 7 Question Answer – Sound

Question 1.
How is the sound produced when we speak ?
Sound is produced due to vibration of our vocal chord.

Question 2.
How does the velocity of sound depend on pressure ?
Velocity of sound is independent of change of pressure. Question 3.
What is musical sound ?
It is a pleasing sound produced by a periodically vibrating body.

Question 4.
How does intensity of sound depend on density of medium ?
Intensity of sound increases in a denser medium than in a rarer medium.

Question 5.
What is note of sound ?
It is a sound of mixed frequencies.

Question 6.
What are the characteristics of a musical sound ?
Characteristics of a musical sound are, it has

• Intensity
• Pitch
• Quality or Timbre

Question 7.
Which characteristic of musical sound depends on frequency ?
Pitch depends on frequency. Question 8.
Find the optical analogous of note and tone,
Note is analogous to white or polychromatic light and tone is analogous to light of a single colour.

Question 9.
What is fundamental tone ?
In a note, the sound of least frequency is known the fundamental tone.

Question 10.
What should be the frequency of audible sound ?
We can hear sound only if the number of vibrations per second, called, frequency of a vibrating body lies within the limit 20 to 20,000.

Question 11.
Between a plane mirror and a wail of house, which one will reflect sound?
A wall of house will reflect sound.

Question 12.
What is the minimum distance of the reflector to produce an echo ?
The minimum distance of the reflector is 16.6 m to produce an echo.

Question 13.
What is Hertz ?
The unit, used at present to express frequency is Hertz (Hz) named after the celebrated German physicist Heinrich Hertz.

Question 14.
What is decibel ?
Decibel (dB) is the unit of expressing intensity of sound.

Question 15.
The oscillation of a pendulum cannot be heard. Why ?
The oscillation of a pendulum has a frequency less than 20 Hz. So, we cannot hear the sound of Oscillation. Question 16.
State the relation among velocity, wavelength and frequency of a wave.
Wave velocity = Frequency x Wavelength

Question 17.
What is the unit of frequency ?
Unit of frequency is cycles/second or CPS or Hz.

Question 18.
What is the unit of wavelength in SI system ?
The unit of wavelength in SI system is metre.

Question 19.
What is the velocity of sound at 0°C ?
332 m/sec.

Question 20.
Is medium required for the propagation of sound ?
Medium is required for the propagation of sound.

Question 21.
Which is responsible for the generation of sound ?
Vibration is responsible for the generation of sound. Question 22.
How does pitch depend on frequency ?
Greater the frequency, higher will be the pitch.

Question 23.
Can sound travel through vacuum ?
Sound cannot travel through vacuum.

Question 24.
How many frequencies are there in a tone ?
A tone is a sound of single frequency.

Question 25.
What is called fundamental tone ?
In a note the tone of lowest frequency is called the fundamental tone.

Short Answers type Questions Marks for each 2

Question 1.
What is the definition of sound ?
Sound: Sound is a type of energy, which is generated from a vibrating body, and which travels through an elastic medium and finally reaches our ear to create a special sensation in the brain.

Question 2.
How is the sound produced ?
The material bodies capable of producing sound are called the sources of sound or sounding bodies. Sound is produced due to vibrations of the sounding bodies.

Question 3.
What is ultrasonic sound ?
Ultrasonic sound : We cannot hear sound generated from the sources having frequency 20,000/sec or more. These sounds are known as ultrasonic sound.

Question 4.
What is subsonic sound ?
Subsonic sound : We cannot hear the sound generated from the sources having frequency 20/sec or less. These sounds are known as subsonic sound.

Question 5.
What is audible sound ?
Audible sound : We can hear only the sounds of the sources producing about 20 to 20,000 vibrations per second. These sounds are called audible sound.

Question 6.
Can sound travel through vacuum ?
Explanation : For propagation of sound, a material medium (solid, liquid or gas) is necessary. Sound cannot travel through vacuum.

Question 7.
If a person talks or even screams on the surface of the moon, the sound will not be audible. Explain why.
Explanation : The moon has no atmosphere or gas surrounding the moon. It is absolutely vacuum all around the moon. That is why if a person talks or even screams on the surface of the moon, the sound will not be audible.

Question 8.
Define wave.
Definition of wave : It is defined as a disturbance that moves through a medium transferring energy from a point to another point without any physical transportation of the material between the points.

Question 9.
What is transverse wave ?
Transverse wave: Transverse wave motion is that wave motion in which the individual particles of the medium execute vibrations about their mean positions in a direction perpendicular to the direction of motion of the wave. Question 10.
What is longitudinal wave ?
Longitudinal wave : Longitudinal wave motion is that wave motion in which the individual particles of the medium execute vibrations about their mean positions along the direction of motion of wave.

Question 11.
What do you mean by oscillation ?
Oscillation: An oscillation of a particle is its trajectory in which the particle starting from a certain position moves through some distance and retracing the path it returns to its starting point with the same direction of motion with which it started initially. Question 12.
What do you mean by amplitude ?
Amplitude: The maximum displacement of a particle in a medium on either side of its mean position is called amplitude.

Question 13.
What is time period ?
Time period : The time required by a particle to make one complete oscillation is known as time period. It is denoted by T.

Question 14.
What is frequency ?
Frequency : The number of complete oscillations of a source of sound per second is known as its frequency.
Unit of frequency is Hertz, its symbol is Hz.

Question 15.
What do you mean by wavelength ?
Wave length :

• It is the distance a wave or an energy travels through during one complete oscillation of its source.
• It is the distance between the two consecutive crests or troughs in a wave.
It is normally represented by A. Its SI unit is metre (m).

Question 16.
What is wave velocity ?
Wave velocity : In a medium, the distance travelled by the wave in one second is defined as the velocity of propagation of the wave or wave velocity. It is normally represented by c.

Question 17.
Establish a relation connecting wave velocity, wavelength and frequecy.
Relation among wave velocity, wavelength and frequency :
Let the frequency and wavelength of a wave be v and λ respectively.
According to the definition :
‘v’ numbers of complete waves are generated in 1 sec. Since wavelength is λ, so in 1 sec, wave advances by v x λ = vλ distance.
Now, the distance traversed by wave in 1 sec is its velocity (v).
So,v = vλ
∴ Wave velocity = Frequency x Wavelength
This equation is applicable for all types of waves.

Question 18.
What do you mean by free or natural vibration ?
Free or Natural vibration : When a body vibrates with its own natural frequency, it is said to execute natural or free vibrations. The frequency of free vibration depends on the density, shape and elasticity of the body.
Example : The frequency of a tuning fork depends on the length and cross-section of its arms as well as density and elasticity of the material of tuning fork.

Question 19.
What is called forced vibration ?
Forced vibration: When a body is maintained in a state of vibration by a strong periodic force, the vibration is called forced vibration.

Example : Intensity of sound generated from a tuning fork is not very high and hence cannot be heard from a distance. Now when this tuning fork is touched on a table, it causes forced vibration on the table and thereby sound is generated from the table also. Intensity of this sound is relatively high and hence can be heard from a distant location.

Question 20.
What is called resonant vibration ?
Resonant vibration: It is a particular case of forced vibration when the frequency of the driving force is equal to the natural frequency of the body. When such condition is fulfilled amplitude of oscillation becomes very high.
Example: A vibration is generated in the air column. When a vibrating tuning fork is placed on the mouth of a pipe filled with air, a shrill sound will be produced. Air being forced vibrated creates this sound.

Question 21.
What is meant by reflection of sound ?
Reflection of sound : When sound waves hit on the boundary separating two homogeneous media, a portion of sound changes its direction from the surface of separation and returns to the first medium. This is known as reflection of sound. Question 22.
State the laws of reflection of sound.
Laws of reflection of sound :

• The incident sound, reflected sound and the normal on the surface of separation through the point of incidence remain on the same plane.
• The angle of incidence is equal to the angle of reflection.

Question 23.
What is mach number ?
Mach number: The ratio of speed of a body to the speed of sound is called the mach number of the body. If the mach number of a body be greater than 1, then the velocity of the body becomes supersonic velocity.

Question 24.
How does speaking tube operate ?
Speaking tube : If we talk at one end of a long tube, it can be heard clearly on the other end. The sound generated at one end does not get scattered. On the contrary, it undergoes several reflections within the tube and finally reaches the other end. In this way, a speaking tube operates.

Question 25.
How does a stethoscope function ?
Stethoscope : Doctors use this gadget for chest examination. This is basically a reflecting tube. This gadget consists of two rubber tubes, with a metallic plate at the junction of the tube. The sound of the heart beat reaches from the metallic plate to the ears of the doctor through multiple reflections of the sound within the rubber tubes.

Question 26.
What is echo ? What is the condition for its occurrence ?

• Echo : When a sound after reflection from some reflector is again heard separately from the original sound, then this reflected sound is known as an echo.
• Condition : For occurrence of an echo the minimum distance between the source of sound and a reflector of sound producing echo should be arround 16.6 m.

Question 27.
Deduce the minimum distance of a reflector that can produce echo of a transient sound.
Minimum distance of the reflector for listening the echo of a transient sound : The sensation of a transient sound remains in our brain is $$\frac{1}{10}$$ sec. after it reaches our ear. This period is known as persistence of hearing. If a second sound reaches our brain within this sec. of time, the later one cannot be identified by the brain separately. Obviously a gap of at least sec. $$\frac{1}{10}$$ should be there between the original sound and reflected sound so that the brain can recognize them separately.

Now, if the velocity of sound in air be 332 m/sec, $$\frac{1}{10}$$ in sec. it travels through 33.2 m. For producing an echo, the total length of path traversed by a sound from the source to the reflector and back should be at least 33.2 m. Hence for hearing echo of a transient sound, the minimum distance of the reflector from the source of sound is half of 33.2 m i.e. 16.6 m.

Question 28.
Give two applications of echo.
Applications of echo :

• Determination of velocity of sound in air with the help of stopwatch and a pistol.
• Depth of sea, height of an aeroplane above ground may also be determined with the help of echo.

Question 29.
What is musical sound ?
Musical sound : A continuous sound pleasing to the ear and produced by regular and periodic vibrations of a source is known as musical sound.
Example : Sounds generated from different musical instruments like violin, piano, flute etc. are musical sounds.

Question 30.
What is noise ?
Noise : A noise is a sound which is discontinuous, unpleasant to the ear and produced by irregular and non-periodic vibrations of a source.
Example : The sounds of moving cars, trains, machines, sound of explosions etc. are the examples of noise.

Question 31.
What do you mean by tone ?
Tone : A musical sound having a single frequency is known as tone.
Example : The sound generated from a tuning fork has a unique frequency. Hence the sound generated from a tuning fork is a tone.

Question 32.
What is meant by Note ?
Note: A musical sound however generally consists of number of components having different frequencies. Such a sound is known as note.

Question 33.
What is fundamental tone and what is overtone ?
Fundamental tone : In a note the tone of lowest frequency is known as fundamental tone.
Overtone : All tones, except fundamental tone, present in a note are known as overtones.

Question 34.
What is harmonics and what is an octave ?

• Harmonics : Those overtones whose frequencies are integral multiples of the frequency of the fundamental tone are called harmonics.
• Octave : The particular harmonic whose frequency is double the fundamental frequency is known as the second harmonic or octave of the fundamental tone.

Question 35.
What are the characteristics of musical sound ?
Characteristics of musical sound: Musical sound has three important characteristics. These are :

• Intensity
• Pitch and
• Quality or timbre

Question 36.
What is intensity of a musical sound ?
Intensity : It means how forcefully or how strongly the sound reaches to a listener. Intensity or loudness of a sound depends on the energy contained per unit volume of the medium through which sound passes.

Question 37.
What are the factors which affect intensity of a musical sound ?
The intensity of a musical sound depends upon the following factors :

• Amplitude of the vibration of the source
• Size of the sounding body
• Distance of the observer from the source
• Presence of other bodies
• Density of the medium.

Question 38.
What is pitch ? On what factory pitch of a note depends ?

• Pitch: The characteristic of a musical sound by which a sharp or a shrill sound is distinguished from a dull, flat sound of the same intensity is known as its pitch.
• Pitch of a note depends: Pitch of a note depends on its frequency. Greater the frequency, higher will be the pitch. Wavelength being inversely proportional to frequency, the sound of low wavelength has higher pitch and vice versa.

Question 39.
What is quality of a musical sound ?
Quality of a musical sound : It is the characteristic of a musical sound which enables us to distinguish one sound from another even though the two sounds may have the same intensity or pitch. Richer the quality, more pleasant is the sound.
Example : If harmonium and violin are played simultaneously, one can easily distinguish between these two even without seeing. This distinction is felt out of the property called quality of sound.

Question 40.
What is sound pollution ?
Sound pollution : Any unpleasant and unwanted sound is noise. Sound pollution means creation of discomfort, disturbance and irritation which result to ill effects to mental and physical health.

Question 41.
What are the sources of sound pollution ?
Sources of sound pollution :

• The sound of vehicle causes sound pollution.
• The machinery used in industries creates sound of intensity more than tolerance limit and thereby causes sound pollution.
• Loudspeakers, amplifiers, air horn played at top volume are major sources of sound pollution.
• The sound crackers cracked in different functions e.g. Puja ceremony, Marriage ceremony etc. causes sound pollution.

Question 42.
What are the harmful effects of sound pollution ?
Harmful effects of noise pollution :

• Sound pollution may cause hinderance in the growth of nervous system of a child in the womb.
• One may become deaf due to sound pollution.
• The sound of high intensity and pitch may cause blood pressure, nervous breakdown, heart disease, loss of memory, loss of concentration etc.

Question 43.
What are the remedial measures of sound pollution ?
Remedial measures of sound pollution :

• Factory should be prohibited in residential area.
• Sound resistant measures may be taken in industry.
• Unnecessary use of horn in the vicinity of hospital, school etc should be restricted.
• Airport should be away from residential area.
• Sound pollution awareness programme should be taken up at regular interval.

Question 44.
Why does the sound of explosion occuring in the sun not reach the earth ?
Explanation: The continuous explosive sounds occuring in the sun because of conversion of hydrogen to helium by nuclear fusion is not heard from the earth. This is due to the presence of a large space beyond the earth’s atmosphere without any material medium. The sound of explosion cannot transmit to the earth since sound cannot propagate without the help of a material medium.

Question 45.
Why it is not advisable to regulate a watch on hearing the sound of a distant siren ?
Explanation: The sound from the siren at a distance takes some time to reach a place. If the person adjusts his watch with the idea that the sound of the siren reaches him just at the time of its production, his watch will be slow by a few seconds that is taken by the sound to reach him.

Question 46.
Why are voices of females or babies more sharp than that of a grown up male person ?
Explanation : Vocal cords of females or babies are tender, so, while they speak or sing the cords vibrate with high frequency, for this reason their voices are very sharp. Vocal cords of male persons are usually stiff, so their frequency of vibration is low and hence male voices are generally flat.

Question 1.
Why sound waves are called elastic waves ?
Reason :

• Sound waves require some time to travel from one place to another. Just like on elastic wave.
• Vibration of sources are necessary for the production of elastic waves.
Sound waves also require vibration of the sources for their production.
• The particles of the medium through which elastic waves pass, are not bodily moved when the waves pass through then. The same thing happens for the sound waves.
• Like all other elastic waves, sound waves require a material medium for their propagation.
• Sound waves like all other elastic waves undergo reflection, refraction and interference.

Question 2.
Describe in brief an experiment which shows that a material medium is necessary for propagation of sound.
Experiment to prove
the necessity of material medium for propagation of sound :
Experimental setup: A glass jar is placed in airtight condition on a platform provided with a suction pump. An electric bell kept suspended inside the jar can be operated from outside with the help of a battery.

Experiment : The electric bell is set into operation. Air from inside the jar is gradually drawn out.

Observation : A sound of the bell is heard when the jar is full of air but the sound gradually fades away on withdrawal of air. The sound is again heard clearly when air is re-introduced.

Inference : Since sound from the electric bell is not heard in absence of air within the jar, it proves that sound from a source cannot propagate in absence of a material medium. Numerical Problems :

Working formula]
(i) υ = vλ (υ = wave velocity,
v = frequency, λ = wavelength)

(ii) s = υ x t (s = Distance,
υ = velocity of sound
t = time)

(iii) υt = ν0 (1 + 0.00 183t)
ν>0 = velocity of sound at 0°C
υt= velocity of sound at t°C)

(iv) Height of an aeroplane above ground (h) = $$\frac{t}{2} \sqrt{v^2-u^2}$$
(υ = velocity of sound
u = velocity of aeroplane
t = time interval of sound and echo)

(v) $$v=\frac{s}{2}\left(\frac{1}{t_1}+\frac{1}{t_2}\right)$$
(v = velocity of sound
s = distance, t1 initial time t2 final time)

Problem 1:
The velocity of sound in air is 350 m/s. Frequency of a tuning fork is 700/s. Find the wavelength of the sound generated by the tuning fork.
We know,
n = 700sec
λ = ?
y = 350 m/sec.
υ = vλ
Or, λ — $$\frac{v}{v}$$
= $$\frac{350}{700}$$=0.5m

Problem 2:
The frequency of a tuning fork is 440 Hz. If the wave length of the wave is 0.73 m then find the velocity of sound.
v = 440 Hz
λ = 0.73m
υ =?
We know,
υ = vλ
or, v = 440 x 0.73
= 321.2 m/s

Problem 3:
A boat approaches a high vertical cliff. When the anchor is dropped, an echo is heard 2s later. How far is the boat from the cliff? (velocity of sound = 332 m/s)
As the velocity of sound is 332 m/s so total distance travelled by sound in 2 s is = 2 x 332 = 664 m.
For an echo, the sound must go to the cliff and return through the same distance.
So the distance of the boat from the cliff = $$\frac{664 \mathrm{~m}}{2}=332 \mathrm{~m}$$

Problem 4 :
In measuring the depth of an ocean, it was found that the sound of an explosion and its echo by the bed had a time interval of 20s. between them
As the velocity of sound in water = 1436 m/s, so total distance
travelled by the sound in 20s = 20 x 1436 m
The depth of the ocean = $$=\frac{20 \times 1436}{2} \mathrm{~m}$$
= 14360m

Problem 5 :
Find the frequency of a tuning fork whose wave length is 17m.? (velocity of sound in air = 340 m/s)
υ= 340m/s
λ = 1.7m
v = ?
We know,
υ = vλ
or $$v=\frac{v}{\lambda}$$
= $$\frac{340}{1.7}$$
= 200 Hz
∴ Frequency of the tuning fork = 200 Hz

Problem 6 :
Two persons A and B are standing 1 km apart in open air. ‘A’ fires a gun and ‘B’ will receive the sound after 3 second of the flash. Calculate the velocity of sound in air.
s = 1km = 1000m
t = 3
v = ?
We know,
$$v=\frac{s}{t}$$
or $$v=\frac{1000}{3}$$
∴ velocity of sound in open air = 333.3 m/s

Problem 7.
What should be the minimum distance of the reflector from the source of sound to hear the echo of the word ‘Beautiful’? (velocity of sound = 340 m/s)
The word ‘beautiful’ is a trisyllabic one. So to hear it distinctly, one would require $$\left(3 \times \frac{1}{5}\right)$$  s = $$\frac{3}{5}$$ = s to pronounce it.
In $$\frac{3}{5}$$ sec sound travels $$\left(340 \times \frac{3}{5}\right)$$ m = 204m
= $$\frac{204 \mathrm{~m}}{2}$$ = 102m