Exercise-7a
Question 1. What are mechanical waves?
Solution:
The waves that need a material medium to transmit energy are known as mechanical waves.
Question 2. (i) Define amplitude term in relation to a wave.
(ii) Define frequency term in relation to a wave.
(iii) Define wavelength term in relation to a wave.
(iv) Define wave velocity term in relation to a wave.
Solution:
(i) Amplitude: The amplitude of a wave is the largest displacement of a medium particle on either side of its mean location. The metre is its S.I. unit (m).
(ii) Frequency: The frequency of the waves is the quantity of vibrations produced by a wave-producing particle of the medium in a second.
It may also be thought of as the quantity of waves that travel through a spot in a second. Hertz is its SI unit (Hz).
(iii) Wavelength: The wavelength of a wave is the distance it travels when a medium particle vibrates in one unit of time. The metre is its S.I. unit (m).
(iv) Wave velocity: The wave velocity is the distance a wave travels in one second. It uses meters per second as its SI unit (ms-1).
Question 3. A wave passes from one medium to another medium.
(i) Mention the property of the wave which changes.
(ii) Mention the property of the wave which does not change.
Solution:
(i) When a wave moves from one medium to another, its wavelength (or speed) changes.
(ii) A wave’s frequency does not vary as it travels across different media.
Question 4. State two factors on which the speed of a wave travelling in a medium depends.
Solution:
Two factors on which the speed of a wave travelling in a medium depends are:
(i) Density: The square root of the gas’s density determines how fast travels sound across space.
(ii) Temperature: As the temperature rises, so does the sound speed.
Question 5. State two differences between the light and sound waves
Solution:
(i) Unlike sound waves, which must propagate through a material medium, light waves may move through vacuum.
(ii) Sound waves are mechanical waves, whereas light waves are electromagnetic waves.
Question 6. What do you mean by reflection of sound? State one condition for the reflection of a sound wave Name a device in which reflection of sound wave is used.
Solution:
(1) Sound waves reverberate when they contact an obstruction, much like light rays do.
(2) This is referred to as sound reflection.
(3) Sound is subject to the same laws of reflection that light is.
(4) An echo is a repeating sound that results from a sound wave’s reflection off of an obstruction.
(5) One such tool utilized by those with hearing loss is a hearing aid. Here, sound waves that the hearing aid picks up are reflected into a more confined space that leads to the ear.
Question 7. (i) What is meant by an echo?
(ii) What is the condition necessary for an echo to be heard distinctly?
Solution:
(i) If a person makes a sharp sound while standing some distance away from a wall or a hillside, he or she will hear two distinct sounds: the first is the original sound, which is heard almost instantly, and the second is the echo, which is heard after the wall or hillside has reflected the original sound.
(ii) An echo is a sound that is heard after the original sound hits a solid object. The circumstances are-
a) The reflecting surface ought to have a bigger reflective area than the sound wavelength.
b) The formula specifies that a minimum of 17 meters in the air is required to hear the echo.Vt/2
Question 8. A man is standing at a distance of 12 m from a cliff. Will he be able to hear a clear echo? Give a reason for your answer.
Solution:
The man won’t be able to hear the echo because
1 seconds. This is due to the fact that our ears continue to perceive sound for 0.1 seconds after the stimulating stimulus stops working.
Question 9. State two applications of echo
Solution:
Below are the applications of echo:
(i) Dolphins generate ultrasonic waves and listen for their echoes to identify their adversaries and obstacles.
(ii) The echo technique of ultrasonic waves is utilized in medical science to image human organs including the liver, gall bladder, etc. The term for this is ultrasonography.
Question 10. Explain how the speed of sound can be determined by the method of echo.
Solution:
Sound is generated from a location that is known to be at least 50 meters away from the reflecting surface. A stop watch with the smallest count, 0.01 seconds, is used to record the time period t during which the echo travels to the location from whence the sound was originated. Then the following relationship is used to compute the speed of sound:
Question 11. State the use of echo by a bat, dolphin and fisherman.
Solution:
By producing and delivering ultrasonic waves and hearing and detecting the echoes, bats, dolphins, and fishermen may all discover their adversaries, barriers, or the location of fish.
Question 12. How do bats avoid obstacles in their way, when in flight?
Solution:
Bats are capable of producing and listening to extremely high-frequency sounds up to roughly 1000 kHz. Any obstruction in its path will reflect the noises made by flying bats. Bats can locate obstacles even in the dark by listening for the echoes. So that they may securely fly without running into the obstructions.
Question 13. (i) What is meant by sound ranging?
(ii) Give one use of sound ranging.
Solution:
(i) Sound ranging is the technique for using echo to detect barriers.
(ii) Animals like dolphins and bats utilize it to find their adversaries.
Question 14. Name the waves used for sound ranging. Why are the waves mentioned by you audible to us?
Solution:
The sound ranging process makes use of ultrasonic pulses. Although human hearing can detect frequencies between 20 Hz and 20,000 Hz, ultrasonic waves have a frequency higher than that.
Question 15. What is sonar? State the principle on which it is based.
Solution:
Sonar provides accurate range and navigation. The spacecraft emits ultrasonic waves in all directions, and they are picked up upon return after being reflected by the obstructions. They employ the echo technique.
Question 16. State the use of echo in medical science.
Solution:
Ultrasonography is the term used in medical science to describe the echo technique of using ultrasonic waves for examining human organs including the liver, gall bladder, uterus, and womb.
Exercise-7a
Multiple Choice Types:-
Question 1. The minimum distance between the source and the reflector in air So that an echo is heard is approximately equal to:
a) 10 m
b) 17 m
c) 34 m
d) 50 m
Solution: 17 m
Question 2. To detect the obstacles in their path, bats produce:
a) Infrasonic waves
b) Ultrasonic waves
c) Electromagnetic waves
d) Radio waves
Solution: b) Ultrasonic waves
Exercise-7a
Numerical
Question 1. The wavelength of waves produces on the surface of water is 20 cm. If the wave velocity is 24 ms-1, Calculate:
(i) The number of waves produces in one second
(ii) The time in which one wave is produced.
Solution:
Question 2. Calculate the minimum distance in air required between the source of sound and the obstacle to hear an echo. Take speed of sound in air = 350 m s-1
Solution:
Question 3. What should be the minimum distance between source and reflector in water so that echo is heard distinctly? (The speed of sound in water = 1400 m s-1)
Solution:
Question 4. A man standing 25 m away from a wall produces a sound and receives the reflected sound.
(a) Calculate the time after which he receives the reflected sound if the speed of the sound in air is 350 ms-1.
(b) Will the man be able to hear a distinct echo? Explain the answer.
Solution:
(b) Yes, because the reflected sound reaches the man 0.1 seconds after the original sound is heard and the original sound persists only for 0.1 seconds.
Question 5. Radar sends a signal to an aeroplane at a distance 45 km away with a speed of 3×108 ms–1. After how much time is the signal received back from the aeroplane?
Solution:
Speed of sound transmitted from RADAR = 3×108 m/s
Distance at which the signal is transmitted = 300km = 300000m
Since, the signal travels twice that is, when transmitted
And when it is being received by the RADAR, the distance is calculated twice
Total distance the signal travels = 2×300000m
Time taken to receive the signal back after reflecting from the aeroplane
Question 6. A man standing 48 m away from a wall fires a gun calculate the time after which an echo is heard. (The speed of sound in air is 320 m s-1).
Solution:
Question 7. A ship on the surface of water sends a signal and receives it back from a submarine inside water after 4 s. Calculate the distance of the submarine from the ship. (The speed of sound in water is 1450 ms-1)
Solution:
Question 8. A pendulum has a frequency of 5 vibrations per second. An observer starts the pendulum and fires a gun simultaneously. He hears echo from the cliff after 8 vibrations of the pendulum. If the velocity of sound in air is 340 ms-1, find the distance between the cliff and the observer.
Solution:
It is given that,
5 vibrations by pendulum in 1 sec
Question 9. A person standing between the two vertical cliffs produces a sound. Two successive echoes are heard at 4s and 6s. Calculate the distance between the cliffs. (Speed of sound in air = 320 ms-1)
Solution:
The distance of first cliff from the person,
2D1=velocity × time
D2 = 960m
Distance between cliffs = D1 + D2
Distance between cliffs = 640 + 960
Distance between cliffs = 1600 m
Question 10. A person standing at a distance x in front of a cliff fires a gun. Another person B standing behind the person A at a distance y from the cliff hears two sounds of the fired shots after 2s and 3s respectively. Calculate x and y(take speed of sound 320m/s).
Solution:
The person B hears two of the fired shots; the first one is direct from the gun while other sound comes after reflection from the cliff
Speed of sound 320m/s
Time taken by the sound to reach from A to B directly
y + x = 960____________(2)
Adding equation (1) and (2)
y – x + y + x = 640 + 960
2y = 1600
y = 800m
Value of y put in equation (1)
800 – x = 640
-x = 640-800
-x = -160
x = 160 m
Question 11. On sending an ultrasonic wave from a ship towards the bottom of a sea, the time interval between sending the wave and receiving it back is found to be 1.5s. If the velocity of wave in sea water is 1400 ms-1, find the depth of the sea.
Solution:
Question 12. Figure below shows the distance-displacement graph of two waves A and B. Compare (i) the amplitude, (ii) the wavelength of the two waves.
Solution:
Amplitude is the maximum displacement from the mean position. For A the maximum displacement = 10cm and for B the maximum displacement = 5cm.
The ratio of maximum amplitude is A1/A2
The ratio of maximum amplitude = 10/5
The ratio of maximum amplitude = 2:1
Wavelength of A = 8cm
Wavelength of B = 16cm
The ratio of wavelength is λ1/λ2
The ratio of wavelength = 8/16
The ratio of wavelength = 1:2
Exercise-7b
Question 1. What do you understand by free vibrations of a body? Give one example.
Solution:
The term “free vibrations” refers to a body’s vibrations when no outside force is acting on it. For instance, when we press the piano keys, the individual strings begin to vibrate at their natural frequencies.
Question 2. (i) What is meant by the natural frequency of vibration of a body?
(ii) On What factors does the natural frequency of vibration of a body depend?
Solution:
(i) When all vibrating bodies are allowed to move freely and at a frequency of It is the body’s normal vibrational frequency.
(ii) A body’s inherent frequency of vibration is influenced by its size and form.
Question 3. Draw a graph between displacement from mean position and time for a body executing free vibrations in a vacuum.
Solution:
Displacement-time graph for the free vibrations:
Question 4. State one condition for a body to execute free vibrations
Solution:
Only in a vacuum can a body vibrate freely since any other medium would introduce some resistance, causing the vibration’s amplitude to fluctuate and eventually stop growing altogether.
Question 5. (a) Name one factors on which the frequency of sound emitted due to vibrations in an air column depends.
(b) How does the frequency depend on the factor stated in part (a).
Solution:
a) The length of the air column affects the frequency of sound that is produced as a result of air column vibration.
b) The length of the air column is a factor that affects sound frequency. The frequency drops as the air column’s length grows. We may thus say that they are inversely related to one another.
f ∝ l/i
Question 6. State one way of increasing the frequency of a note produced by an air column
Solution:
By shortening the air column, the frequency of the note generated in it may be made more frequent.
Question 7. State two ways of increasing the frequency of vibrations of a stretched string
Solution:
Increasing the tension in the string and shortening the string will both increase the frequency of vibration of the stretched string.
Question 8. How does the frequency of sound given by a stretched string depend on its (a) length, (b) tension?
Solution:
(A) The length of the string has an inverse relationship with the frequency of sound.
f ∝ l/i
(b) The square root of the string’s tension has a direct relationship with the frequency of sound.
f ∝ √T
Question 9. What adjustments would you make for tuning a stringed instrument for it to emit a note of a desired frequency?
Solution:
A stringed instrument has a feature that allows the string tension to be changed. We may get the required frequency by adjusting the tension.
Question 10. The following diagram shows three ways in which the string of an instrument can vibrate.
(a) Which of the diagram shows the principal note?
(b) Which has the frequency four times that of the first?
(c) What is the ratio of the frequency of the vibration in (i) and (ii)?
Solution:
(a) The principal note is depicted in the diagram in (i)
(b) Diagram occurs four times as frequently as the first.
(c) It is 1:2.
Question 11. Explain why strings of different thicknesses are provided on a stringed instrument.
Solution:
Since the natural frequency of vibration of a stretched string is inversely related to the radius (thickness) of the string, different-thickness strings are used on stringed instruments to generate sound waves of various frequencies.
Question 12. A blade, fixed at one end, is made to vibrate by pressing its other end and then releasing it. State one way in which the frequency of vibrations of the blade can be lowered
Solution:
By extending the blade’s length or placing a little weight on the blade’s free end, the frequency of the blade’s vibrations can be reduced.
Question 13. How does the medium affect the amplitude of free vibrations of a body?
Solution:
The medium’s existence provides some resistance to motion, which causes the vibrating body to continually lose energy and thus see a continuous decline in the amplitude of the vibration. Continual, amplitude-decreasing vibrations of a body in the presence
Question 14. What are damped vibrations? How do they differ from free vibrations? Give one example of each.
Solution:
We refer to resistive force as dampened vibrations. The free vibrations’ amplitude stays constant, and they keep vibrating indefinitely. However, as time passes, the amplitude of damped vibrations decreases, and eventually they stop. A thin limb of a tree, for instance, produces dampened vibrations when it is tugged and then released. When a tuning fork vibrates in the air, the vibrations are dampened.
Question 15. The following diagram shows the displacement – time graph of the vibrating body.
(i) Name the kind of vibrations
(ii) Give one example of such vibrations
(iii) Why is the amplitude of vibrations gradually decreasing?
(iv) What happens to the vibrations of the body after some time?
Solution:
(i) Damped vibrations
(ii) As an illustration, consider the damped vibrations produced when a thin limb of a tree is tugged and then let go.
(iii) Because of the frictional (or resistive) force that the surrounding medium applies to the vibrating body, the amplitude of vibrations steadily diminishes. The vibrating body continually loses energy as a result of exerting effort against the force of friction, which results in a reduction in amplitude.
(iv) The body that is vibrating eventually runs out of energy and stops.
Question 16. A tuning fork is set into vibration in air. Name the kind of vibrations it executes.
Solution:
The damped oscillations cause the tuning fork to oscillate.
Question 17. Draw a sketch showing the displacement of a body executing damped vibrations against time.
Solution:
Displacement time graph of damped vibrations
Question 18. (i) What are forced vibrations?
(ii) Give one example to illustrate forced vibrations.
Solution:
(i) The term “forced vibrations” refers to a body’s vibrations that occur as a result of an external periodic force operating on the body.
(ii) Example: When a guitarist plays the guitar, they make the strings vibrate against their will.
Question 19. On keeping the stem of a vibrating tuning fork on the surface of a table, a loud sound is heard. Give reason.
Solution:
Only when a unique case of forced vibration (resonance) has taken place can a loud sound be heard.
Resonance is a loud sound that occurs when the periodic force acting on a body has a frequency equal to the natural frequency of that body.
When the stem of a vibrating tuning fork is placed on the surface of a table, resonance causes a loud sound to be heard.
Question 20. Distinguish between the free (or natural) and forced vibrations.
Solution:
(i) A body’s free vibrations are its vibrations when there is no resistive force acting on it. Forced vibrations are the kind of vibrations that occur when an outside force is present.
(ii) The size and form of the body affects the frequency of vibration in free vibrations. The frequency of forced vibrations is the same as the frequency of the applied force.
Question 21. (i) What is meant by resonance?
(ii) Describe a simple experiment to illustrate the phenomenon of resonance and explain it.
Solution:
(i) A specific instance of forced vibrations is resonance. When a body is subjected to an external periodic force with a frequency equivalent to its natural frequency, the body starts to vibrate more intensely and quickly. Resonance is the name given to this phenomenon.
(ii) Mount the two identical tuning forks A and B, which have the same frequency, such that their open ends are facing one another on the two different sound boxes
The prong A begins to vibrate when tapped on a rubber pad. Tuning fork B begins vibrating and emitting a loud sound when tuning fork A is placed on its sound box. Resonance is what causes the vibrations in B.
Question 22. State the condition for the occurrence of resonance.
Solution:
Condition for resonance: When the frequency of the applied force and the natural frequency of the vibrating body are exactly equivalent, resonance occurs.
Question 23. Complete the following sentence: Resonance is a special case of …………….. Vibrations, when frequency of the driving force is ……… natural frequency of the body.
Solution:
Resonance is a special case of forced vibrations, when frequency of the driving force is equal to the natural frequency of the body.
Question 24. Differentiate between the forced and resonant vibrations.
Solution:
Question 25. Why is a loud sound heard at resonance?
Solution:
The body vibrates loudly during resonance, which transfers more energy to the ears and produces a loud sound.
Question 26. In following figure shows two tuning forks A and B of the same frequency mounted on separate sound boxes with their open ends facing each other. The fork A is set into vibration. (a) Describe your observation. (b) state the principle illustrated by this experiment.
Solution:
(a) The forced vibrations in the sound box’s air column are created by the tuning fork A’s oscillations. Due to the huge air surface area in the sound box, these vibrations have a large amplitude. They are sent to the fork B’s sound box. Beginning to vibrate at the same frequency as the fork A is the air column of B. The fork B takes up these vibrations and begins vibrating as a result of resonance since the frequency of these vibrations is the same as the fork B’s natural frequency.
(b) The second tuning fork B will begin to vibrate after tuning fork A begins to ring. Resonance is what causes the second tuning fork B to vibrate.
Question 27. In following figure shows A, B, C and D are four pendulums suspended from the same elastic string XY. Lengths of pendulum A and D are equal, while the length of pendulum B is smaller and the pendulum C is longer. The pendulum A is set into vibration. (a) What is your observation? (b) Give reason for your observation.
Solution:
(a) Shift the pendulum A to one side in a direction normal to its length to start it vibrating. The pendulum D is shown to begin vibrating initially with tiny amplitude until eventually gaining the same amplitude that pendulum A did. Since the total energy is constant, when the amplitude of pendulum D reaches its highest, the amplitude of pendulum A reaches its minimum. After some time, pendulum D’s amplitude will drop and pendulum A’s amplitude will rise. Because their inherent frequencies are identical, only the pendulums A and D may interchange energy. The amplitudes of the vibrations in the pendulums B and C are quite tiny.
(b) Through XY, the other pendulums B, C, and D receive the forced vibrations generated in pendulum A. While pendulum D enters a state of resonance, pendulums B and C continue to vibrate under tension.
Question 28. A vibrating tuning fork held over an air column of a given length with its one end closed, produces a loud audible sound. Name the phenomenon responsible for it and explain the observation.
Solution:
Resonance is the term for the phenomena that causes a loud audible sound. The forced vibrations in the air column are generated by the tuning fork’s oscillation. You can hear a loud noise for a particular amount of air column. When the tuning fork’s frequency and the air column’s frequency are equivalent, this occurs.
Question 29. In following figure shows A, B, C and D represent test tube each of height 20 cm which are filled with water up to heights of 12 cm, 14 cm, 16cm and 18cm respectively. If a vibrating tuning fork is placed over the mouth if test tube D, a loud sound is heard.
(a) Describe the observations with the tubes A, B and C when the vibrating tuning fork is placed over the mouth of these tubes.
(b) Give the reason for your observation in each case.
(c) State the principle illustrated by the above experiment.
Solution:
(a) While the tubes A and C produce no audible loud noise, the tube B does.
(b) The air column in tube B experiences resonance, whereas the air columns in tubes A and C do not. Because the length of the air column in tube D is 20-18 = 2 cm and that in tube B is 20-14 = 6 cm, the frequency of vibrations of the air column in tube B is the same as the frequency of vibrations of the air column in tube D. (3 times). However, the frequency of the air column vibrations in tubes A and C is not the same as the frequency of the air column vibrations in tube B.
(c) Resonance occurs when the frequency of the vibrating tuning fork and the air column vibrations are equivalent.
Question 30. When a troop crosses a suspension bridge the soldiers are asked to break steps. Explain the reason.
Solution:
The troops must break steps as a troop crosses a suspension bridge. The reason is that forced vibrations of a certain frequency are created in the bridge because as troops march in steps, all of the different periodic pressures they apply are in phase with one another. Now, the bridge will vibrate with huge amplitude owing to resonance and the suspension bridge may collapse if the natural frequency of the bridge occurs to be identical to the frequency of the steps.
Question 31. Why are the stringed instruments like guitar provided with a hollow sound box?
Solution:
The sound box is built such that the natural frequency of the air column inside it is the same as the natural frequency of the stretched strings on it. As a result, when the strings are made to vibrate, the air column inside the box is forced into forced vibrations. The sound box’s huge surface area causes it to vibrate a big volume of air at the same frequency as the string. So a loud sound is generated as a result of resonance.
Question 32. (i) How do you tune your radio set to a particular station?
(ii) Name the phenomenon involved in tuning your radio set to a particular station and defines it
Solution:
(i) To tune a radio receiver, we simply change the electrical components’ values to generate vibrations with a frequency corresponding to the radio waves we want to receive. Due to resonance, the energy of the signal at that particular frequency is picked up by the incoming waves when the two frequencies coincide. The receiver set then amplifies the signal that was received.
(ii)Resonance is the relevant phenomena this is a unique instance of forced vibrations. A body quickly starts to vibrate with greater amplitude when the frequency of externally applied periodic force acting on it is equal to its natural frequency. Resonance is the name for this occurrence.
Exercise-7b
Multiple Choice Types
Question 1. A wire stretched between two fixed supports, is plucked exactly in the middle and then released. It executes (neglect the resistance of the medium):
a) Resonant vibrations
b) Natural vibrations
c) Damped vibrations
d) Forced vibrations
Solution: b) Natural vibrations
Question 2.When a body vibrates under a periodic force, the vibrations of the body are:
a) Natural vibrations
b) Damped vibrations
c) Forced vibration
d) Resonant vibrations
Solution: c) Forced vibrations
Question 3. A tuning fork of frequency 256 Hz will resonate with another tuning fork of frequency:
a) 128 Hz
b) 256 Hz
c) 384 Hz
d) 512 Hz
Solution: b) 256 Hz
Exercise-7c
Question 1. Name three characteristics of a musical sound.
Solution:
The following three characteristics of sound are:
(i) Loudness
(ii) Pitch or shrillness
(iii) Quality or timber
Question 2. (a) Which of the following quality determines the loudness of a sound wave?
(i) Wavelength (ii) frequency and (iii) amplitude
(b) How is loudness related to the quantity mentioned above in part(a)?
Solution:
(a) Amplitude – A wave with larger amplitude produces a louder sound.
(b) Loudness is inversely correlated with amplitude, squared.
Question 3. If the amplitude of a wave is doubled, what will be the effect on its loudness?, how is its loudness affected?
Solution:
Since loudness is inversely proportional to the square of amplitude, the loudness will be four times greater.
Question 4. Two waves of the same pitch have amplitudes in the ratio 1:3.
(i) What will be the ratio of loudness?
(ii) What will be the ratio of frequencies?
Solution:
(i) Ratio of loudness will be 1:9
(ii) The ratio of frequency will be 1:1.
Question 5. How does the wave pattern of a loud note differ from a soft note? Draw a diagram.
Solution:
Question 6. Name the unit in which loudness of sound is measured.
Solution:
Decibel is the unit in which loudness of sound is measured.
Question 7. Why is the loudness of the sound heard by a plucked wire increased when it is mounted on a sound board?
Solution:
Plucking causes the strings to vibrate, and the vibrations are then transmitted to the hollow sound box. As a result, the box’s vast volume of air is likewise made to vibrate, creating a loud sound. The sound that is generated will be louder the more air there is in the sound box.
Question 8. Define the term intensity of a sound wave. State the unit in which it is measured.
Solution:
The quantity of sound energy travelling through a unit area usually per second at any given place on the medium is its intensity. Microwatt per meter2 is its measure.
Question 9. How is loudness of sound related to the intensity of wave producing it?
Solution:
Loudness L and intensity I are related by the formula: L = K log I, where K is a proportionality constant.
Question 10. Comment on the statement ‘loudness of sound is a subjective quantity, while intensity is an objective quantity.
Solution:
The quantity of sound energy travelling through a unit area usually per second at any given place on the medium is its intensity. The energy carried by the sound wave close to the listener’s eardrum determines how loud a sound is. Being a feeling, loudness also depends on how sensitive the listener’s are hearings. As a result, each listener will experience sound differently, even at the same intensity. Furthermore, due to differences in ear sensitivity for distinct frequencies, two sounds with the same strength but different frequency may sound different volumes to the same listener. Therefore, loudness is a subjective quality, but the sound wave’s strength, which can be measured, is an objective quality.
Question 11. State three factors on which loudness of sound heard by a listener depends
Solution:
(i) Loudness is inversely correlated with amplitude, squared.
(ii)Loudness is negatively correlated with square of distance.
(iii) The surface area of the vibrating body affects how loud the sound is.
Question 12. The bells of a temple are big in size. Why?
Solution:
Campanology, the study of bells, states that bigger bells have a lower resonance frequency. The frequency of the sound must be lower for a sound wave to travel across vast distances. Additionally, it has been noted that a bell’s body thickness correlates with its sound quality. The size of a temple’s bells is large because of this.
Question 13. Name the unit used to measure the sound level.
Solution:
The unit used to gauge sound intensity is the decibel.
Question 14. What is the safe limit of sound level in dB for our ears?
Solution:
Up to 120 dB
Question 15. (i) What is meant by noise pollution?
(ii) Name one source of sound causing noise pollution.
Solution:
(i) Noise pollution is the annoyance caused by unwelcome loud and unpleasant sound that is louder than 120 dB and comes from a variety of sources, including loudspeakers, moving automobiles, etc.
(ii) There are many sound sources, such as loudspeakers and moving automobiles that contribute to noise pollution.
Question 16. What determines the pitch of a sound?
Solution:
Sound’s frequency or wavelength determines its pitch. When two notes with the same amplitude and played on the same instrument have vibrations with different wavelengths or frequencies, the pitch of the notes will vary.
Question 17. Name the subjective property of sound related to its frequency.
Solution:
Pitch
Question 18. Name and define the characteristic which enables one to distinguish two sounds of same loudness, but of different frequencies, given by the same instrument.
Solution:
Pitch is a property of sound that allows us to discriminate between various sound frequencies. An acute note may be differentiated from a grave or flat note by its pitch, which is a property of sound.
Question 19. Draw a diagram to show the wave pattern of high pitch note and a low pitch note, but of the same loudness.
Solution:
The first diagram is high pitch note and second one is low pitch note.
Question 20. How is it possible to detect the filling of a bottle under a water tap by hearing the sound at a distance?
Solution:
The length of the air column shrinks as the water level in a bottle submerged in water rises, increasing the frequency of sound generated, or making it ever-shrinker. Thus, one may determine the amount of water in the bottle by hearing sound from a distance.
Question 21. The frequencies of notes given by flute, guitar and 500 Hz. Which one of these has the highest pitch?
Solution:
Trumpet since it occurs most frequently
Question 22. Complete the following sentence:
(i) The pitch of sound increases if its frequency …………..
(ii) If the amplitude of a sound is halved, its loudness becomes ………………
Solution:
(i) Increases
(ii) One-fourth
Question 23. The diagram below shows three different modes of vibration P, Q and R of the same string of a givens length.
(a) Which vibration will produce a louder sound and why?
(b) Which vibration will produce sound of maximum shrillness (or pitch) and why?
(c) What is the ratio of wavelength of vibrations P and R?
Solution:
a) R will make the loudest sound possible since it has the greatest amplitude.
b) P will have the loudest sound since it has the highest frequency.
c) Let’s suppose string has length l. Then wavelength of P = 2I/3 Wavelength of R = 2l
Question 24. Name the characteristic which enables one to distinguish the sound of two musical instruments even if they are of the same pitch and same loudness.
Solution:
Soundness or quality of the wood is the characteristic which enables one to distinguish the sound of two musical instruments even if they are of the same pitch and same loudness.
Question 25. How do the two sounds of same loudness and same pitch produced by different instruments differ? Draw diagrams to illustrate your answer.
Solution:
Due to their varied waveforms, two sounds with the same loudness and pitch but created by separate instruments sound different. Along with the primary note, the waveforms are influenced by the quantity and relative amplitude of the subsidiary notes. The wave patterns of two sounds with the same volume and pitch but coming from two distinct instruments are depicted in the diagram below. Due to their differing waveforms—the first is a sine wave, while the second is a triangle wave—they induce different sensations in the ears.
Question 26. Two identical guitars are played by two persons to give notes of the same pitch. Will they differ in quality? Give a reason for your answer.
Solution:
Since the guitars are comparable, their waveforms and hence their quality will be same.
Question 27. Two musical notes of the same pitch and same loudness are played on two different instruments. Their wave patterns are as shown in following Figure. Explain why the wave patterns are different.
Solution:
Subsidiary notes are produced by various instruments differently. When performed on one instrument, a note has many subsidiary notes, and when played on another, the note has few subsidiary notes. They therefore have various waveforms.
Question 28. How is it possible to recognize a person by his voice without seeing him?
Solution:
The reason behind this is that each person’s vocal cord vibrations have a distinctive waveform that varies depending on the individual.
Question 29. State the factor that determine
(i) The pitch of a note.
(ii) The loudness of the sound heard.
(iii) The quality of the note.
Solution:
(i) Frequency
(ii) Amplitude
(iii) Waveform
Question 30. Name the characteristic of the sound affected due to a change in
(i) Its amplitude.
(ii) Its wave form.
(iii) Its frequency.
Solution:
(i) Loudness
(ii) Quality or timbre
(iii) Pitch
Question 31. The sketches I to IV in following Figure show sound waves, all formed in the same time interval.
Which diagram shows:
(i) a note from a musical instrument
(ii) a soft (not loud) note,
(iii) a bass (low frequency) note.
Solution:
(i) IV
(ii) I
(iii) II
Question 32. Shows the wave patterns of three sounds A, B and C. Name the characteristic of sound which is same between (i) A and B, (ii) B and C, and (iii) C and A.
Solution:
(i) Loudness and quality are equal since both have the same amplitude and waveform.
(ii) The waveform and amplitude are not same. Therefore, no attribute is the same.
(iii) Because both sounds in this example have the same frequency, their pitches are identical.
Question 33. A microphone is connected to the Y-input of a C.R.O Three different sounds are made in turn in front of the microphone. Their traces (a), (b) and (c) produces on the screen are shown in following figure.
(i) Which trace is due to the loudest sound? Give reason for your answer.
(ii) Which trace is due for the sound with the lowest pitch? Explain your answer.
Solution:
(i) Largest amplitude is b
(ii) Lowest amplitude is a
Question 34. In what respect does the wave pattern of a noise and music differ? Draw diagram to explain your answer.
Solution:
While the wave pattern in noise is somewhat erratic, it is regular in music.
Question 35. State one difference between a musical note and a noise
Solution:
Noise is loud, dissonant, and unappealing to the ear, whereas a musical note is lovely, smooth, and acceptable. While the waveform of a musical note is regular, that of a noisy signal is not.
Exercise-7c
Multiple Choice Types
Question 1. By reducing the amplitude of a sound wave, its:
a) Pitch increases
b) Loudness decreases
c) Loudness increases
d) Pitch decreases
Solution: c) Loudness decreases.
Question 2. Two sounds of same loudness and same pitch produced by two different instruments differ in their:
a) Amplitudes
b) Frequencies
c) Waveforms
d) All of the above.
Solution: c) Waveforms
Question 3. Two sounds A and B are of same amplitude, same wave forms but of frequencies f and 2f respectively. Then:
a) B differs in quality from A
b) B is grave, A is shrill
c) B is shrill, A is grave
d) B is louder than A
Solution: c) B is shrill, A is grave
