Sound Class 9 Physics Worksheet Pdf
Class 9 Physics students should refer to the following printable worksheet in Pdf for Sound in standard 9. This test paper with questions and answers for Grade 9 Physics will be very useful for exams and help you to score good marks
Class 9 Physics Worksheet for Sound
CBSE Class 9 Physics Worksheet - Sound - Practice worksheets for CBSE students. Prepared by teachers of the best CBSE schools in India.
Exam Questions NCERT Class 9 Science Chapter 12 Sound
Question. What is the reflection of sound?
Answer. When sound travels in a given medium, it strikes the surface of another medium and bounces back in some other direction, this phenomenon is called the reflection of sound. The waves are called the incident and reflected sound waves.
Question. What type of surfaces are the best for reflecting sound?
Answer. The best surface for the reflection of sound is polished or rough and big obstacle is necessary.
Question. What are good and bad reflectors of sound?
Answer. Good reflectors means which reflect the sound clearly. Bad reflectors means which do not reflect the sound clearly.
Question. A baby recognizes her mother by her voice. Name the characteristic of sound involved
Answer. The characteristic of sound involved in uniqueness of the sound is quality of sound or timber.
Question. What is SONAR? For what it is used?
Answer. SONAR is Sound Navigation And Ranging. It is a technique used to measure the depth of the sea, locate the sunken ships or icebergs and submarines.
Question. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?
Answer. Sound travels faster in solids when compared to any other medium. Therefore, at a particular temperature, sound travels fastest in iron and slowest in gas.
Question. Does sound follow the same laws of reflection as light does? Explain.
Answer. Yes. Sound follows the same laws of reflection as light. The reflected sound wave and the incident sound wave make an equal angle with the normal to the surface at the point of incidence. Also, the reflected sound wave, the normal to the point of incidence, and the incident sound wave all lie in the same plane.
Question. Give two practical applications of reflection of sound waves.
Answer. (i) Reflection of sound is used to measure the speed and distance of underwater objects. This method is called SONAR. (ii) Working of a stethoscope – the sound of patient’s heartbeat reaches the doctor’s ear through multiple reflections of sound.
Question. Flash and thunder are produced are produced simultaneously, but why the flash is seen first and then the sound is heard?
Answer. This is because the speed of sound in air is very less than the speed of light in air. With the increase in Density of the medium, the speed of sound also increases.
Question. A bucket kept under a running tap is getting filled with water. A person sitting at a distance is able to get an idea when the bucket is about to be filled.
(i) What change takes place in the sound to give this idea? (ii) What causes the change in the sound?
Answer. (i) The frequency of sound to increases (f∝l1)when the length (l) of the air column is decreased, i.e., when the bucket is getting filled with water.
(ii) Change in sound is caused because air column is decreased on the filling the water in bucket.
Question. How noise and music can change your mood/ mental happiness?
Answer. Music can relax the mind, energize the body, and even help people better manage pain. Whereas noise will put a negative effect on it .
Question. Define the following characteristics of a sound wave a. Loudness b. Pitch. C. Quality of sound
Answer. A. Loudness: Loudness of sound depends upon the intensity of sound. It is found that. i.e. greater the amplitude, greater will be the intensity and so louder will be the sound. The Unit of loudness is decibels (dB) and The loudness of normal talks is about 60 dB. B. Pitch: Pitch is the characteristic of a wave by which sound wave appears shrill or grave. It depends upon the frequency of the wave. Higher the frequency higher will be the pitch and vice-versa. C. Quality : It is the sensation received by the ear by which, we are able to differentiate two sounds (even if they are of same pitch and loudness).
Question. Which of the following sound waves we can hear : 10 Hz, 500 Hz, 1500 Hz, 12000 Hz, 25000 Hz?
Answer. 500 Hz, 1500 Hz, 12000 Hz.
Question. What are longitudinal waves? Give two examples.
Answer. A wave in which the particles of the medium vibrate back and forth along the same direction, in which the wave is moving, is called a longitudinal wave.
(a) The sound waves in air.
(b) The waves produced in air when a sitar wire is plucked.
Question. State three characteristics of a musical sound. Onwhat factors do they depend?
Answer. Characteristics of musical sound are :
(i) Loudness–Amplitude affects loudness–more amplitude, louder the sound and lesser the amplitude, softer is the sound.
(ii) Pitch-Frequency affects pitch-more frequency more pitch, less frequency less pitch.
Question. How does the sound produced by a musical instrument, reach your ears? Astronauts need radio transmitter to talk to each other on Moon. Why?
Answer. The sound produced by the musical instrument makes the molecules of air vibrate. These vibrations are carried forward by the other molecules till they reach our ear. These then vibrate our eardrum to producesound. Since, sound requires a medium to propagate,
therefore, sound cannot travel between astronauts on the Moon, hence, they use radio transmitters.
Question. State any two characteristics of a wave motion.
Answer. The characteristics of wave motion are :
(i) It is a periodic disturbance.
(ii) Energy transfer takes place at a constant speed.
Question. Will the sound be audible if the string is set into vibration on the surface of the Moon? Give reason for your answer.
Answer. No, we will not hear any audible sound on the surface of the Moon. This is because sound requires a medium to propagate, since there is no atmosphere on the surface of Moon, therefore, the sound will not be heard.
Question. What change, if any, would you expect in the characteristics of musical sound when we increase :
(i) its frequency, and
(ii) its amplitude?
(i) Pitch of sound will increases,
(ii) Loudness of sound will increases.
Question. The stem of a tuning fork is pressed against a table top. Answer the following questions :
(i) Would the above action produce any audible sound?
(ii) Does the above action cause the table to set into vibrations?
(iii) If the answer above is yes, what type of vibrations are they?
(iv) Under what conditions does the above action lead to resonance?
(i) Yes, there is an audible sound produced.
(ii) Yes, the table top is set into ‘forced vibrations’ by this.
(iii) The vibrations are forced vibrations.
(iv) Pressing the stem of a vibrating tuning fork against a table top, would lead to resonance if the frequency of the tuning fork equals the natural frequency of oscillations of the table top.
Question. If the amplitude of a wave is doubled, what will be the effect on its loudness?
Answer. Loudness depends upon the square of the amplitude of the wave, therefore, when the amplitude of wave is doubled, the loudness becomes four times.
Question. How do the frequency and amplitudes affect a musical sound?
Answer. The ‘frequency’ of a musical sound affects its ‘pitch’. The more the frequency of a (musical) sound, the ‘sharper’ and ‘shriller’ the sound becomes.
The ‘amplitude’ of a musical sound affects its loudness, or intensity. The more the amplitude of the sound, the louder (or more intense) the sound is.
Question. Give one example each of natural vibration, forced vibration and resonance.
Answer. (i) Natural vibration : The vibrations of a simple pendulum about its mean position.
(ii) Forced vibration : A sonometer wire, under tension, vibrating under the influence of a vibrating tuning fork.
(iii) Resonance : A correctly adjusted length of a sonometer wire under proper tension, vibrating under the influence of a vibrating tuning fork.
Question. Mention one practical use of echoes.
Answer. Echoes are used in radars to estimate the distance of flying objects.
Question. How does a stretched string on being set into vibration, produce the audible sound?
Answer. On being set into vibrations, the stretched string, forces the surrounding air to vibrate. This vibrating air, in turn, affects our eardrum and produces an audible sound.
Question. Write conditions for the production of an echo.
Answer. Conditions for the production of an echo are :
(i) Time gap between the original sound and the reflected sound.
The echo will be heard if the original sound reflected by an obstacle reaches our ears after 0.1 s.
(ii) Distance between the source of sound and obstacle.
Thus, the minimum distance (in air at 25°C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.
(iii) Nature of the obstacle : For the formation of an echo, the reflecting surface or the obstacle must be rigid such as a building, hill or a cliff.
(iv) Size of the obstacle : Echoes can be produced if the size of the obstacle reflecting the sound is quite large.
Question. A longitudinal wave is produced on a toy slinky. The wave travels at a speed of 30 cm/s and the frequency of the wave is 20 Hz. What is the minimum separation between the consecutive compressions of the slinky?
Answer. Wave speed, v = 30 cm/s
Frequency of the wave, v = 20 Hz = 20 s–1
The minimum separation between the consecutive
compressions is equal to the wavelength. Therefore,
Wavelength=30 cm s–1/20s-1= 1.5 cm
Question. A bat can hear sound at frequencies up to 120 kHz. Determine the wavelength of sound in the air at this frequency. Take the speed of sound in the air as 344 m/s.
Answer. Frequency,n = 120 kHz = 120 × 103 Hz
= 120 × 103 s–1
Velocity of sound in the air, v = 344 m/s
Wavelength of the sound wave = λ
Wavelength,λ= wave velocity/frequency
= 2.87 × 10–3 m = 0.29 cm
Question. Give uses of multiple reflection of sound.
Answer. There are several uses of multiple reflection of sound :
(i) Megaphone is a device used to address public meetings. It is horn-shaped. When we speak through megaphone, sound waves are reflected by the megaphone. These reflected sound waves are directed towards the people (or audience) without much spreading.
(ii) The ceilings of concert halls and auditoriums are made curved. This is done so that the sound reaches all the parts of the hall after reflecting from the ceiling. Moreover, these ceilings are made up of sound absorbing materials to reduce the reverberation.
(iii) Stethoscope is a device used by doctors to listen the sound produced by heart and lungs. The sound produced by heart beat and lungs of a patient reaches the ears of a doctor due to multiple reflection of sound.
(iv) Sound boards are curved surfaces (concave) which are used in a big hall to direct the sound waves towards the people sitting in a hall. The speaker is (i.e., source of sound) placed at the focus of the sound board.
(v) Sound waves from the speaker are reflected by die sound board and these reflected waves are directed towards the people (or audience).
(vi) Hearing aid is used by a person who is hard of hearing. The sound waves falling on hearing aid are concentrated into a narrow beam of sound waves by reflection. This narrow beam of sound waves is made to fall on the diaphragm of the ear. Thus, diaphragm of the ear vibrates with large amplitude. Hence, the hearing power of the person is improved.
Question. Give application of ultrasound (ultrasonic waves).
Answer. Ultrasonic waves have number of uses :
(1) Ultrasonic vibrations are used for homogenising milk. These vibrations break down the larger particles of the fat present in milk to smaller particles.
(2) Ultrasonic vibrations are used in dish washing machines. The vibrating detergent particles rub against the dirty utensils and thus, clean them.
(3) Ultrasonic vibrations produce a sort of depression in rats and cockroaches.
(4) Ultrasonic vibrations are used to study the growth of foetus in mother’s womb.
(5) Ultrasonic vibrations are used in relieving pain in joints and muscles.
(6) Ultrasonic vibrations are used in detecting flaws in articles made from metals. They are also used in finding the thickness of various parts of a metallic component.
Question. A tuning fork produces 1024 waves in 4 seconds.Calculate the frequency to the tuning fork.
Answer. As the tuning fork produces 1024 waves in 4 seconds, hence
Frequency of tuning fork,
n = Number of vibration per second
= 1024/4 = 256 Hz
Question. A human heart, on an average, is found to beat 75 times a minute. Calculate its frequency.
Answer. No. of beats of human heart = 75 min–1
= 75/1 min
= 75 /60 = 1.25 s–1
So, average frequency of human heart beating
= 1.25 s–1.
Question. A boat at anchor is rocked by waves whose consecutive crests are 100 m apart. The wave velocity of the moving crests is 20 m/s. What is the frequency of rocking of the boat?
Answer. Distance between two consecutive crests = 100 m
Wave velocity v = 20 m/s
The distance between two consecutive crests is equal to the wavelength of the wave. So,
Frequency = Wave velocity/Wave length
=20 ms-1/100 m= 0.2 s–1
So, the frequency of rocking of the boat is 0.2 s–1.
1. If you place some small blades on a drum, what will you observe when you beat this drum with a drum stick? Explain.
2. Describe an activity with a (a) metre scale (b) rubber band to show that only a vibrating body produces sound.
3. With the help of tuning fork demonstrate that sound is produced by a vibrating body.
4. What is vibrating in the following musical instruments producing by a musical sound? (a) Violin (b) drum (c) saxophone (d) flute (e) guitar (f) dholak (g) tabla (h) sitar
5. What is medium? What constitutes the medium?
6. Describe an activity to show sound travels faster through a solid medium than through a gaseous medium.
7. Describe an activity to show sound travels faster through a solid medium than through a liquid medium.
8. Describe the experiment to demonstrate that waves do not carry matter while travelling.
9. What are the characteristics of wave motion?
10. Distinguish between compression and rarefaction.
11. Draw a density – distance graph in case of a longitudinal wave and indicate the position of compression and rarefaction.
12. A tuning fork has a number 384 marked on it. What does this number signify?
13. Ocean wave of time period of 10 sec have a speed of 15m/s. what is the wavelength of these waves>
14. A boat at anchor is rocked by wave whose compression is 100 m apart. The wave velocity of the moving crest is 20m/s. what is the frequency of rocking of the boat?
15. A sound wave of frequency 640 Hz travels 8oom in 2.5 s. calculate: (a) speed of sound (b) wavelength of sound wave
16. What will happen to the loudness of the sound if:
(a) Amplitude of a wave is tripled
(b) Surface area of a vibrating body is decreased
(c) The distance between the source and the observer is doubled.
(d) The density of the medium decrease
(e) The wind is blowing in the direction of propagation of sound
17. What is meant by reflection of sound?
18. Name any three devices based on reflection of sound.
19. What is the megaphone? Name the principle on which it is based.
20. What is the sound board?
21. Name a place where you can experience echo.
22. What do you mean by echo?
23. What are the conditions necessary for the formation of an echo?
24. Give reasons for the following:
(a) A person pressed his ears against the railway track to find whether the train is approaching or not.
(b) Speed of sound in solid is more than speed of sound in gas.
(c) Speed of sound is more during a summer day than a cold winter night.
(d) The speed of sound is affected by the direction of moving wind.
(e) Sound travels faster than in a rainy day than a dry day.
(f) How flying supersonic aircraft shatters glasses of the window panes of house.
(g) People living near the airports having problems
(h) Lighting is seen much earlier than the thunder is heard
(i) A sound board is placed behind a speaker in an auditorium
25. Explain the structure and working of human ear.
26. Give reason for the following:
(a) We cannot hear sound on vibrating our hands forward and backward.
(b) Some animals get disturbed by earthquake
(c) We cannot hear the sound produced by our heart beat.
(d) Owners of dogs use gallant on are whistle.
27. Why is the velocity of sound more than that in water or air?
28. How can we know from a distance the car is approaching us?
29. Why do we not get echo at all places?
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