Can sound wave travel in vacuum? Examples of electromagnetic waves Factors affecting the speed of sound Speed of sound in solid, liquid, and gas

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1 Sound Get the CBSE Class 9th Science chapter, Sound: Important Questions & Preparation Tips This will provide you with a very clear idea about what type of questions are being framed for the exam and from which topics. The pattern in which the Question Paper is set is quite different from the style in which the course structure is actually defined. Consider the below mentioned terms/questions at the time of preparation. Sound and its importance for us. Characters of a sound wave Mechanical wave and its property What type of energy sound is? Definitions of longitudinal and transverse waves Define wave front Sound is a mechanical wave. Justify

2 Can sound wave travel in vacuum? Examples of electromagnetic waves Factors affecting the speed of sound Speed of sound in solid, liquid, and gas (very important) Audible range for humans and other animals Propagation of sound. Can sound propagate through vacuum? How does the sound produced by a vibrating object in a medium reach your ear? Why sound needs a medium to travel? What type of wave is represented by density-distance graph? Transverse wave cannot travel through gases or air. Why? Definition of wave length, frequency, time period, amplitude, and oscillation What is wave number? Relation between speed, frequency, and wave length Concept and formation of compressions (C) and rarefactions (R) What do you mean by quality of sound? Which wave property determines (a) Loudness (b) Pitch A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source? What is intensity of sound? Intensity and loudness are same or different. Explain A sound wave has frequency of 2 KHz and wavelength 35 cm. How long will it take to travel 1.5 Km Is there any difference between reflection of sound and reflection of light? What is echo? Echo is related to reflection or refraction? Numerical based on echo.(very important) An echo returned in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m s 1? Reverberation and its applications

3 Uses of multiple reflections of sound Ceilings of cinema halls and conference halls are always curved. Why? Frequency range of ultrasound and its uses (Important) Expand SONAR and mention its uses A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound through seawater is 1531 m/s, what is the distance of the seabed from the ship? What type of sound is emitted by prey? Neat and clean diagram of human ear with all audible parts. Functions of all the parts given below: 1. Pinna 2. Auditory canal 3. Tympanic membrane 4. Cochlea Name the three bones found in the middle ear Important Questions are as below: Q. What happens when a wave travels through a medium 1) Particles are transferred from one place to another 2) Energy is transferred in a periodic manner 3) Energy is transferred at a constant speed 4) None of the above statements is applicable Q. Bats detect the obstacles in their path by receiving the reflected 1) Ultrasonic waves 2) Electro-magnetic waves 3) Radio waves 4) Infrasonic waves Q. The physical quantity, which oscillates in most waves, is 1) Wave length

4 2) Energy 3) Amplitude 4) Mass Q. The distance between a compression and the next rarefaction of a longitudinal wave is. 1) λ/8 2) 2l 3) λ/4 4) λ/2 Q. When is echo heard? 1) If the time interval between original sound and reflected sound is more than 1/10 sec. 2) If the time interval between original sound and reflected sound is less than 1/20 sec. 3) If the time interval between original sound and reflected sound is less than 1/10 sec. 4) If the time interval between original sound and reflected sound 1/20 sec. Q. What type of sound wave can propagate in solids? 1) Longitudinal only 2) Transverse only 3) Either longitudinal or transverse 4) Non mechanical wave only Question Bank chapter sound for class 09 Two marks questions 1. Explain how echoes are used by bats to judge the distance of an obstacle? 2. State the special properties of ultrasound that make it useful to us.in general, how these properties are utilized.

5 3. Why is soft furnishing avoided in concert halls? 4. Draw a diagram depicting low pitched sound and high pitched sound and write main difference between the two? 5. Distinguish between longitudinal and transverse waves. Give one example each. 6. An explosion takes place at the moon. After what time would it be heard at the earth? Three marks questions 7. Two sources A and B vibrate with the same amplitude. They produce sounds of frequencies 1 khz and 30 khz respectively. Which of the two waves will have greater power? 8. Find the time period of the source of a sound wave whose frequency is 400Hz. 9. A sound wave travels at a speed of 340m/s. If its wavelength is 2 cm, what is the frequency of the wave? Will it be in the audible range? 10. The grandparents and parents of a two year girl are playing with her in a room. A sound source produces a 28 khz sound. Who in the room is most likely to hear the sound? Five marks questions

6 11. Sound cannot travel in vacuum. Describe an experiment to demonstrate this. 12. With the help of a diagram describe how compression and rarefaction pulses are produced in air near a source of sound. 13. Explain briefly how a flaw in a mental component can be detected using ultrasound? 14. Explain the working and application of SONAR. 15. A monkey drops a coconut from the top of a tree. He hears the sound of the coconut hitting the ground seconds after dropping it.if the monkey was 19.meters above the ground, what is the speed of sound in air?(take g = 9.8m/s2). 8th/9th Chapter-Sound Physics CBSE Solved paper IX Class Question solution for Physics Chapter Sound 1. How sound is produced? Answer: Sound is produced by the vibration of vibrating object. 2. What do you understand by a wave?

7 Answer: A periodic disturbance created in material medium that transfer sound and light energy is called wave. 3. Write three differences between sound and light waves. Answer: sound wave is Mechanical wave that require material medium for propagation where as light waves are electromagnetic wave that can travel in vacuum. 4. What do you understand by sound energy cannot be produced? Answer: Sound energy cannot be produced on its own. Some mechanical energy is required to make an object vibrate. It is the mechanical energy of the vibrating object which travels through a medium and ultimately reaches the ear. 5. What is the name of the wave that can travel through vacuum? Answer: Electromagnetic wave 6. Explain by some experiment that sound waves require medium for their propagation. Answer: An electric bell is suspended inside an airtight glass bell jar connected to a vacuum pump. As the electric bell circuit is completed, the sound is heard. Now if the air is slowly removed from the bell jar by using a vacuum pump, the intensity of sound goes on decreasing and finally no sound is heard when all the

8 air is drawn out. We would be seeing the hammer striking the gong repeatedly. This clearly proves that sound requires a material for its propagation. 7. How sound waves travel through some medium? Answer: Sound waves work travel through some medium by passing vibrations from molecule to molecule. If there is no medium, then there no molecules to pass vibrations. 8. Why sound waves do not propagate through vacuum? Answer: If there is no medium, then there no molecules to pass vibrations. 9. What are the transverse waves? Give two examples. Answer: Transverse waves: A wave in which the particles of the medium vibrate at right angles to the direction, in which the wave is moving, is called transverse wave. Example: Light waves. 10. What are longitudinal waves? Give two examples. Answer: Longitudinal waves: A wave in which the particles of the medium vibrate back and forth in the same

9 direction in which the wave is moving is called longitudinal wave. Example: sound waves. 11. Give two points of difference between longitudinal and transverse waves. Answer: Differences between the transverse wave and longitudinal wave:- Transverse wave :- (1)A transverse wave is a wave in which variation of the amplitude of a wave is perpendicular to the direction of propagation of wave. (2) Light wave, wave in string, wave developed in water etc. (3)In light wave electric and magnetic field oscillates normally to the direction of motion of the wave. (4) It may propagate in the vacuum. Longitudinal wave :- (1)A longitudinal wave is a wave in which variation of the amplitude of wave is in the direction of the propagation of wave. (2)sound wave, oscillation in the spring. (3) In spring wave elongation or compression occurs along the direction of motion of the wave. (4) It can not propagate in the vacuum. 12. How will you prove that the sound waves exhibit longitudinal behaviour? Answer: Take a tuning fork and a hard pad. Allow the tuning fork to strike the

10 pad which makes the prongs to vibrate. When it starts vibrating, the inward and outward movements takes place in prong which causes the movement of inward and outward towards the mean position.the tuning fork is given as, The vibaration of the tuning fork produces the compressions and refractions of the sound in the air, When the tuning fork vibrates in air, they force the particles of the air to vibrate back and forth by a small distance. While vibrating, when the prong moves to the right side, it sends out a compression and when the prong moves to the left, it produces a rarefraction in air. The longitudinal waves in series produce compressions and rarefractions in air from the tuning fork. These compressions and rarefractions of sound waves is formed by the vibrating particles causing vibration in the ears, the eardrum vibrates for reproduction of sound. 13. What are rarefaction and compression in case of sound waves?

11 Answer: Compressions: Areas in the wave where the air molecules are pushed close together and so at a slightly higher pressure. Rarefaction: Areas in the wave where the air molecules are further apart and so at a slightly lower pressure. 14. Distinguish between crests and troughs. Answer: The highest point of a wave is known as its crest while the trough is the lowest point of the wave. Wavelength is the horizontal distance between successive crests or troughs. 15. Write the SI unit of velocity of a wave. Answer: m/s 16. What are the factors that describe the sound wave and define them? Answer: 17.Why is a thundering sound heard later than lightening? Answer: This because the speed of sond is much lesser than that of light.

12 18. Sound travels with different speeds in different media. Comment. Answer: Sound waves need to travel through a medium such as a solid, liquid, or gas. The sound waves move through each of these mediums by vibrating the molecules in the matter. The molecules in solids are packed very tightly. Liquids are not packed as tightly as solids. And gases are very loosely packed. The spacing of the molecules enables sound to travel much faster through a solid than a gas. Sound travels about four times faster and farther in water than it does in air. This is why whales can communicate over huge distances in the oceans. Sound waves travel about thirteen times faster in wood than air. They also travel faster on hotter days as the molecules bump into each other more often than when it is cold. 19. How far are a compression and its nearest rarefaction in a longitudinal wave? Answer: Wavelenth/2 20. Define sound ranging. Answer: The method of the determination of the location of a source of sound waves by measuring the time lapse between their transmission and their reception 21. What is the frequency range of sound for human beings? Answer: 20Hz to 20000Hz

13 22. What are the ultrasonic and supersonic waves? Answer: Ultrasonic waves refers to sound of frequencies greater than those that can be heard (usually frequencies above 20 khz). Supersonic refers to sound of velocities faster than the speed of sound (in the medium under consideration). 23. What type of waves are produced by animals like bats and dolphins? Answer: Ultrasound above 20000Hz 24. Explain two applications of ultra sound waves? Answer: Ultrasound is a wave with frequency greater than the upper limit of human hearing. These waves travel along well -defined paths and can even penetrate obstacles. Some important applications of ultrasound are: a. It is used for medical diagnosis and therapy and also to clean parts located in hard-to-reach places, for example, spiral tube, electronic components etc. b. Ultrasound is used to detect cracks and flaws in metal blocks. Such as aeroplane wings can be checked for cracks that would be invisible on the surface.

14 c. Its use in scanning goes far beyond pregnancies. Many other parts of the body are analysed using it (bladder gallstones, the heart, etc.) d. Detection of developmental/ structural abnormalities in the fetus. Evaluation of the heart and diagnosis of cardiac problems. This technique is called Echocardiography. 25. Ultrasound is also used to break small stones formed in the kidneys into fine grains.25. Explain how ultrasound waves are used to detect a flaw in an object? Answer: The ultrasound waves are allowed to pass through metal block to which are fitted detectors to detect the waves. Ultrasounds can be used to detect minor cracks or flaws in metal block. For this, ultrasonic waves are allowed to pass through metal blocks and detectors are used to detect the transmitted waves. If there is a crack in metal block, these waves get reflected back, thus indicating the presence of defects or flaws like cracks in the metal block. 26. Which sound wave is used in ECG (echocardiography)?answer: ultra sound wave 27. Give the full form of SONAR. Answer: SONAR: Sound Navigation And Ranging 28. Name the technique used to measure the depth of a sea. Answer: Sound ranging 29. How will you determine the depth of a sea using SONAR? Answer: ONAR is an acronym for Sound Navigation And Ranging. It is an acoustic device used to measure the depth, direction, and speed of under-water

15 objects such as submarines and ship wrecks with the help of ultrasounds. It is also used to measure the depth of seas and oceans. A beam of ultrasonic sound is produced and transmitted by the transducer (it is a device that produces ultrasonic sound) of the SONAR, which travels through sea water. The echo produced by the reflection of this ultrasonic sound is detected and recorded by the detector, which is converted into electrical signals. The distance ( d ) of the under-water object is calculated from the time ( t ) taken by the echo to return with speed ( v ) is given by 2d = v t. This method of measuring distance is also known as echo-ranging. 30. How do the bats fly in dark? Answer: Bats fly in the darkness of night without colliding with other objects by this method called echolocation. Bats emit high-frequency ultrasonic squeaks while flying & listen to the echoes produced by the reflection of their squeaks from the objects ( or obstacles ) in their path. From the time taken by the echo to

16 be heard, bats can judge the distance of the object (or obstacle) in their path and hence avoid it by changing the direction. Notes: Bats search their prey at night by this method only.this happens as follows : Bats emit high-frequency ultrasonic squeaks while flying & listen to the echoes produced by the reflection of their squeaks from the prey like a flying insect.from the time taken by the echo to be heard, bats can judge the distance of the insect & hence can catch it. 31. How RADAR is different from SONAR? Answer: SONAR (Sound Navigation and Ranging) uses sound wave which is mechanical energy to "look" through water or other media, and RADAR (Radio Direction and Ranging) uses electromagnetic radiation(radio waves) to "look" through the air or other media. RADAR signals are mostly used for ground or atmospheric observations whereas SONAR signals are ideal for navigation and measurement under water. 32. Explain the function of ear by explaining its each part? Answer: The outer ear is called pinna collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the ear drum or tympanic membrane. When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches it. In this way the eardrum vibrates. The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear. The middle ear

17 transmits the amplified pressure variations received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound. Note: The stirrup bone of the middle ear is the smallest bone in human body. 33. Distinguish between loudness and intensity of sound. Answer: Intensity depends on the energy per unit area of the wave and it is independent of the response of the ear, but the loudness depends on energy as well as on the response of the ear. 34. Why are ceilings of concert halls curved? Answer: The ceilings of concert hall are curved, so that sound after reflection from it reaches all the corners of the hall, and hence, is audible to everyone in the hall. 35. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute? Answer: Number of vibrations produced in 1 second = 100 Number of vibrations produced in 1 minute (60 s) = = Give two practical applications of the reflection of sound waves. [2010] Answer:

18 (i) In stethoscope the sound of patient s heartbeat reaches the doctor s ears by multiple reflections in the tubes. (ii) Megaphones are designed to send sound waves in particular direction are based on the reflection of sound. 37. Why are longitudinal waves called pressure waves? Answer: Sound waves travels in the form of compression and rarefactions, which involve change in pressure, and volume of the air. Thus they are called pressure waves. 38. What are harmonics? Answer: harmonics are notes of frequency which integral multiple of the fundamental frequency produced by a device. For e.g. third harmonic is 3 times the fundamental frequency. 39. What are fundamental note and overtones? Answer: When a sound is produced, there are tones of different frequencies. The tone of lowest frequency is called fundamental note and the tones of higher frequency are called overtones. Q.40 Sound travels faster on a rainy day than on a dry day. Why? Answer: Sound travels faster on rainy day because the velocity of sound increases with increase in humidity. On rainy day humidity is more thus velocity of sound is aso more.

19 Q.41 Why are the window panes of houses sometimes cracked when a bomb explodes even at large distance? Answer: The windows breaks because of the generation of shock waves in the explosion. Q42 If the tension in the wire is increased four times how will the velocity of wave in a string varies? Answer: velocity of the wave in string is directly proportional to the square root of the tension thus if tension is increased 4 times the velocity will be doubled. 1. How is sound produced? Ans: Sound is produced by a vibrating body e.g. when a tightly stretched bond is plucked, it vibrates and produced sound. When it stops vibrating, it does not produced any sound. 2. How is sound produced by humans? Ans: In humans, the sounds is produced by the voice box or the larynx. Put your fingers on the throat and find a hard bump that seem to move when you swallow. This part of the body is known as the voice box. This is at the upper end of the wind pipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the suit the vocal cords vibrate, producing sound. 3. why the voices of men, women and children are different.

20 Ans: The vocal cords in men are about 20 mm long. In women these are about 15 mm shorter. Children have very short vocal cords. This is the reason why the voices of men, women and children are different. 4. What is oscillatory motion Ans: The to and fro motion of an object is known as vibration. This motion is also called oscillatory motion. The number of oscillations per second is called the frequency. It is expressed in hertz. Its symbol is Hz. Amplitude and frequency are important properties of any sound. 5. On what factor loudness of sound deepens on? Ans: Loudness is property of sound that distinguish loud sound from bass. The loudness of sound depends on its amplitude. It is proportional to the square of the amplitude of the vibration producing the sound. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small the sound produced is feeble. The loudness is expressed in a unit called decibel(db). Above 80 db the noise becomes physically painful. 6. What is pitch? Ans: It is the characteristics of sound distinguish shrillness and pitch of the sound. When the frequency of the vibration is higher, then the sound has a higher pitch. When the frequency of vibration is lower, then the sound has a low pitch. The frequency of the voice of a child is higher than that of an adult. The voice of a woman has higher frequency and is shriller than that of a man. 7. What is range of the frequency of audible sound? Ans: The sounds of the frequencies less than about 20 vibrations per second (20 Hz) are

21 called inaudible sounds or infrasonic. Such sound can not be detected by the human ear. The sound of frequencies higher than about 20,000 vibrations per second (20 khz) are called the ultrasonic sound. For human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz 8. Why are sound waves called mechanical waves? Ans: Sound waves force the medium particles to vibrate. because of the interaction of the particles present in that medium 9. What is SONAR? Ans: SONAR stands for Sound Navigation and Ranging and it works on the principle of reflection of sound waves. The SONAR technique is used to determine the depth of the sea and to locate under water hills, valleys, submarines, icebergs sunken ships etc. 10. What is the intensity of sound? Ans: The amount of sound energy passing each second through unit area is called the intensity of sound. 11. Define : (i) Reverberation of sound. (ii)echo (iii) Time period (iv)wavelength. (v) wave velocity (vi) compression. (vi) rarefaction Ans: (1)The persistence of sound due to repeated reflection and its gradual fading away is called reverberation of sound. (2) Echo is a repetition of sound due to the reflection of original sound by a large and hard obstacle.

22 (3) Frequency is defined as the number of oscillations per second. The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T. (4) The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength. (5)The distance traveled by a wave in one second is called wave velocity. It depends upon the nature of the medium through which it passes. (6) The part or region of a longitudinal wave in which the density of the particles of the medium is higher than the normal density is known as compression. (7) The part or region of a longitudinal wave in which the density of the particles of the medium is lesser than the normal density is called a rarefaction. 13. What through and crest. The point of maximum positive displacement on a transverse wave is known as crest. The point of maximum negative displacement on a transverse wave is known as through. Q.1 How does a sound making object differ from one that is silent? Answer: Sound making object can vibrate easily. Q.2 How does sound from a sound producing body travel through air to reach our ears? Answer: Sound from a sound producing body travel through air to reach our ears by making regular compression and rarefaction. Q.3 Why are the voices of men, Women and children different? Answer: The voices of men, women and children differ from each other due to the following factors: (i) The pitch is one of the most essential factors to determine the voice of the individual. For example, men have lower-pitched voices while women have higher-pitched voices.

23 (ii) Second factor which brings in the difference in the voices is the length and the thickness of vocal cords. Q.4 Why a sound cannot be heard on the moon? Answer: A sound cannot be heard on the moon due to absence of medium air Q.5 How do astronauts talk to one another on the surface of moon and Why? Answer: Astronauts communicate with the help of radio waves on moon because these waves do not need a medium to travel Q.6 If you want to hear a train approaching from far away, why is it more convenient to put the ear to the track? Answer: This is because sound can travel through railway track (solid ) faster than through air. Q.7 State one observation from everyday life which shows that sound travels much more slowly than light? Answer: We hear thunder after lightining. Q.8 What is the scientific name for the following? The number of vibrations made per second. Answer: Frequency Q.9 Why do we not hear the screams of a bat? Answer: This is because the screams are ultra sonic(above 20,000 hertz) which human ear cannot hear. Q.10 Which of the following frequency of sound can be heard by a dog but not by a man? (a) 50,000 hertz (b) 15,000 hertz Answer: A sound of frequency 50,000 hertz can be heard by a dog but not by a man Q.11 Explain how, noise pollution {or excessive loud noise} is harmful to human beings.

24 Answer: Noise pollution can lead to a number of health-related problems. Some of them are as follows: (i) Hearing loss (ii) inability to sleep (iii) Hypertension (iv) Severe headache (v) Stress Q.12 State the various measures which can be taken to control {or reduce} noise pollution in our surroundings. Answer: Noise pollution can be reduced by using silencers in vehicles, industrial machines, and home appliances reducing use of vehicle horns, running TV, radio and music systems at low volumes. Planting of trees along roads and buildings also help to reduce noise pollution. Q.13 Give two causes of noise pollution from the homes. Answer:. In the home noise pollution is caused by television radio and music systems at high volume, some kitchen appliances, desert coolers, air conditioners etc. Q.14 Sound of different pitch can be produced using a flute. Explain, how? Answer: If we blow a flute, the air column vibrates and produces sound. Flute contains small air columns, when air is bowed through the mouth piece the air columns start vibrating which produces sound. Q.15.How does Sound produced by humans? Answer: In humans sound is produced by the vioce box or larynx. It is the upper part of the wind pipe. Two vocal cords are stretched across the voice box leaving a narrow slit. When the lungs force air through the slit, it vibrates and produces sound. Q. 16. How Jal Tarang produces sound? Answer: In Jal-tarang musical instrument the cup containing minimum water produces the sound of lowest frequency or lowest pitch. As the amount of water in the cup goes on increasing, the frequency (or the pitch) of the sound produced also goes on increasing. Q. 17. If 125 oscillations are produced in 5 seconds, what is the frequency in hertz? Answer: in 5 seconds 125 oscillations are produced

25 So, in 51 seconds 125/5 = 25 oscillations are produced Hence, the frequency = 25Hz Q. 18. What is the frequency of a vibrating body whose time-period is 0.05 second? Answer: f = 1/t = 1/0.05 = 100/5 = 20HZ Q. 18. State true or false (i) Sound can be produced by beating an object irrespective of whether there are vibrations or not. (ii)the sound of a tabla normally has a lower pitch than the sound of a violin. (ii)a short flute will produce sound of lower pitch than a long flute. Answer: (i) False [Sound is produced from a vibrating object.] (ii) -True [The sound of a tabla normally has a lower pitch than the sound of a violin.] (iii) True[A short flute will produce sound of lower pitch than a long flute.] Q.19. Give reasons Why Trees planted along the roads reduce noise? Answer: Trees are planted along the roads and around buildings so that it could cut off sound vibrations produced from vehicles speakers etc. by absorbing them, hence leading in prevention of Noise pollution. Q.20. what are transverse and longitudinal waves? Answer: Longitudinal waves: A wave in which the particles of the medium vibrate back and forth in the same direction in which the wave is moving is called longitudinal wave. Example: sound waves. Transverse waves: A wave in which the particles of the medium vibrate at right angles to the direction, in which the wave is moving, is called transverse wave. Example: Light waves. Q.21. Explain how bats use ultrasounds to catch a prey? Ans: Bats search out prey and fly in dark night by emitting and detecting reflections of ultrasonic waves. The high pitched ultrasonic squeaks of the bat are reflected from the obstacles or prey and returned to bat s ear. The nature of reflection tells the bat where the obstacle or prey is and what it is like.

26 Q.22. Explain how moths of certain families are able to escape capture from a bat? Ans: Moths of certain families have very sensitive hearing equipment. These moths can hear the high frequency squeaks of the bat and know where a bat is flying nearby and are able to escape capture. Q. 23.How does the ear drum of human ear vibrate? When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly the eardrum moves outward when a rarefaction reaches it. In this way the eardrum vibrates. Q. What is the role of hammer bone, anvil bone, stirrup bone and cochlea of human ear in hearing a sound Ans: The vibrations due to the ear drum is amplified several times by three bones in the middle ear. The pressure variations in the inner ear turned in to electrical signals by the cochlea. Q.24. How does the stethoscope help the doctors in listening to the sound of the patients heart beat? Ans. In a stethoscope, the sound produced within the body of a patient is picked up by a sensitive diaphragm and then reaches the doctor's ear by multiple reflections. Q.3. Explain how defects in a metal block can be detected using ultrasound. Ans. To detect minor cracks or flaws in metal block, ultrasonic waves are allowed to pass through metal blocks and detectors are used to detect the transmitted waves. If there is a crack in metal block, these waves get reflected back Q.25. How is ultrasound used for cleaning? Ans. The object to be cleaned is put in a tank fitted with ultrasonic vibrator. The tank is filled with cleaning solution. As the ultrasonic vibrator is switched on, high frequency vibrations are set up and the dust, grease and dust particles get detached and the object gets thoroughly cleaned. Numerical

27 Physics class 9th Numerical for chapter sound 1. A person observe the smoke from a gun 1.4 sec before he hear the bang. If the gun is 476m away from the person,find the speed of sound? 2. A person standing near the cliff fires the gun and heared the echo after 1.5 sec. If the speed of sound in air is 340m/sec, how far is person from the cliff? 3. A sound wave of frequency 5000 Hz travel in air with speed of 350m/sec. Calculate the wave length. 4. If u hear thunder 7 sec after u see yhe lightining, how far u from the lighting occurred? 5. A bat emits ultrasonic wave of frequency 30 Khz..If its speed is 350m/s and bats hear its echo after 0.6 sec after emitting the, find how far is bat from obstacle and wave length of wave? 6. Meera is standing between two hills.she shouted loudly and hears first echo after 0.5 sec and second echo after 1 sec. what is distance between two hills? 7. If 2000 ripples produced in 5 sec in a pond find time period and frequency of ripples formation. 8. A man standing in a valley between two parallel mountains fires a gun and hears echo at an interval of 2 s and 3.5s. what is a) the distance between two mountains b) the location of the man with respect to the mountain. 9. Calculate the wavelength of a sound whose frequency is 220 Hz and speed is 440 in a given medium. 10. A body is vibrating 6000 times is 1 minute. If the velocity of sound in air is 360 m/s, find (i) Frequency in Hz (ii) wavelength of sound.

28 11.. A stone is dropped from a 500 m tall building into a pond. When is sound splash heard? Given g=10 m/s2, speed of sound = 340 m/s. 12. An echo is heard in 3 sec after the emission of sound. If speed of sound in air is 342 m/s, what is the distance of the reflecting surface from the source? Question 21 Back to Top Question: Calculate a) the wavelength b) the time period of a tuning fork of frequency 512 Hz which is set to vibrate. Velocity of sound in air is 320 m/s. Answer: Frequency of the tuning fork (u) = 512 Hz. Velocity of sound (v) = 320 m/s = s Question 22 Back to Top Question: Sound waves travel with a speed of 330 m/s. What is the wavelength of sound, whose frequency is 550 Hz? Answer: Speed of the sound wave (v) = 330 m/s Frequency of the sound wave ( ) = 550 Hz

29 Wavelength = 0.6 m Question 23 Back to Top Question: Derive a relation between wave-velocity, frequency and wavelength. Answer: Wave velocity is defined as the distance travelled by the wave in one second. or Wave velocity = Wavelength x frequency Question 24 Back to Top Question: Radio waves of speed m s -1 are reflected off the moon and received back on earth. The time elapsed between the sending of the signal and receiving it back at the earth station is 2.5 s. What is the distance of the moon from the earth? Answer: = = m Question 25 Back to Top

30 Question: (a) A sound wave of wave length m has a time period of 10-3 s. If the time period is decreased to 10-4 s. Calculate the wave length and frequency of the new wave. (b) Name the subjective property of sound related to its frequency and of light related to its wavelength. Answer: time taken to travel = = = 330 m s -1 Time period of 2 nd wave = 10-4 s Therefore wavelength = V T = 330 x 10-4 = m Frequency = = = 10 3 Hz (b) Pitch is related to the frequency of sound and colour is related to the wavelength of light. Question 26 Back to Top Question: (a) State two characteristics of wave motion. (b) What is the relation between frequency, wavelength and speed of a wave? Answer: (a) A wave motion is periodic in nature. The particles of the medium do not move from their mean position but execute vibration but only the energy is transmitted from one point to another. (b) Speed = wavelength frequency v =

31 Question 27 Back to Top Question: A longitudinal wave of wavelength 1 cm travels in air with a speed of 330 ms -1. Calculate the frequency of the wave. Can this wave be heard by a normal human being? Answer: = 33,000 Hz The sound is not audible to human ear because frequency 20 to 20,000 Hz is the audible range of human ear. Question 28 Back to Top Question: A person standing between two vertical cliffs and 640 m away from the nearest cliff shouted. He heard the 1 st echo after 4 seconds and the second echo 3 seconds later. Calculate (i) the velocity of sound in air and (ii) the distance between the cliff. Answer: Velocity of sound v = = = 320 m s -1 Distance the farthest cliff, d = = = 1120 m Therefore the distance between the cliffs, D = = 1760 m Question 29 Back to Top

32 Question: A ship on the surface of water sends a signal and receives it back after 4 seconds from a submarine inside the water. Calculate the distance of the submarine from the ship (The speed of sound in water is 1450 m s -1 ). Answer: = 2900 m or 2.9 km Question 30 Back to Top Question: A man fires a gun and hears its echo after 5 seconds. The man then moves 310 m towards the hill and fires his gun again. This time he hears the echo after 3 seconds. Calculate the speed of sound. Answer: Let d be the distance between the man and the hill in the beginning. =?. (1) He moves 310 m towards the hill. Therefore the distance will be (d - 310)m.. (2) Since velocity of sound is same, equating (1) and (2), we get 3d = 5d d = 1550 d = 775 m = 310 m s -1