Important Questions for CBSE Class 9 Science Chapter 12 Sound

IMPORTANT QUESTIONS

1 MARK QUESTIONS
Question.1 What is SONAR? [SAII-2014]
Answer. SONAR (Sound Navigation And Ranging) is a technique for determining water depth and locating underwater objects, such as reefs, submarines, and schools of fish.

Question.2 Define one hertz. [SAII-2013]
Answer. One hertz is one vibration per second.

Question.3 Define wavelength. [SAII-2013]
Answer. It is the distance between two nearest points in a wave which are in the same phase of vibration.

Question.4 Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
Answer. No, I will not be able to hear sound, because moon has no atmosphere. Therefore, no sound waves can travel to your ears and, therefore, no sound is heard.

Question.5 Guess which sound has a higher pitch; guitar or car horn ?
Answer. Car horn has a higher pitch than a guitar, because sound produced by the former is shriller than the latter.

Question.6 How are the wavelength and frequency of a sound wave related to its speed?
Answer. Speed of sound = Frequency x Wavelength.

Question.7 In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature ?
Answer. Sound travels fastest in iron as compared to water and air.

Question.8 What is the audible range of the average human ear? [SAII-2011]
Answer. An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.

Question.9 What is sound and how is it produced?
Answer. Sound is mechanical energy which produces a sensation of hearing. When an object is set into vibrations, sound is produced.

Question.10 Why is sound wave called a longitudinal wave?
Answer. Sound wave is called longitudinal wave because the particles of the medium vibrate in the direction of the propagation of wave.

Question.11 Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a darkroom?
Answer. The characteristic of sound is quality or timbre.

Question.12 Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why ? [SAll-2012]
Answer. The speed of light is  3 x 108 ms-1 whereas that of sound is 344  ms-1 in air. Thus, flash of lightning is seen at once, but sound takes few seconds to reach our ears.

Question.13 The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
Answer.
No. of vibrations produced in 1 s = 100
No. of vibrations produced in 60 (sec) = (1 min) = 100 x 60 = 6000.

Question.14 Name the two types of mechanical waves.
Answer. The two types of mechanical waves are:
(i) Transverse wave and (ii) Longitudinal wave.

Question.15 What is a wave?
Answer. A wave is a disturbance that travels in a medium due to repeated periodic motion of particles about their mean position,- such that the disturbance is handed over from one particle to the other without the actual motion of the medium.

Question.16 What is a transverse wave? [SAII-2010]
Answer. It is a wave in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave. ,

Question.17 What is a longitudinal wave?
Answer. It is a wave in which the particles of the medium vibrate in the direction of propagation of the wave.

Question.18 What is a trough? [SAII-2011]
Answer. A trough is a depression in a wave /.e., maximum displacement in the negative direction (below the mean position).

Question.19 What do you understand by*the term infrasonic vibrations?
Answer. The sounds of frequency lower than 20 Hz are called the infrasonics or subsonics.

Question.20 Which of the following sound  wavers  we can hear: 10 Hz, 500 Hz, 1500 Hz, 12000 Hz, 25000 Hz ?
Answer. 500 Hz, 1500 Hz, 12000 Hz.

Question.21 What do you understand by the term ultrasonic vibrations?
Answer. Sounds of frequency higher than 20,000 Hz are called the ultrasonics.

Question.22 What do you understand by the term echo? [SAII-2012]
Answer. The sound heard after reflection from a rigid obstacle is called an echo.

Question.23 Name the term associated with the travelling disturbance in a medium.
Answer. Wave.

Question.24 Do waves transport energy?
Answer. Yes.

Question.25 Do waves transport matter?  
Answer. No.

Question.26 Do the particles of the medium move from one place to another in a medium?
Answer. No.

Question.27 Does the velocity of wave motion depend on the nature of the medium ?
Answer. Yes.

Question.28 Does the velocity of wave motion depend on the nature or motion of the source?
Answer. No.

Question.29 What is the other name of a long flexible spring?
Answer. Slinky.

Question.30 Can you produce both types of waves (i.e., longitudinal and transverse) on a slinky?  [SAII-2010]
Answer. Yes.

Question.31 Where is the density of air higher; at compressions or at rarefactions?
Answer. At the compression.

Question.32 Name the quantity that represents the length of one complete wave.
Answer. Wavelength.

Question.33 What is the distance between two consecutive crests in a wave called?
Answer. Wavelength.

Question.34. Is the amplitude of a wave the same, as the amplitude of the vibrating body producing the wave? [SAII-2011]
Answer. Yes.

2-MARKS QUESTIONS
Question.1 How moths of certain families are able to escape  captures  from bats ? What is the range of frequencies assciated with
(a) infra sound ? (b) ultrasound ? [SAII-2014]
Answer.  They have very sensitive hearing equipment, can hear the squeaks (ultrasound) of bat and know when a bat is flying nearby.
(a) Less than 20 Hz. (b) More than 20,000 Hz.

Question.2 A person fires a gun standing at a distance of 55 m from a wall. If the speed of sound is 330 ms-1, find the time for an echo to be heard. [SAII-2014]
Answer.  Given S = 55 m, v = 330  ms-1 t = ?
2d = v x t
or t = 2d/v = 2 x 55/330 = 0.3 s

Question.3 The pulse rate of a man is 80 beats in one minute. Calculate its frequency. [SAII-2014]
Answer.
No. of beats per minute = 80
No. of beats per second = 80/60 = 1.3
So, frequency = 1.3 Hz

Question.4 Which wave property determines: (a) loudness, (b) pitch ? [SAII-2011]
Answer.
(a) The amplitude of the wave determines the loudness. More the amplitude of a wave, more is the loudness produced.
(b) The pitch is determined by tl e frequency of the wave. Higher the frequency of a wave, more is its pitch and shriller is the sound.

Question.5 Distinguish between loudness and intensity of sound.
Answer. The loudness depends on energy per unit area of the wave and on the response of the ear, but intensity depends only on the energy per unit area of the wave and is independent of the response of the ear.

Question.6 Why are the ceilings of concert halls curved?
Answer. The ceilings of concert halls are curved because sound after reflection from it reaches all the corners of the hall and is audible to each person in the hall.

Question.7 What is the range of frequencies associated with :
(a) Infra sound? (b) Ultrasound?
Answer.
(a) Infra sound : Sound waves between the frequencies 1 to 20 Hz.
(b) Ultrasound : Sound waves of the frequencies above 20,000 Hz.

Question.8 When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day? [SAII-2012]
Answer. If the temperature rises, the speed of sound will increase. This in turn will increase the minimum distance required for hearing an echo. No echo is heard because the distance between the source of sound and reflecting body does not increase.

Question.9 Give two practical applications of reflection of sound waves.
Answer.
(i) Megaphones are designed tp send sound waves in particular direction are based on the reflection of sound.
(ii) In stethoscope, the sound of patient’s heartbeat reaches the doctor’s ears by multiple reflections in the tubes.

Question.10 What is reverberation? How can it be reduced?
Answer. Reverberation is the repeated multiple reflections of sound in any big enclosed space. It can be reduced by covering the ceiling and walls of the enclosed space with some absorbing materials like fibre board, loose woolens etc.

Question.11 Explain how bats use ultrasound to catch their prey. [SAII-2011]
Answer. The bats produce high pitched ultrasonic waves which are not heard by human beings. The ultrasonic waves on striking the insect send back an echo, which is heard by the bat. As the echo is heard by the bat it hovers on the insect and catches it.

Question.12 How is ultrasound used for cleaning?
Answer. The object to be cleaned is put in a tank fitted with ultrasonic vibrator. The tank is filled with water containing detergent. As the ultrasonic vibrator is switched on the detergent rub against the object at a very high speed and hence cleans it.

Question.13 Explain how defects in a metal block can be detected using ultrasound.
Answer. The ultrasound waves are allowed to pass through metal block to which detectors are fitted. If there is a small defect in the metal block like an air bubble or a crack, then the ultrasound waves are reflected from such spots. Metal block if defective is indicated by the reflected ultrasonic waves.

Question.14 Explain why can echoes not be heard in a small room. [SAll-2012]
Answer. For hearing echo, there should be at least a distance of 17 m between the source of sound and the body from which’ sound is reflected. In small rooms this is not the case, hence echoes are heard.

Question.15 Sound is produced due to a vibratory motion, then why a vibrating pendulum does not produce sound? [SAII-2010]
Answer. The frequency of the vibrating pendulum does not lie within the audible range (20 Hz to 20,000 Hz) and hence it does not produce audible sound.

Question.16 Prove that: v = fλ, where the symbols have their usual meanings.
Answer. The particle takes time t equal to the time period to complete one vibration. Let v be the velocity of the wave, then
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Question.17 Two sound waves A and B are shown in the figure. Identify the sound wave hiving :
(i) high frequency
(ii) low frequency.
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Answer.
(i) Wave B has high frequency as it repeats itself after smaller intervals of time.
(ii) Wave A has low frequency as it repeats itself after longer intervals of time.

Question.18 Two sound waves A and B are shown in figure. Identify the sound wave having :
(i) small amplitude
(ii) large amplitude
important-question-for-cbse-class-9-science-sound-2-3
Answer.
(i) Sound wave B has small amplitude.
(ii) Sound wave A has large amplitude.

Question.19. A sound wave travelling in a medium is represented as shown in figure,
(i) Which letter represents the amplitude of the sound wave?
(ii) Which letter represents the wavelength of the sound wave?
important-question-for-cbse-class-9-science-sound-2-4
Answer.
(i) Letter P represents the amplitude of the sound wave.
(ii) Letter Q represents the wavelength of the sound wave.

3-MARKS QUESTIONS
Question.1 Distinguish between transverse and longitudinal waves. (Three points) [SAII-2014]
Answer.
Transverse waves
(a) Particles the medium vibrate at right angles.
(b) Alt. crests and troughs formed.
(c) e.g., water wavesv
Longitudinal waves
(a) Particles vibrate parallel to the direction of waves.
(b) Alt. compressions, rarefaction formed.
(c) e.g., sound waves.

Question.2 State three characteristics of a musical sound. On what factors do they depend. [SAII-2014]
Answer.
Characteristics of musical sound are:

  •  Loudness-Amplitude affect loudness-more amplitude loud/Less amplitude soft sound.
  •  Pitch-frequency affects pitch-more frequency more pitch, Less freq. less pitch.
  •  Quality or Timhpr.

Question.3 A stone js dropped into a well 44 m deep and  the sound of splash is heard after 3.12 sec. Find the speed of sound in air. [SAII-2014]
Answer.
important-question-for-cbse-class-9-science-sound-2-5
Question.4
(a) The sound of which of the following frequencies are audible to human ears : 2 Hz, 5 Hz, . 20 Hz, 200 Hz, 2000 Hz.
(b) How [paths of, certain families are able to escape capture ? [SAII-2014]
Answer.
(a) 20 Hz, 200 Hz and 2000 Hz.
(b) They do not reflect the ultrasounds sent towards them by bats.

Question.5 How does the sound produced by a musical instrument, reach your ears? Astronauts need radio transmitter to talk to each other on moon. Why ? [SAII-2014]
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 produce sound. Since sound requires a medium to propagate, therefore, sound cannot travel between astronauts on the moon, hence they use radio transmitters.

Question.6 How does the sound produced by a vibrating object in a medium reach your ear?
Answer.  Air is the commonest material through which sound propagates. When vibrating objects, like prongs of a tuning fork move forward, they push the molecules of the air in front of them. This in turn compresses the air, thus, creating a region of high pressure and high density called compression. This compression in the air travels forward. When the prongs of the tuning fork move backward, they create a region of low pressure in the air, commonly called rarefaction. This region has low pressure, low density and more volume. As the tuning fork continues to vibrate, the regions of compression in the air alternate with the regions of rarefaction. These regions alternate at the same place. The energy of vibrating tuning fork travels outward. This energy which reaches the ears, makes the eardrums to vibrate and thus we hear sound.

Question.7 What are wavelength, frequency, time period and amplitude of a sound wave? [SAII-2013]
Answer.  Wavelength: It is the linear distance between two consecutive compressions or two consecutive rarefactions.
Frequency: The number of compressions or rarefactions taken together passing through a point in one second is called frequency.
Time period : It is the time taken by two consecutive compressions or rarefactions to cross a point.
Amplitude: It is the magnitude of maximum displacement of a vibrating particle about its mean position.

Question.8 Does sound follow the same laws of reflection as light does? Explain. [SAII-2010]
Answer.  Yes, sound and light follow the same laws of reflection given below :
(a) Angle of incidence at the point of incidence = Angle of reflection.
(b) At the point of incidence, the incident sound wave, the normal and the reflected sound wave lie in the same plane.

Question.9 Explain the working and application of SONAR ?
Answer.  SONAR is a device for determining water depth and locating underwater objects like reefs, submarines and schools of fish. To find the depth of an ocean, a strong ultrasonic wave is sent from the ship towards the bottom of ocean.
On striking the bottom of the ocean, the ultrasonic wave is reflected upward toward the ship. This wave is received by a suitable receiver. The time of travel from the source of sound to the receiver is noted. We can calculate the depth of ocean floor if the velocity of sound in water is known :
Depth of ocean floor = v x t/2
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Question.10 What are longitudinal waves? Give two examples. [SAII-2011]
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. Examples :
(a) The sound waves in air.
(b) The waves produced in air when a sitar wire is plucked.

Question.11
(a) Draw the sound waves for a low pitched and the high pitched sound.
(b) Write one use of ultrasonography.
(c) Which wave property determines pitch? [SAII-2013]
Answer.
(a) The diagram is as shown :
important-question-for-cbse-class-9-science-sound-2-7
(b) Ultrasonography is used for examination of the foet us during pregnancy to detect congenial defects and growth abnormalities.
(c) Frequency.

5 MARKS QUESTIONS
Question.1 With the help of a labelled diagram show that sound needs a material medium for its propagation.
Answer.  A well labelled diagram is as shown:  Take an electric circuit which consists of a cell, a switch and an electric bell arranged inside a bell jar, which stands on the platform of an evacuating pump.
The switch Of the bell is pressed to close the electric circuit. When there is air within the bell jar, sound is heard. Air is now pumped out of the bell jar. When the air is completely removed from the bell jar, no sound is heard as it is obvious from fig. because the medium of air which has to carry energy from the bell to the bell jar is removed. It shows that sound needs material medium for its propagation.
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Question.2
(a) A particular transmitter of Aakashvani broadcasts at 420.5 m wavelength. Given the speed of radio waves  3 x 108 ms-1. Calculate the frequency at which the radio station broadcasts its programme.
(b) What is the direction of oscillations of the medium particles through which a :
(i) transverse wave is propagating ?
(ii) longitudinal wave is propagating ? [SAII-2014]
Answer.
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Question.3 Draw a well labelled diagram of the human ear. Explain its working.
Answer.  A well labelled diagragi of the human ear is as shown :
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The sound waves are collected by the pinna and then are directed through ear canal. The sound waves on striking the eardruip make it vibrate exactly the same way as the given sound emitting object. The bones in the middle ear start vibrating when the eardrum vibrates. It helps in magnifying the vibrations. When the magnified vibrations reach the cochlea in the inner ear, the fluid in it starts vibrating. These vibrations are picked up by sensory receptors and are converted into electrical signals. These electrical signals then travel to the brain which interprets sound.

Question.4
(a) What is reverberation ? Write two ways of reducing reverberation.
(b) Distinguish between tone and note.
(c) With the help of a simple diagram, explain how defects in a metal block can be detected using ultrasound. [SAII-2013]
Answer.
(a) The persistence of sound in an auditorium as a result of repeated reflections of sound is called reverberation.
(b) A sound of single frequency is called a tone. The sound which is produced due to a mixture of several frequencies is called a note.
(c) Ultrasounds can be used to detect cracks and flaws in metal blocks. The cracks or holes
inside the metal blocks, which are invisible from outside reduce the strength of the structure. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of the flaw or defect.
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APPLICATION BASED QUESTIONS

Question.1 The given graph (Fig.)shows the displacement versus time relation for a disturbance travelling with velocity of 1500  ms-1. Calculate the wavelength of the disturbance.
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Answer.
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Question.2 Which of the below two graphs (a) and (b) (Fig.) representing the human voice is likely to be the male voice? Give reason for your answer.
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Answer.  The difference in male and female voice lies in its frequency. Female voice is of higher frequency. Hence, graph ‘a’ represents a male voice.

Question.3 A stretched wire 0.5 m long is made to vibrate in two different modes as shown in diagram (A) and (B) given below :
(i) If the wavelength of the wave produced in mode (A) is 1 m, what is the wavelength of the wave produced in mode (B) of the following diagram?
(ii)In which case is the note produced louder? Give a reason for your answer.
(iii) In which case is the pitch of the note produced higher? Give a reason for your answer.
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Answer.
(i) The wavelength of the wave produced in mode B is half of that in mode A.
(ii) The note produced in mode A is louder. This is because the amplitude of the wave in mode A is more than that of mode B. Loudness, being proportional to the square of the amplitude, would/ therefore/Tje-more in mode A.
(iii) The pitch of the note produced in mode B is higher. This is because pitch becomes higher with an increase in frequency. As seen from the figure, the frequency of note in mode B is twice as much as that of the note in mode A.

Question.4 Two musical notes of same pitch and loudness are played on a violin and a piano.-The waveforms are as shown in figures below. Explain why the wave patterns are different. [SAI1-2010]
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Answer. The number, and nature, of harmonics and overtones present, affects the quality of the sound. The different combinations of the number and nature of harmonics and overtones present in the ‘notes’ give these different wave patterns.

Higher Order Thinking Skills (HOTS) Questions

Question.1 How do you account for the fact that two strings can be used to give notes of the same pitch and loudness but of different quality?
Answer. The ‘quality’ of a given note is determined by the overall effect of the harmonics present in it. The harmonics are multiples of the fundamental or basic frequency of the ‘note’. Depending on the conditions under which vibrations are taking place, sometimes we get one set of harmonics and sometimes another set. The quality of the two notes will, therefore, different even though their fundamental frequencies may be the same.

Question.2 A person standing between two vertical cliffs and 640 m away from the nearest cliff shouted. He heard the first 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 cliffs. [SAII-2011]
Answer.
important-question-for-cbse-class-9-science-sound-2-17

Question.3 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?
Answer.
(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.4 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. 5 A longitudinal wave of wavelength 1 cm travels in air with a speed of 330 { ms }^{ -1 }. Calculate the frequency of the wave. Can (his wave be heard by a normal human being?
Answer.  Given λ = 1 cm = 0.01m
v = 330 ms-1
Using v = v λ, we have v = v/λ = 330/0.01 = 33000 Hz
No, this cannot be heard by a normal human being.

Question.6 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.7 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 thd sound, the louder, (or more intense) the sound is.

Question. 8 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.9 Mention one practical use of echoes.
Answer. Echoes are used in radars to estimate the distance of flying objects.

Question.10 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.11 Will the sound be audible if the string is set into vibration on the surface of the moon? Give reason for your answer. [SAII-2012]
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.12 What change, if any, would you expect in the characteristics of musical sound when we increase:
(i) its frequency, and (ii) its amplitude?
Answer. (i) Pitch of sound will  increases (ii) Loudness of sound will increases

Reasoning Questions

Question.1 Name the subjective property of sound related to its frequency and of light related to its wavelength. [SAII-2013]
Answer.  The subjective property of sound waves, related to its frequency, is known as its pitch. The name of the similar subjective property of light, related to its wavelength, is the colour of light

Question.2 Two friends were playing on identical guitars whose strings were adjusted to give notes of the same pitch. Will the quality of the fwti notes be the same? Give a reason for your answer.
Answer.  The quality of the two notes need not to be the same. This is because whereas the pitch of a note is determined solely by its frequency, the quality is determined by its harmonic or overtone contents. It is the number, distribution, and relative intensity of the different harmonics and overtones that determine its quality. All these factors need not to be the same when the two notes have the same pitch.

Question.3 Sound made in front of a tall building 18 m away is repeated. Name the phenomenon and briefly explain it.
Answer.  The phenomenon is known as Echo.  We get echoes because of the reflection of sound. The reflected sound reaches the observer after a time 2d/v, where d is the distance of the reflecting wall from the source and v is its velocity. For the human ear, this time must be at least 0.1 second to ensure that the reflected sound does not get mixed up with the direct sound. Since the velocity of sound in air is nearly  340  ms-1, we must have 2d/340 > 0.1 or d > 17 m. Since the building is at a distance of 18 m, we can hear the reflected sound in the form of an echo clearly.

Question.4 A sound source produces 40 crests and 40 troughs in 0.4 s. Find the frequency of the wave.
Answer.
Given, Number of crests = 40, number of troughs = 40, time = 0.4 s.
We know that, one wave consists of one crest and one trough.
Therefore, 40 crests and 40 troughs will be equivalent to 40 waves.
These 40 waves are being produced in 0.4 s.
We define frequency as the number of waves per second. Therefore, the frequency of the wave will be
v = 40/0.4 =100 Hz

Question. 5 A Sonar emits pulses on the surface of water which are detected after reflection from the bottom. If the time interval between the emission and detection of the pulse is 2 s, find the depth of water. Take velocity of sound in water as 1531 ms-1. [SAII-2012]
Answer.
important-question-for-cbse-class-9-science-sound-2-18

Important Topics/Areas/Questions which are
frequently asked in the examination

Question.1. 20 waves pass through a point in 2 seconds. If the distance between one crest and adjacent trough is 1.5 m. Calculate (a) the frequency. (b) the wavelength.
Answer.
important-question-for-cbse-class-9-science-sound-2-19

Question.2
(a) The sound of an explosion on the surface of lake is heard by a boatman 100 in away and a diver 100 m below the point of explosion. Of the two persons mentioned (boatman or diver), who would hear the sound first and why?
(b) Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is  440  ms-1 in a given medium.
Answer.
(a) Diver will hear the sound first because the speed of sound is more in water than in air.
(b) v = Frequency x Wavelength
or Wavelength λ= v/v = 440/220 = 2 m

Question.3 A source is producing 1500 sounds waves in 3 seconds. If the distance covered by a compression and an adjacent rarefaction be 68 cm, find (a) frequency (b) wavelength and (c) velocity of sound wave. [SA II-2011]
Answer.
important-question-for-cbse-class-9-science-sound-2-20

Question.4 In a submarine equipped with sonar, the time between transmission of ultrasonic signals and the reception of the echo after reflection is found to be 80 sec. Find the distance of the obstacle (from submarine), speed of sound in water is 1530 m s’1 [SA II-2014]
Answer.
important-question-for-cbse-class-9-science-sound-2-21

Question.5 If the frequency of a tuning fork is 400 Hz and the speed of sound is air is 340 ms-1. Find how far sound travels when tuning fork makes 16 vibrations. [SA II – 2014]
Answer.
important-question-for-cbse-class-9-science-sound-2-22

Question.6 A laboratory uses an ultrasonic scanner to trace tumour in tissue. The operating frequency of the scanner is 4.2 x 106 Hz. Calculate the wavelength of sound in the tissue if the speed of the sound is 1700  ms-1. [SA II- 2014]
Answer.
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NUMERICAL PROBLEMS

Question.1 A human heart on an average is found to beat 75 times a minute. Calculate its frequency.
Solution.
important-question-for-cbse-class-9-science-sound-2-24

Question.2 A,person has hearing range of 20 Hz to 20 kHz. Calculate the wavelengths of sound waves in air corresponding to above frequencies? Take speed of sound in air as 340 ms-1. [SAII-2010]
Solution.
important-question-for-cbse-class-9-science-sound-2-25
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Question.3 What is the frequency of a wave whose time period is 0.05 s?
Solution.
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Question.4 If 25 waves were produced per second, what is the frequency in Hz?
Solution.
important-question-for-cbse-class-9-science-sound-2-28

Question.5 A source is producing 15 waves in 3.0 s. The distance between a crest and a trough is
10.0 cm. Find : (a) the frequency, (b) the wavelength, and (c) the velocity of the wave.
Solution.
important-question-for-cbse-class-9-science-sound-2-29

Question.6 A body is vibrating 6000 times in one minute. If the velocity of sound in air is 360 ms-1, find (r) frequency of the vibration in Hz, (ii) wavelength of the sound produced. [SAII-2011]
Solution.
important-question-for-cbse-class-9-science-sound-2-30

Question.7. A source of a wave produces 30 crests and 30 troughs in 0.3 seconds. What is the time period of the wave?
Solution.  One wave cycle comprises of one crest and one trough. As the given source produces 30 crests and 30 troughs, the given source produces 30 full wave cycles in 0.3 s.
Number of wave cycles produced in 0.3 s = 30 and number of wave cycles produced in
1s – 30/0.3 = 100.
But the number of wave cycles per second represents frequency. Therefore, the frequency of the wave is 100 Hz.
Also, time period (T) of the wave – 1/v = 1/ 100 = 0.01 s.

Question.8 A boat at anchor is rocked by waves, whose consecutive crests are 100 m apart. If the wave velocity of moving crests is 20 ms-1, calculate the frequency at which the boat will rock?
Solution.
important-question-for-cbse-class-9-science-sound-2-31

Question.9 A longitudinal wave is piroduced on a toy slinky, such that frequency of wave is 20 Hz and the speed of the wave is 30 cms-1. What is the minimum separation between the consecutive compressions on the slinky? [SAII-2012]
Solution.
important-question-for-cbse-class-9-science-sound-2-32

Question.1. A child hears an echo from a cliff 4 seconds after the sound from a powerful cracker is produced. How far away is the cliff from the child? (Take velocity of sound in air as 340 ms-1)
[SAII-2011]
Solution.
important-question-for-cbse-class-9-science-sound-2-33

Question.11 Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is
440 ms-1 in a given medium.
Solution.
important-question-for-cbse-class-9-science-sound-2-34

Question.12 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?
Solution.
important-question-for-cbse-class-9-science-sound-2-35

Question.13 An echo is returned in 3 s. What is the distance of the reflecting surface from the source, given the speed of sound is 342 ms-1?
Solution.
important-question-for-cbse-class-9-science-sound-2-36

Question.14 A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 ms-1, how far away is the cliff?
Solution.
important-question-for-cbse-class-9-science-sound-2-37

Question.15 A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies ? Take the speed of sound in air as 344 ms-1.
Solution.
important-question-for-cbse-class-9-science-sound-2-38

Question.16 Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound Wave in air and in aluminium to reach the second child. Given, velocity of sound in air and aluminium are 346 ms-1 and 6420 ms-1 respectively. [SAII-2010]
Solution.
important-question-for-cbse-class-9-science-sound-2-39

Question.17 A sound wave travels at a speed of 399 ms-1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible? [SAII-2013]
Solution.
important-question-for-cbse-class-9-science-sound-2-40

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