Oops! It appears that you have disabled your Javascript. In order for you to see this page as it is meant to appear, we ask that you please re-enable your Javascript!

Sound : S.Chand


pg 185
Very Short Answer Type Questions :-

Q.1    Can sound travel through- (a) iron (b) water ?

Sol.      a)Yes                         b) Yes.


Q.2    Can sound travel through vacuum ?
Sol.      No.


Q.3    Name the type of waves which are used by astronauts to communicate with one another on moon (or in outer space).
Sol.      Electromagnetic waves (Radio waves).


Q.4    Name one solid, one liquid and one gas through which sound can travel.
Sol.     Solid - iron, liquid - water, gas - oxygen.


Q.5    Which of the following cannot transmit sound ?
          Water, Vacuum, Aluminium, Oxygen gas

Sol.      Vacuum.


Q.6     Name the physical quantity whose SI unit is 'Hertz'.
Sol.       Frequency.


Q.7     What is the SI unit of frequency ?
Sol.       Hertz.


Q.8     What type of wave is represented -
               (a) by density-distance graph ?
               (b) by displacement-distance graph ?
Sol.        (a)   Longitudinal wave       
               (b)  Transverse wave.


Q.9     Is the speed of sound more in water or in steel ?
Sol.       In steel.


Q.10   In which medium sound travels faster : air or iron ?
Sol.      Iron.


Q.11   In which medium sound travels fastest : air, water or steel ?
Sol.      Steel.


Q.12   Out of solids, liquids and gases :
              (a) in which medium sound travels slowest ?
              (b) in which medium sound travels fastest ?
Sol.        (a) Gases
               (b) Solids


Q.13   Which of the following is the speed of sound in copper and which in aluminium ?
              (a) 5100 m/s
           (b) 1500 m/s
           (c) 3750 m/s

Sol.       Copper - 3750 m/s ; Aluminum - 5100 m/s.


Q.14    If you want to hear a train approaching from far away, why is it more convenient to put the ear to the track ?
Sol.       Since sound travel much faster in solids, It will be heard much sooner through a track than through air.


Q.15    What is the speed of sound :
               (a) in air ?
            (b) in water ?
            (c) in iron ?

Ans.        (a) In air - 346 m/s
               (b) In water - 1490 m/s
               (c) In iron - 5950 m/s.


Q.16   What name is given to those aircrafts which fly at speeds greater than the speed of sound?
Sol.        Supersonic jets.


Q.17   A jet aircraft flies at a speed of 410 m/s. What is this speed known as ?
Sol.        Supersonic speed.


Q.18   What is meant by supersonic speed ?
Sol.        When the speed of an object is greater than the speed of sound, it is called supersonic speed.


Q.19   State one observation from everyday life which shows that sound travels much more slow than light.
Sol.        Thunderstorm is heard after lightning is seen.


Q.20   Name the two types of waves which can be generated in a long flexible spring (or slinky).
Sol.        Longitudinal and transverse waves can be produced.


Q.21   A stone is dropped on the surface of water in a pond. Name the type of waves produced.
Sol.        Transverse waves.


Q.22   Name the type of waves produced when a tuning fork is struck in air.
Sol.        Longitudinal waves.


Q.23   What is the general name of the waves consisting of :
              (a) compressions and rarefactions ?
              (b) crests and troughs ?
Sol.        (a) Longitudinal waves
               (b) Transverse waves.


Q.24   State the general name of the waves in which the particles of the medium vibrate:
              (i) in the same direction as wave.
              (ii) at right angles to the direction of wave.
Sol.        (a) Longitudinal waves
               (b)Transverse waves.


Q.25   What type of waves are illustrated by the movement of a rope whose one end is fixed to a pole and the other end is moved up and down ?
Sol.       Transverse wave.


Q.26   What should an object do to produce sound ?
Sol.       Vibrate.


Q.27   What is the name of the strings which vibrate in our voice box when we talk ?
Sol.       Vocal cords.


Q.28   Name the device which is used to produce sound in laboratory experiments.
Sol.      Tuning fork.


Q.29   What is the nature of sound waves in air ?
Sol.       Longitudinal Waves.


Q.30   What conclusion can be obtained from the observation that when the prongs of a sound making tuning fork touch the surface of water in a beaker, the water gets splashed?
Sol.        That the prongs of tuning fork are vibrating.


Q.31   State whether the following statement is true or false :-
              Sound produced by a vibrating body travels to our ears by the actual movement of air.
Sol.      False.


Q.32   Which of the following travels slowest in air and which one fastest ?
              Supersonic aircraft, Light, Sound
Sol.        Slowest - Sound  and Fastest - Light.


Q.33   Which term is used to denote a speed greater than the speed of sound ?
Sol.       Supersonic.


Q.34   In which medium sound travels faster : air or hydrogen ?
Sol.      Hydrogen.


Q.35   A tuning fork has a number 256 marked on it. What does this number signify ?
Sol.      Frequency of the tuning fork .


 Q.36  What is the time-period of a tuning fork whose frequency is 200 Hz ?
Sol.       \nu= {1 \over T} = 5 \times 10^{ - 3} \,s.


Q.37   Calculate the frequency of a wave whose time-period is 0.02 s.
Sol.       \nu= {1 \over T}
                 = {1 \over {0.02}} = 50 Hz.


Q.38   What will be the change in the wavelength of a sound wave in air if its frequency is doubled ?
Sol.       Wavelength is halved.


Q.39   If 20 waves are produced per second, what is the frequency in Hertz ?
Sol.      20 Hz.


Q.40   Fill in the following blanks with suitable words :

(a) Sound is caused by.............
(b) A sound wave consists of places of higher pressure called ............. and places of........ pressure called.......
(c) Wave speed in metres per second equals frequency in........... multiplied by.............in...............
(d) Sound cannot travel through..................
(e) The speed of sound in a solid is................... than the speed of sound in air.
(f) When the frequency of the sound is increased, the wavelength................

Sol.

(a) Vibrations
(b) Compressions : lower pressure ; rarefactions
(c) Hertz ; wavelength ; metres
(d) Vacuum
(e) Greater
(f) Decreases.


Short Answer Type Questions :-

Q.41   What is vacuum ? Explain why, sound cannot travel through vacuum ?
Sol.        Region where there are no particles of matter is called vacuum. Sound cannot travel through vacuum because it needs a medium to travel.


Q.42   Explain the term 'amplitude' of a wave. Draw the diagram of a wave and mark its amplitude on it.
Sol.       The maximum displacement of a particle from its mean position is called amplitude. In below figure A is amplitude.

1


Q.43   (a) Distinguish between longitudinal and transverse waves.
              (b) Are sound waves longitudinal or transverse ?
Sol.        (a) Transverse wave
                  1. The particles move or vibrate about their mean position in a direction perpendicular to the direction of
                       propagation.

                  2. It travels in the form of crests and troughs.
                  3. Light waves are transverse waves and may travel in vacuum.

                Longitudinal waves
                 1. The particles vibrate about their mean position in a direction parallel to the direction of propagation.
                 2. It travels in the form of compression and rarefaction.
                 3. Sound waves are longitudinal waves and can be produced only in a material medium.

                (b) Sound waves are longitudinal.


Q.44  A cricket ball is seen to hit the bat first and the sound of hitting is heard a little later. Why?
Sol.      Since light travels much faster than sound, we see the ball hit the bat and hear the sound later on.


Q.45   Explain why, the flash of lightning reaches us first and the sound of thunder is heard a little later.
Sol.       Since light travels at a very great speed as compared to the speed of sound, we see the flash before the sound.


Q.46   Explain why, the flash of a gun shot reaches us before the sound of the gun shot.
Sol.        Since light travels at a very great speed as compared to the speed of sound, we see the flash before the sound.


Q.47   Which of the following terms apply to sound waves in air and which to water waves?
              Transverse, Rarefaction, Trough, Crest, Compression, Longitudinal
Sol.        Sound wave - rarefaction, compression, longitudinal
               Water wave - transverse, trough, crest.


Q.48   (a) Name four ways in which sound can be produced.
              (b) Calculate the speed of a sound wave whose frequency is 2 kHz and wavelength 65 cm.
Sol.        (a) Sound can be produced by vibration of vocal chords, blowing air in the flute, striking a drum , explosion.
              (b) \nu = 2 KHz = 2000 Hz
                     λ = 65 cm = 0.65 m
                     V = \nu\times\lambda = 2000 × 0.65 = 1300 m/s.


Q.49    If a ringing bicycle bell is held tightly by hand, it stops producing sound. Why ?
Sol.       Sound is produced due to vibration in the bell. When the bell is held tightly in the hand, the vibrations stop. Hence sound stops.


Q.50    Which object is vibrating when the following sounds are produced ?
            (i) The sound of a sitar
            (ii) The sound of a tabla
            (iii) The sound of a tuning fork
            (iv) The buzzing of a bee or mosquito
            (v) The sound of a flute

Sol.         (i) string
               (ii) membrane of the tabla
               (iii) prongs of the tuning fork
               (iv) wings of the bee
               (v) The air column inside the flute.


Q.51    Give reason for the following :
                In most of the cases, we cannot see the vibrations of a sound producing object with our eyes.
Sol.        The lowest frequency that we can hear is 20 Hz. The time period of this vibration will be {1 \over {20}}s or 0.05s. Due to persistence of vision in our eyes we cannot see vibration of less than 0.1 s.Therefore we cannot see the vibrations of any sound producing object.


Q.52    Describe a simple experiment to show that the prongs of a sound producing tuning fork are vibrating.
Sol.       Suspend a tennis ball on a rigid support by means of a string. Touch the vibrating tuning fork. You will are that the tennis ball is set into motion by the vibration.

112_1


Q.53    When we open a gas tap for a few seconds, the sound of escaping gas is heard first but the smell of gas comes later Why ?
Sol.       The speed with which the gas moves is much lower than the speed of sound. So we can hear the sound of escaping gas much before we smell it.


Q.54    A sound signal of 128 vibrations per second has a wavelength of 2.7 m. Calculate the speed with which wave travels.
Sol.       \nu = 128 Hz ; λ = 2.7 m
              V = \nu\times\lambda = 345.6 m/s


Q.55    A wave is moving in air with a velocity of 340 m/s. Calculate the wavelength if its frequency is 512 vibrations/sec ?
Sol.        V = 340 m/s ; \nu = 512 Hz
               \lambda= {V \over \nu } = {{340} \over {512}} = 0.67 m


Q.56   Define the 'frequency' and 'time-period' of a wave. What is the relation between the two?
Sol.       Frequency is the number of vibration produced in one second (\nu ).

              Time period is the time of one vibration,
              Time period (T) ={1 \over \nu }


Q.57   Explain why, a ringing bell suspended in a vacuum chamber cannot be heard outside.
Sol.      As sound is a mechanical wave it cannot travel in vacuum. So the bell ringing inside a vacuum chamber cannot be heard.


Q.58   The frequency of the sound emitted by the loudspeaker is 1020 Hz. Calculate the wavelength of the sound wave in air in cm where its velocity is 340 m/s.
Sol.       \nu = 1020 Hz,V = 340 m/s
              \lambda= {V \over \nu }= {{340} \over {1020}}= 0.33 m


Q.59    What is the difference between a compression and a rarefaction in a sound wave ? Illustrate your answer with a sketch.
Sol.        Compression : Particles of the medium are pushed close to each other. It is a region of high density and pressure.
               Rarefaction : Particles of the medium move away. It is a region of low density and pressure.

1111111 


Long Answer Type Questions :-

Q.60   (a) What is sound ? What type of waves are sound waves in air ?
              (b) Describe an experiment to show that sound cannot pass through vacuum.
Sol.

(a) Sound is a form of energy which gives the sensation of hearing waves. Sound waves are longitudinal waves.
(b) Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. The bell jar is connected to a vacuum pump,as shown in Fig. If you press the switch, you will be able to hear the bell. Now start the vacuum pump. When the air in the jar is pumped out gradually, the sound becomes fainter, although the same current is passing through the bell. After some time when less air is left inside the bell jar you will hear a very feeble sound. When all air is removed no sound is heard. This shows that sound can not pass through vacuum.
122


Q.61   (a) How is sound produced ? Explain with the help of an example.
                 (b) How does sound from a sound producing body travel through air to reach our ears ? Illustrate your answer with the help of a labelled diagram.
Sol.

(a) Sound is produced by the vibrations of any object. These vibrations are transferred to air particles and reach to us. For example - In school bell, when the gong strikes the bell, vibrations are produced in the bell which are transmitted through the air to our ears. These vibration produce sensation of sound in our ears.

(b) When a body vibrates the air in its neighbourhood is alternately compressed and rarefied. The compressed air has higher pressure than surrounding air. It therefore pushes the air particles near it causing compression to move forward. A rarefaction or low pressure is created at the original place.
These compressions and rarefaction cause particles in the air to vibrate about their mean position. The energy is carried forward in these vibration. This is how sound travels.
1111111


Q.62   (a) An electric bell is suspended by thin wires in a glass vessel and set ringing. Describe and explain what happens if the air is gradually pumped out of the glass vessel.
              (b) Why cannot a sound be heard on the moon ? How do astronauts talk to one another on the surface of moon ?
Sol.

(a) As the air inside the vacuum tube is pumped out, it becomes fainter. When all air is pumped out , no sound is heard. This is because air requires a material medium to travel.
(b) There is no atmosphere in the moon, so sound cannot propagate. Astronauts talk through radio wave.


Q.63   (a) Define the terms 'frequency','wavelength' and 'velocity' of a sound wave. What is the relation between them ?
              (b) A body vibrating with a time-period of {1 \over {256}}\,s produces a sound wave which travels in air with a velocity of 350 m/s. Calculate the wavelength.
Sol.

(a) Frequency : One compression and one rarefaction constitutes one vibration. The number of vibration in a second is called frequency. Its unit is Hertz.
Wavelength : The distances between two consecutive compressions or rarefaction of a wave. Its S.I unit is meter.

Velocity : Velocity is defined as distance travelled by wave in 1 s. It is calculated by the formula v=λT [where λ is distance travelled by 1 wave, and T is the time taken by 1 wave.]

(b) T = {1 \over {256}}\, s , v = 350 m/s
V= {\lambda\over T} , therefore λ = v x t = 350 \times {1 \over {256}}\, = 1.37 m.


Q.64   (a) What are longitudinal waves and transverse waves ? Explain with the help of labelled diagrams.
              (b) Give two examples each of longitudinal waves and transverse waves.
Sol.       (a)Longitudinal waves
             
1. The particles vibrate about their mean position in a direction parallel to the direction of propagation.
              2. It travels in the form of compression and rarefaction.
              3. Sound waves are longitudinal waves and can be produced only in a material medium.

Distance              Transverse wave
                   1. The particles moves or vibrate about their mean position in a direction perpendicular to the direction of
                        propagation.
                  
2. It travels in the form of crests and troughs.
                   3. Light waves are transverse waves and may travel in vacuum.
crest              (b) Longitudinal wave : Sound waves, Compression of spring causes longitudinal waves.
                    Transverse wave : Light, waves on the surface of water.


Q.65    (a) Explain the terms 'compressions' and 'rarefactions' of a wave. What type of waves consist of compressions and rarefactions ?
               (b) A worker lives at a distance of 1.32 km from the factory. If the speed of sound in air be 330 m/s, how much time will the sound of factory siren take to reach the worker ?
Sol.

(a) Compression : Particles of the medium are pushed close to each other. It is a region of high density and pressure.
Rarefaction : Particles of the medium move away. It is a region of low density and pressure.

1111111

Longitudinal waves consist of compressions and rarefaction.

(b) Distance of factory = 1.32 km =1.32 × 1000 m
Speed of sound = 330 m/s
Time for sound to reach work = {{dis\tan ce} \over {speed}} = {{1.32 \times 1000} \over {330}}s = 4\ s


Q.66    (a) Explain the terms 'crests' and 'troughs' of a wave ? What type of waves consist of crests and troughs?
            (b) The flash of a gun is seen by a man 3 seconds before the sound is heard. Calculate the distance of the gun from the man. (Speed of sound in air is 332 m/s).

Sol.

(a) When a wave propagates , the particles of the medium vibrate perpendicular to the direction of the wave. The maximum displacement of particle on one side of the wave (+ve side) is called crest and on the other is called trough. Crests and troughs occurs in transverse waves.

(b) Time taken by flash to reach observer = 3 s
speed of sound in air = 332 m/s
So distance travelled = v × t = 332 × 3 = 996 m


Q.67    (a) When we put our ear to a railway line, we can hear the sound of an approaching train even when the train is far off but its sound cannot be heard through the air. Why ?
               (b) How could you convince a small child that when you speak, it is not necessary for air to travel from your mouth to the ear of a listener ?
Sol.

(a) Since sound travels faster in solids, we can hear the sound of train by putting our ear on the solid and only later we can hear though air.

(b) Take a slinky end tie one end to the wall and give the other end a push or pull alternately. Compressions and rarefactions move towards the wall i.e. wave moves but the parts of the slinky just vibrates about its mean position.
In the same way compressions and rarefactions are produced in air and no net movement of air particles is required for sound to travel.

sund
soundco1soundco2


Multiple Choice Questions (MCQs) :-

Q.68    Which of the following statement best describes frequency ?
           
(a) the maximum disturbance caused by a wave
           
(b) the number of complete vibrations per second
           
(c) the distance between one crest of a wave and the next one
           
(d) the distance travelled by a wave per second
Sol.        (b)


Q.69    Which of the following vibrates when a musical note is produced by the cymbals in an orchestra ?
                (a) stretched strings       (b) stretched membranes
                (c) metal plates              (d) air columns
Sol.        (c)


Q.70    If the speed of a wave is 340 m/s and its frequency is 1700 Hz, then λ for this wave in cm will be :
            (a) 2          (b) 0.2        (c) 20       (d) 200

Sol.        (c)


Q.71    A musical instrument is producing a continuous note. This note cannot be heard by a person having a normal hearing range. This note must then be passing through :
               (a) water                    (b) wax
            (c) vacuum                (d) empty vessel

Sol.       (c)


Q.72    Which one of the following does not consist of transverse waves?
               (a) light emitted by a CFL
            (b) TV signals from a satellite

               (c) ripples on the surface of a pond
            (d) musical notes of an orchestra

Sol.        (d)


Q.73    Sound travels in air :
               (a) if particles of medium travel from one place to another
               (b) if there is no moisture in the atmosphere
               (c) if disturbance moves
               (d) if both, particles as well as disturbance move from one place to another
Sol.         (c)


Q.74    In the sound wave produced by a vibrating turning fork shown in the diagram, half the wavelength is represented by :
           (a) AB      (b) BD     (c) DE     (d) AE

Q74finalSol.         (b)


Q.75    The maximum speed of vibrations which produce audible sound will be in
                (a) dry air              (b) sea water
             (c) ground glass     (d) human blood

Sol.          (c)


Q.76    The sound waves travel fastest :
                (a) in solids             (b) in liquids
             (c) in gases              (d) in vacuum

Sol.          (a)


Q.77    The speeds of sound in four different media are given below. Which of the following is the most likely speed in m/s with which the two under water whales in a sea talk to each other when separated by a large distance ?
                (a) 340       (b) 5170     (c) 1280   (d) 1530
Sol.          (d)


Q.78    When the pitch of note produced by a harmonium is lowered, then the wavelength of the note :
          
(a) decreases
           (b) first decreases and then increases

              (c) increases
           (d) remains the same

Sol.        (c)


Q.79    The velocities of sound waves in four media P, Q, R and S are 18,000 km/h, 900 km/h, 0 km/h, andd 1200 km/h respectively. Which medium could be a liquid substance ?
              (a) P         (b) Q         (c) R      (d) S
Sol.        (d)


Q.80    Which of the following can produce longitudinal waves as well as transverse waves under different conditions?
                (a) water                (b) TV transmitter
             (c) slinky               (d) tuning fork

Sol.          (c)


Questions Based on High Order Thinking Skills (HOTS) :-

Q.81    Draw the sketches of two waves A and B such that wave A has twice the wavelength and half the amplitude of wave B.
Sol. 2

3


Q.82    A device called oscillator is used to send waves along a stretched string. The string is 20 cm long, and 4 complete waves fit along its length when the oscillator vibrates 30 times per second. For the waves on the string :
               (a) what is their wavelength ?
               (b) what is their frequency ?
               (c) what is their speed ?
Sol.

Length of string = 20 cm containing 4 waves
Therefore, length of one wave= {{20} \over 4}\,cm = 5\,cm = wavelength
Frequency = 30/s

(a)wavelength, λ = 5 cm = 0.05 m
(b)frequency \nu = 30 Hz
(c)speedV = \nu\times \lambda= .05 \times 30 = 1.5\,m/s


Q.83    Through which of the following materials can sound travel ?
                Wood, air, water, steam, ice, hydrogen, steel, diamond.
Sol.         All the materials.


Q.84    A sound producing body is at considerable distance from a man. There can be four different media W,X,Y and Z between the sound producing body and the man. The medium X brings the sound to man most quickly whereas medium Z takes the maximum time. The time taken by medium W in bringing sound to man is less than that of X but more than that of Z. The medium Y, however, fails to bring the sound from the sound producing body to the man. Which medium could be the one :
          
(a) having no fixed shape and no fixed volume ?
              (b) having a fixed volume but no fixed shape ?
              (c) having the same composition as that on the moon ?
              (d) having a fixed shape and a fixed volume ?
Sol.

(a) Z(gas)
(b) W (liquid)
(c) Y (vacuum)
(d) X (solid)


Q.85    The longitudinal waves travel in a coiled spring at a rate of 4 m/s. The distance between two consecutive compressions is 20 cm. Find :
              (i) Wavelength of the wave
           (ii) Frequency of the wave

Sol.        V = 4 m/s
              (1) Wavelength λ = 20 cm = 0.20 m
              (2) Frequency \nu = {v \over \lambda } = {4 \over {.2}} = 20 Hz


Short Answer Type Questions Pg 206

Q.1    Which property of sound leads of the formation of echoes ?
Sol.       Reflection of sound.


Q.2    What name is given to the repetition of sound caused by the reflection of sound waves ?
Sol.       Echo.


Q.3    What name is given to the persistence of sound in a big hall or auditorium ?
Sol.       Reverberation.


Q.4    Name three devices which work on the reflection of sound.
Sol.      Loud hailers, stethoscopes, and sounding boards.


Q.5    What is the other name of a loud-hailer ?
Sol.      Megaphone.


Q.6    Name the three characteristics of sound.
Sol.      Frequency, Amplitude, Speed.


Q.7    Name the unit used to measure the loudness of sound. Also write its symbol.
Sol.      Decibels , its symbol is dB.


Q.8    Name the characteristic which helps us distinguish between a man's voice and a woman's voice, even without seeing them.
Sol.     Pitch


Q.9    How does the pitch of a sound depend on frequency ?
Sol.      Higher the frequency , higher will be the pitch.


Q.10   Name the characteristic of sound which depends on (a) amplitude (b) frequency, and (c) waveform.
Sol.       (a) Loudness (b) pitch (c) quality.


Q.11   Name the characteristic of sound which can distinguish between the 'notes' (musical sounds) played on a flute and a sitar (both the notes having the same pitch and loudness).
Sol.       Quality or timbre.


Q.12   Name the organs of hearing in our body.
Sol.        Ears.


Q.13   Name that part of ear which vibrates when outside sound falls on it.
Sol.        Tympanic membrane.


Q.14   Name the three tiny bones present in the middle part of ear.
Sol.        Hammer , anvil , stirrups.


Q.15   There are three small bones in the middle ear : anvil, hammer and stirrup :
               (a) Which of these bones is in touch with ear-drum ?
               (b) Which of these bones is in touch with oval window ?
Sol.         (a) hammer

                (b) stirrup.


Q.16    What is the function of three tiny bones in the ear ?
Sol.         To amplify the vibrations of sound.


S.Chand pg 207.

 Q.17    Name the tube which connects the middle ear to throat.
Sol.          Eustachian tube.


Q.18    Name the nerve which carries electrical impulses from the cochlea of ear to the brain.
Sol.         Auditory nerve.


Q.19    What is the name of passage in outer ear which carries sound waves to the ear-drum?
Sol.         Auditory canal.


Q.20    Why should we not put a pin or pencil in our ears ?
Sol.         It may damage the ear drum.


Q.21    What type of scans are used these days to monitor the growth of developing baby in the uterus of the mother ?
Sol.         Ultrasound.


Q.22    How is an ultrasound scan for fetus (unborn baby) better than X-ray ?
Sol.         X-rays can damage the body cells of an unborn baby.


Q.23    What is the name of the device which is used to find the depth of sea (or ocean) by using ultrasonic sound waves ?
Sol.         SONAR.


Q.24    Write the full name of 'SONAR'.
Sol.         Sound , Navigation and Ranging.


Q.25    Name the principle on which a soundboard works.
Sol.         Reflection of sound.


Q.26   Name the device which is used to address a small gathering of people.
Sol.        Megaphone.


Q.27   Name the device used by doctors to listen to our heartbeats.
Sol.        Stethoscope.


Q.28   What is the shape of a soundboard kept behind the speaker on the stage of a big hall ?
Sol.        Concave.


Q.29   Name two sound absorbing materials (or objects) which can make our big room less echoey.
Sol.        Curtain, Carpets.


Q.30   Can we hear (a) infrasonic waves (b) ultrasonic waves ?
Sol.        No.


Q.31   What name is given to the sound waves of frequency too low for humans to hear ?
Sol.        Infra sound.


Q.32   What name is given to the sound waves of frequency too high for humans to hear ?
Sol.        Ultrasound.


Q.33   What type of sound waves are produced by a vibrating simple pendulum ?
Sol.        Infra sound.


Q.34   What happens to the pitch of a sound if its frequency increases ?
Sol.        Increases.


Q.35   What happens to the loudness of a sound if its amplitude decreases ?
Sol.       Decreases.


Q.36   What name is given to sound waves of frequencies higher than 20 kHz ?
Sol.       Ultrasound.


Q.37   Fill in the following blanks with suitable words :
              (a) An echo is simply a.......sound.
              (b) Pitch of sound depends on......
              (c) Loudness of sound depends on.....
              (d) Quality of sound depends on.....
              (e) Echoes are caused by the........of sound.
Sol.     (a) Reflected

            (b) Frequency
            (c) Amplitude
            (d) Waveform
            (e) Reflection


Q.38   On which day, a hot day or a cold day, an echo is heard sooner ? Give reason for your answer.
Sol.        On a hot day, echo is heard sooner since velocity of sound increases with temperature.


Q.39   In which medium, air or water, an echo is heard much sooner ? Why ?
Sol.       In water , since speed of sound is greater in liquids.


Q.40   What is reverberation ? What will happen if the reverberation time in a big hall is too long?
Sol.        The multiple reflection of sound from the walls of a room or hall causing sound to persist for some time is called reverberation. If the hall is too long , the reflection will become an echo.


Q.41   How can reverberations in a big hall or auditorium be reduced ?
Sol.        Reverberation can be reduced by covering the walls and ceiling with sound absorbent materials like curtains and false ceilings.


Q.42   Why do we hear more clearly in a room with curtains than in a room without curtains ?
 Sol.       There is less reverberation in rooms with curtains so we can hear sound more clearly as the sound is absorbed by the curtains.


Q.43   What is a megaphone ? Name the principle on which a megaphone works.
Sol.        Megaphone is a device to direct sound waves in one direction so that the intensity of sound in that direction increases. It s based on the principle of reflection of sound.
12


Q.44    What is a bulb horn ? Name the principle on which a bulb horn works.
Sol.         A bulb horn is a tube followed by a conical opening. As the bulb is pressed the waves of air come out through the opening and creates sound. It is based on the principle of multiple reflection of sound.


Q.45    What is a stethoscope ? Name the principle on which a stethoscope works.
Sol.         Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs.
Principle - multiple reflection of sound.


Q.46    What is a soundboard ? Explain the working of a soundboard with the help of a labelled diagram.
Sol.         In an auditorium or cinema halls the wall behind the speaker's platform is curved in such a way that sound after reflection moves parallel in the direction of the audience. Due to which the loudness of sound is increased.

source


Q.47    (a) What is meant by the 'loudness' of sound ? On what factor does the loudness of a sound depend ?
                (b) Draw labelled diagrams to represent
                  (i) soft sound, and
                  (ii) loud sound, of the same frequency.

Sol.       (a) Loudness is the physiological response of our ears to sound. It depends on amplitude and frequency and also age of the person. Human beings can hear sound of frequency between 20 Hz to 20,000 Hz.

soft


Q.48    (a) Explain the term 'pitch' of a sound. On what factor does the 'pitch' of a sound depend?
           
(b) Draw labelled diagrams to represent sound of
                  (i) low pitch, and
                  (ii) high pitch, of the same loudness.

Sol.       (a) Pitch is that characteristic of sound that determines the shrillness of sound. It depends on frequency of sound.
             (b)

sound


Q.49    What is meant by the quality (or timbre) of sound ? On what factor does the quality (or timbre) of a sound depend?
Sol.        The quality or timbre of sound is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness. The sound which is more pleasant is said to be of a rich quality.


Q.50    Explain why, if we strike a table lightly, we hear a soft sound but if we hit the table hard, a loud sound is heard.
Sol.         If we strike a table lightly the vibrations produced have less amplitude than when we strike the table hard. Since loudness depends on amplitude, soft sound is heard when we strike lightly.


Q.51   Give one use of ultrasound in industry and one in hospitals?
Sol.        Use of ultrasound
               (1) To clean parts of machines hard to reach.
               (2) Echocardiography is used to form image of heart.


Q.52   How is it that bats are able to fly at night without colliding with other objects?
Sol.        Bats are able to fly at night without colliding with other objects as they use reflection of ultrasound to know what object are around them.


Q.53   Explain how, bats use ultrasound to catch the prey.
Sol.        Bats emit ultrasound. These are reflected by various obstacles and return to the bat’s ear. The nature of reflection tells the bat where the obstacles or prey is and accordingly the bat is able to catch its prey.


Q.54   Explain how, flaws (or defects) in a metal block can be detected by using ultrasound.
Sol.       Ultrasound is passed through the metal block which has to be tested. In case of flaws ultrasound does not pass through it but is reflected back. A detector on the other side of the block does not receive all the transmitted waves and hence the flow is detected .
detectprs


Q.55   Why are the ceilings of concert halls made curved ? Draw a labelled diagram to illustrate your answer.
Sol.      The walls and ceilings are made curved so that sound reaches all parts of the hall after reflection.

boys


Q.56    Draw a labelled diagram to show the multiple reflections of sound in a part of the stethoscope tube.
Sol.
final

              Multiple reflection within a stethoscope


Q.57    What is the range of frequencies associated with
           (a) infra sound
           (b) audible sound, and
           (c) ultrasound ?

Sol.        (a) less than 20 Hz
               (b) 20 Hz to 20,000 Hz
               (c) Above 20,000 Hz


Q.58    (a) What is the difference between infrasonic waves and ultrasonic waves ?
                   (b) Choose the infrasonic waves and ultrasonic waves from the following frequencies :
                             (i) 10,000 Hz          (ii) 30,000 Hz       (iii) 18 Hz
                       (iv) 50,000 Hz
          (v) 10 Hz
Sol.        (a) Infrasonic - less then 20 Hz
                           Ultrasonics- more than 20,000 Hz
                    (b) Infrasonic - 10 Hz, 18 Hz
                            Ultrasonic - 30,000 Hz, 50,000 Hz


Q.59    (a) What is the frequency range of hearing in humans ?
                   (b) Which of the following sound frequencies cannot be heard by a human ear ?
                            (i) 10 Hz                (ii) 100 Hz             (iii) 10,000 Hz
                      (iv) 15 Hz               (v) 40,000 Hz

Sol.        (a) 20 Hz - 20,000 Hz
               (b) (i), (iv) and (v)


Q.60    The echo of a sound is heard after 5 seconds. If the speed of sound in air be 342 m/s, calculate the distance of the reflecting surface.
Sol.        Echo time = 5 s
               Speed of sound = 342 m/s
               Distance{{v \times t} \over 2} = {{342 \times 5} \over 2} = 855m


Q.61   The speed of sound in water is 1500 meters per second. How far away from an under-sea rock should a deep sea diver be so that he can hear his own echo ?
Sol.      Speed of sound in water = 1500 m/s
             Minimum time of sound for the echo to be heard = 0.1 s
             Minimum distance = {{V \times T} \over 2} = {{1500 \times 0.1} \over 2} = 75\,m


Long Answer Type Questions

Q.62   (a) What is meant by 'reflection of sound' ? What type of surfaces are the best for reflecting sound ?
              (b) Name any two objects which are good reflectors of sound.
              (c) State the laws of reflection of sound.
Sol.       (a) The bouncing back of sound wave is called reflection . Hard surfaces are the best reflection surfaces
              (b) Walls, cliffs
              (c) The laws of reflection of sound are -
                    (1) The incident wave, reflected wave and the normal all lie in the same plane.
                    (2) The angle of incidence = angle of reflection


Q.63   (a) What is an echo ? How is echo formed ?
              (b) What is the minimum distance in air required from a sound reflecting surface to hear an echo (at 20ºC)
          
(c) A man standing 825 meters away from a cliff (steep rock) fires a gun. After how long will he hear echo? Speed of sound in air is 330 m/s.
Sol.       (a)  The repetition of sound due to reflection is called echo. It is formed if reflection of sound is heard after 0.1 s.
              (b)  At 20o , speed of sound = 344 m/s
                     Time for echo to reach = 0.1 s
                     Minimum distance = v × t2 = 344 × 0.12 = 17.2m
              (c)  distance d = 825m
                     speed V = 330 m/s
                     t = {{2d} \over v} = {{2 \times 825} \over {330}} = 5\,s


Q.64   (a) What is ultrasound ? What is the difference between ordinary sound and ultrasound ?
          
(b) Write any three applications (or uses) of ultrasound.
Sol.       (a) Sound having frequency greater then 20,000 hz is called ultrasound. Ultrasound cannot be heard by human beings like ordinary sound.
              (b) Uses of ultrasound -
                     (1) To check flaws and cracks in metal
                     (2) In ultrasonography
                     (3) In echocardiography


Q.65    (a) What are infrasonic waves ? Name two animals which produce infrasonic waves.
               (b) What are ultrasonic waves ? Name two animals which can produce ultrasonic waves
               (c) The audible range of frequencies of an average human ear is from 20 Hz to 20 kHz. Calculate corresponding wavelengths. (Speed of sound in air is 344ms−1 ).
Sol.        (a) Infrasonic are vibration of frequency less then 20 Hz. Whales and elephants produced infrasonic.
               (b) Sound waves of frequencies greater then 20,000 Hz are called ultrasonic waves. Porpoises and bats produce ultrasonic waves.
               (c) \lambda _1 = {v \over \nu } = {{344} \over {20}} = 17.2\,m
                     \lambda _2= {{344} \over {20,000}} = 0.0172\,m


Q.66    (a) Define the following terms :
                        (1) Echolocation (2) Echocardiography (3) Ultrasonography.
               (b) Name an animal which navigates and finds food by Echolocation.
               (c) Which of the two produces ultrasonic waves : porpoise or whale ?
Sol.        (a)
               (1)
Echolocation is the technique by which objects are located by sensing time of echo of the sound emitted

               (2) Formation of image of the heart by reflection of ultrasound from heart is called echocardiography.
               (3) Creating images of different parts of the body by means of reflection of ultrasound is called Ultrasonography.
               (b) Bats
               (c) Porpoises


Q.67   (a) What is sonar ? Explain its use.
              (b) A sonar station picks up a return signal after 3 seconds. How far away is the object ? (Speed of sound in water = 1440 m/s).
Sol.        (a) SONAR stand for Sound, Navigation And Ranging. It is a device used to measure distance, direction and speed of underwater objects. It has a transmitter and detector near its base. The transmitter transmits ultrasonic signals which get reflected by various underwater objects. These are received by the detector which can convert these waves into appropriate electrical signals and give us the required information.
              (b) Time of echo = 3 s

                     speed of sound = 1440 m/s
                     Distance = = {{v \times t} \over 2} = {{1440 \times 3} \over 2} = 2160\,m


Q.68    Draw a neat and labelled diagram of the human ear. With the help of this diagram, explain the construction and working of the human' ear?
Sol.       The human ear consists of three parts – the outer ear, middle ear and inner ear.
              Outer ear : This is also called ‘pinna’. It collects the sound from the surrounding and directs it towards auditory canal.
             
Middle ear : The sound reaches the end of the auditory canal where there is a thin membrane called eardrum or tympanic membrane. The sound waves set this membrane to vibrate. These vibrations are amplified by three small bones- hammer, anvil and stirrup.
             
Inner ear : These vibration reach the cochlea in the inner ear and are converted into electrical signals which are sent to the brain by the auditory nerve, and the brain interprets them as sound.

Untitled-111111111111


Multiple Choice Questions (MCQs)

Q.69    In SONAR we use :
                (a) ultrasonic waves           (b) infrasonic waves
             (c) radio waves
                              (d) audible sound waves
Sol.       (a)


Q.70   When we change a feeble sound to a loud sound, we increase its :
                (a) frequency                      (b) amplitude
             (c) velocity                          (d) wavelength

Sol.      (b)


Q.71   Which kind of sound is produced in an earthquake before the main shock wave begins?
                (a) ultrasound                     (b) infra sound
             (c) audible sound                (d) none of the above

Sol.      (b)


Q.72    Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the strings suitably. By doing so he is adjusting
                (a) intensity of sound only
                (b) amplitude of sound only
                (c) frequency of the sitar string with the frequency of other musical instruments
                (d) loudness of sound
Sol.       (c)


Q.73    'Note' is a sound :
                (a) of a mixture of several frequencies
             (b) of mixture of only two frequencies

                (c) of a single frequency
             (d) always unpleasant to listen to

Sol.          (c)


Q.74    A key of mechanical piano is first struck gently and then struck again but much harder this time. In the second case :
               (a) sound will be louder but pitch will not be different
               (b) sound will be louder and the pitch will also be higher
               (c) sound will be louder but pitch will be lower
               (d) both loudness and pitch will remain unaffected
Sol.        (a)


Q.75   One of the following can hear infra sound. This one is :
              (a) dog                   (b) bat
           (c) rhinoceros        (d) humans

Sol.        (c)


Q.76   An echo-sounder in a trawler (fishing boat) receives an echo from a shoal of fish 0.4 s after it was sent. If the speed of sound in water is 1500 m/s, how deep is the shoal ?
              (a) 150 m         (b) 300 m         (c) 600 m         (d) 7500 m
Sol.        (b)


Q.77   The speed of highly penetrating ultrasonic waves is :
               (a) lower than those of audible sound waves
               (b) higher than those of audible sound wave
               (c) much higher than those of audible sound waves
               (d) same as those of audible sound waves
Sol.        (d)


Q.78   The ultrasound waves can penetrate into matter to a large extent because they have :
               (a) very high speed
            (b) very high frequency

               (c) very high wavelength
            (d) very high amplitude

Sol.         (b)


Q.79   The frequencies of four sound waves are given below. Which of these sound waves can be used to measure the depth of sea by the echo method ?
               (a) 15,000 Hz                   (b) 10 kHz
            (c) 50 kHz                        (d) 10,000 Hz

Sol.         (c)


Q.80   Which of the following frequency of sound can be generated by a vibrating simple pendulum as well as by the vibrating vocal cords of a rhinoceros?
               (a) 5 kHz                           (b) 25 Hz
            (c) 10 Hz                           (d) 15,000 Hz

Sol.         (c)


Q.81    Which of the following device does not work on the multiple reflections of sound waves ?
                (a) stethoscope
             (b) hydrophone
             (c) soundboard
             (d) megaphone

Sol.          (b)


Q.82   What type of waves are generated by SONAR device fixed to a fishing ship ?
                   (a) water waves
           (b) radio waves
           (c) sound waves
           (d) infrared waves

Sol.       (c)


Q.83   We can distinguish between the musical sounds produced by different singers on the basis of the characteristic of sound called :
               (a) frequency                (b) timbre
            (c) pitch                         (d) loudness

Sol.         (b)


Q.84   At 20°C, the minimum distance of a person from a sound reflecting surface to hear an echo is :
          
(a) 12.2 m   (b) 17.2 m      (c) 15.2 m       (d) 34.4 m
Sol.        (b)


Questions Based on High Order Thinking Skills (HOTS)

Q.85   The drawing shows a ship 800 in from a cliff. A gun is fired on the ship. After 5 seconds the people at the front of the ship hear the sound of the gun again.

final2

(a) What is the name of this effect ?
(b) What happens to the sound at the cliff ?
(c) How far does the sound travel in 5 seconds ?
(d) Calculate the speed of sound ?

Sol.         (a)   Echo
               
(b)   The sound gets reflected
                     (c)   1600 m
                     (d)   speed of sound  = {{dis\tan ce} \over {time}} = {{1600} \over 5} = 320\,m/s


Q.86    Consider the following sound waves marked A, B, C and D:
                  (a) Which two waves represent sounds of the same loudness but different pitch?
                  (b) Which two waves represent sounds of the same frequency but different loudness?
                  (c) State whether all these sound waves have been produced by the same vibrating body or different vibrating bodies ?
          
(d) Which vibrating body/bodies could have generated the sound waves shown here?
Q86(i)Q86(ii)Sol.         (a) A and D
                 (b) B and D
                (c) Different vibrating bodies
                (d) Tuning forks of different frequencies.


 Q.87    In an experiment, Anhad studies sound waves. He sets up a loudspeaker to produce sound as shown below :
Q87

          Anhad adjusts the signal to the loudspeaker to give a sound of frequency 200 Hz.
                 (a) What happens to the air in-between Anhad and the loudspeaker ?
                 (b) Explain how Anhad receives sound in both ears.
Sol.          (a) The air starts vibrating at 200 Hz.
                      (b) Anhad receives sound by his right ear through loudspeaker and by his left ear by the reflected sound.


Q.88     Figure X shows a trace of a sound wave produced by a particular tuning fork :

syz                  (a) On the graph paper given in Figure Y, draw a trace of the sound wave which has a higher frequency than that shown in Figure X.
                  (b) On the graph paper shown in Figure Z, draw a trace of the sound wave which has a larger amplitude than that shown in Figure X.
Sol.
211


Q.89    Three different vibrating objects produce three types of sounds X, Y and Z. Sounds X and Y cannot be heared by a man having normal range of hearing but sound Z can be heard easily. The sound X is used in hospital to break kidney stones of a patient into fine grains which then get:flushed out with Urine. The sound similar to that which is produced during an earthquake before the main shock wave is generated.
                (a) What type of sounds are (i) X (ii) Y, and (iii) Z ?
                (b) Name one device which can produce sound like X.
                (c) Name one device in a science laboratory which can produce sound like Y.
                (d) Name one device in our homes which can produce sound like Z.
            
(e) What is the frequency range of sounds like Z?

Sol.          (a) Ultrasound (ii) Infra sound (iii) Audible sound
                 (b) SONAR
                 (c) Simple pendulum.
                 (d) Doorbell.
                 (e) 20 - 20,000 Hz


Q.90    A man is kidnapped, blindfolded and imprisoned in a big room. How could the man tell if he was in :
               (a) a city (b) a village (c) a bare room (d) a furnished room?
Sol.         (a) In a city lot of sound from traffic would be heard.
               (b) In a village much less noise.
               (c) In a bare room if too much reverberation is produced.
               (d) In a furnished room if sound of a person speaking is clear