NCERT solution for class 9//Science - Chapter 12 - Sound

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NCERT solution for class 9//Science -  Chapter 12 - Sound

Chapter 12 -Sound 

Page number 162 

Q1. How does the sound produced by A vibrating object in a medium reach your ear? 

Answer- When an object vibrate  the particles of the medium around it vibrating object from its equilibrium position after displacing the adjacent particles to all over the medium it comes back in its original position this process continuous in a medium the sound reaches  your ear

Page number 163 

Q2. Explain how sound is produced by your school bell? 

Answer - When the bell continuous to move forward and backward then it create the series of the compression and rarefaction and it produced sound

Q3. Why are sound waves called mechanical waves ? 

Answer- Sound rays propagate only through the material medium so it is called mechanical waves

Q4. Suppose you and your friend are on the moon will you be able to hear any sound produced by your friend ? 

Answer - We cannot hear the sound produced by bhai aur friend because there is no atmosphere in the moon so in vacuum sound unable to travel

Page number 166 

Q5 . Which wave property determines loudness and pitch.

Answer - Loudness - the loudness or softness of a sound is determined its amplitude .

Pitch - The pitch of a sound is determined by its frequency 

Q6. Guess which sound has a higher pitch guitar or a car horn 

Answer -Car horn sound has a higher pitch. 

Q7 . What are the wavelength frequency time period and amplitude of a sound wave? 

Answer - Wavelength 

The distance between two consecutive compression and two consecutive Ray diffraction is known as the wavelength its SI unit is meter 

Frequency 

The number of complete oscillation per second is known as the frequency of a sound wave it is measured in hertz ( Hz) 

Amplitude 

The maximum height reached by the crest or trough of the sound wave is known as its  amplitude

Q8. How are the wavelength and frequency of a sound wave related to its speed? 

Answer -  Speed wavelength and frequency of a sound wave are related by the following equation 

Speed = wavelength x frequency 

    V =   𝝀 xv 

Q9. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/ S in a given medium ? 

Answer- Frequency of a sound wave v  = 220 Hz 

Speed of sound wave v =  440 m/ s 

Speed = wavelength x frequency. 

  v   =   𝝀 x v  

440/ 220 = 2m 

Q10. 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 the successive compressions from the source? 

Answer- The time interval between two successive compression is equal to the time period of the wave. And it is a reciprocal of the frequency of the wave 

T= 1/ frequency 

1/ 500 = 0.002 second

Page number 167 

Q11. In which of the three medium air water and iron does sound travel the fastest at a particular temperature? 

Answer- The speed what sound depends on the nature of the medium sound fastest travel in solid decreases in liquid and slowest in gaseous  

Therefore for the given temperature sound travels fastest in iron

Page number 168 .

Q12. An eco return in 3 seconds what is the distance of the reflecting surface from the source given that the speed of the sound is 342 m/s? 

Answer- Speed of sound V= 342 m/s  

Eco returns in time t =  3 s 

Distance travelled by sound = v x t 

342 x 3 = 1026 m  

The distance of the defective surface from the source = 1026/ 2 = 513 m.

Page number 169 

Q14. Why are the ceiling of concert hall curved ? 

Answer- Ceilings of the concern hall are curved so that the sound after reflection spreads uniformly in all directions in halls 

Q15. What is the audible range of the average human ear? 

Answer- The audible range of an average human ear lies between 20 Hz to 20000 Hz

Q16. What is the range of frequencies associated with a)  infrasound b)  ultrasound  

Answer- Infrasound - Its frequency is less than 20 Hz 

Ultrasound- Its frequency is more than 20000 Hz

Page number 172 

Q17. Submarine emits a sonar plus which returns from an underwater relief in 1.02 seconds if the speed of sound in salt water is 1531m/s  How far away is the away  Cliff ? 

Answer- Time taken by the Sonar pulse to return t =  1.02 s 

Speed of the sound in saltwater v  = 1531 m/s 

Distance of the Cliff from the submarine =  v x t 

1.02 x 1531  =1561.62m 

Actual distance d = distance of the clip from the submarine /2 

= 1561/2 

= 780.31m 

Exercise 

Q1. What is sound and how is it produced ?

Answer- Sound is a form of energy which gives the sensation of hearing that is produced by the vibration of particles  by vibrating object

Q2. Describe with the help of a diagram how compression and rarefaction are produced in a year for source of sound ?

Answer-  When A vibrating body moves forward it creates the region of high pressure this high pressure region is known as compression when it moves backward it creates the region of low pressure it is known as rarefaction as the body continuously to move forward and backward it produces the series of compression and rarefaction  

Q3. Cite an experiment to show that sound needs a material medium for its propagation ? 

Answer- Take electric bell and put it in air tight glass Bell jar connected to the vacuum pump put the bell inside the jar and press the switch of the bell we are able to hear the bell ring now remove the air from the bell jar buy vacuum pump the volume of the bail becomes lower and lower and after sometime you will not be heard this is because due to the air has been pumped out. This shows that the sound needs a material medium to travel. 

Q4. Why is sound wave called a longitudinal wave? 

Answer-  Sound wave is called longitudinal wave because it is produced by the compression and rarefaction in the air and it moves parallel to the direction of propagation

Q5. Which characteristics of the sound help you to identify your friends by his voice while sitting with others in a dark room? 

Answer- The quality of a sound enables us to identify our friends by his voice

Q6. Flash and thunder are produced simultaneously but thunder is heard a few second after the flash is seen why  ? 

Answer - Flash and thunder are produced simultaneously but thunder is heard few second after then the flash because the speed of sound is less than the speed of light

Q7. A person has are hearing range from 20hz to 20khz what are the typical wavelength of sound waves in air corresponding to these two frequencies take the speed of sound in air as 344 m/s ? 

Answer- For sound wave  

Speed = wavelength x frequency 

Speed of sound in air  = 344 m/s 

V = 20 Hz  

Frequency = v/V = 344/20 = 17.2 m 

v = 20,000Hz 

Frequency = v/ V 

       344/ 20,000= 0.172m 

Q8. Two children are at opposite ends of an aluminium rod one strike 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?

Answer- velocity of sound in air  = 346 m/ S 

Velocity of sound wave in aluminium  = 6420m/s 

Let length of rod 1 cm

Time taken for sound wave in air t 1 = 1/ velocity in air 

Time taken for sound wave in aluminium t2 = 1/ velocity in aluminium 

= t1/t2  = velocity in aluminium /velocity in air 

6420/ 346 = 18.55: 1

Q9. The frequency of a source of sound is 100 Hz how many times does it vibrate in a minute? 

Answer- Frequency = 100 Hz 

Means the source of sound vibrate hundred times in one second 

1 min = 60 second 

100 x 60 = 6000 times 

Q10. Does sound follow the same laws of reflection as light does explain? 

Answer-  Yes sound follows the same laws of reflection light does 

1. The incident sound waves and reflected sound waves make equal angle with the normal to the reflecting surface 

2. The direction of incident sound direction of reflected sound and the normal lie in same plane

Q 11. What is the work done by the force of gravity on a satellite moving around earth justify your answer? 

Answer - Time is inversely proportional to the velocity of light in any medium if we increases the temperature the speed of  sound also increases does it hot day due to high temperature the speed of sound increases and the time decreases and we can hear the echo  

Time = Distance/ speed

Q12. Can there be displacement of an object in the absence of any force acting on it think discuss this question with reference and teacher 

Answer- 1. Instead to scope used by doctors the sounds of the patient had B reaches to the doctor near by multiple reflection of sound 

2. Ceilings of the auditoriums are curved so that sound after reflection reaches all corners of the auditorium equally

Q13. A stone is dropped from the top of the town 500m high into a pond of water at the base of the tower when is the splash is heard at the top given g= 10m/s and speed of sound 340m/s

Answer-  

Distance covered by stone = 500 m 

Initial velocity u = 0 m/s 

Acceleration due to gravity g  =10m/s 

S = ut+ 1/2 gt 2 

500 = 0+ 1/2 x 10 x t2 

500= 5t2 

t2 = 100 

t= 10 second  

Thus in 10 second the stone reaches the pond 

Speed of sound = 340 m/s 

Time taken by sound to cover 

500 m = 500/ 340 = 1.47 

Total time taken = 10+ 1.47 

                              = 11.47 second  

At 11.47 second splash will be heard on top 

Q14. Sound wave travels at a speed of 339 m s if its wavelength is 1.5 cm what is the frequency of the wave will be audible

Answer- Speed = 339 m/ s 

Wavelength = 1.5 cm = 1.5/100 = 0.015 m 

Speed = wavelength x frequency 

339. = 0.015 x frequency 

Frequency = 339/0.015 

                     = 22600Hz

Q15. What is reverberation how can it be reduced

Answer- The repeated multiple reflection of sound in a enclosed space is known as reverberation it can be reduced by covering the ceiling and walls of their includes space with sound absorbing materials like fibre boards , loose woolens etc

Q16. What is loudness of sound what factors does it depend on ? 

Answer- The loudness of the sound is depends upon its amplitude . The amplitude of sound wave depends upon the force which an object is made to vibrate loud sound can travel on larger distance with less energy. Loudly depends upon amplitude intensity of sound and sensitivity of ear.

Q17. Explain how bats use ultrasound to catch a prey

Answer- Bats are nocturnal animal and their eyesight is weak so they produced high pitch ultrasonic sound waves these are reflected by object  and return to their ear and this allow that to know the distance of a prey

Q 18. How is ultrasound used for cleaning? 

Answer- Object to be cleaned airport in the cleaning solution and ultrasonic sound waves are passed through that solution due to high frequency of these ultrasound waves the detached from the objects and dissolved in solution.

Q19 . Explain the working and application of a SONAR

Answer- Sonar is a device which uses ultrasonic waves to measure distance direction and speed of underwater objects the distance of the object can be calculated by knowing the speed of sound in water and time taken between the transmission and reception of ultrasound the eco produced by reflection of the ultrasonic sound is detected by recorder my detector and it converted into electrical signals the distance of the underwater object is calculated from the time taken by the equal to return with speed V is given by 2 d =  v x t 

Q20 . A sonar device on a submarine send out a signal and receives echo 5 second later . calculate the speed of sound in water with the distance of the object from the submarine is 3625 m

Answer-  Time taken to hear the echo t = 5 second 

Distance of the object from the submarine d= 3625m 

Total distance travelled by the solar waves during the transmission and reception in water  = 2d 

Velocity of sound in water v = 2d/ t 

       = 2 x 3625/ 5 = 1450m/s

Q21. Explain how defects in a metal block can detect in metal box ?

Answer- In metal box do not allow the ultrasound pass through them they reflected back and this nature of sound waves can be used to detect the defects in metal block ultrasound is passed through one end of the metal block and the detector is attached to the another end of the metal block when the ultrasound is passed through  it then the defective part of the  block reflect the sound and it can be detected by detector .  

Q22. Explain how the human ear works? 

Answer- The human ear consists of three parts which are combined rework the outer ear middle ear and the inner ear  

Outer ear . 

It is also called pinna it collects the sound waves from surrounding and transfer it in auditory canal 

Middle ear  

Thin membrane attached to the end of the auditory canal is called ear drum . Sound waves vibrate these ear drum and these vibrations are amplified by three small bones hammer anvil and stirrup .

Inner ear  

Middle ear vibration reaches to the cochlea of inner ear and it converted it into electrical signals which are sent to the brain by auditory nerve and brain interpret them as a sound