We are providing you with the short and detailed notes of class 9th physics chapter Sound along with the ncert solutions and answers to the inbuilt questions. Students would get help in case they need to solve the back exercises and get their concepts clear. In this article, we have penned down the important formulas and diagrams to help the students.
Let us start with our class 9 12th chapter Sound
A sound is a form of energy that produces a sensation of hearing in our ears.
Propagation of Sound- Sound is produced by vibrating objects. When any object is struck it produces sound.
Medium- The matter or substance through which sound is transmitted is called a medium. Sound can travel through solid, liquid, air but the air is the most common medium for sound propagation.
Wave- A wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. They in turn produce similar motion in others.
The particles of the medium do not move forward themselves, but the disturbance is carried forward. This is what happens during the propagation of sound in a medium, hence sound can be visualized as a wave. Sound waves are characterized by the motion of particles in the medium and are called mechanical waves.
When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C), as shown in the following figure. This compression starts to move away from the vibrating object. When the vibrating object moves backward, it creates a region of low pressure called rarefaction (R).
As the object moves back and forth rapidly, a series of compressions and rarefactions are created in the air. These make the sound wave that propagates through the medium. Compression is the region of high pressure and rarefaction is the region of low pressure.
Observation- The sound of the bell can be heard when air is inside the jar. When air is taken out then a sound cannot be heard.
Conclusion- This shows that the presence of a medium is necessary for the propagation of sound waves.
Properties of Sound Waves
Frequency- The number of oscillations of sound waves is called its frequency. The number of peaks and troughs per unit of time will give frequency. It is represented by v (nu) and its SI unit is Hertz (Hz).
Time Period- The time taken to complete one oscillation is called its time period. Its unit is second and is represented by T.
Relation of frequency and time period:
v = 1/T
Amplitude- The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A. Its unit is a meter.
Softness or Loudness of Sound- If the amplitude is smaller then the sound will be softer and if it is larger the sound will be louder. Higher amplitude helps the sound wave is traveling faster.
Speed of sound- It is the distance which compression or a rarefaction travels per unit of time.
So, Speed = Wavelength Frequency
The speed of the sound remains almost the same for all frequencies in a given medium under the same physical condition.
Speed of Sound in Different Media at 25° C.
Reflection of Sound- Sound reflects off a surface in the same way as light reflects and follows the same rules of reflection. The incident sound and the reflected sound make equal angles with the normal and all three are in the same plane.
Echo- In bigger monuments and large empty halls you can hear reflections of your sound after you speak something. This effect is known as echo. The sensation of sound persists in our brain for 0.1 seconds, so to hear an echo the time difference between the original sound and reflected sound should be more than that.
Reverberation- Repeated reflections of sound result in the persistence of sound and are called reverberation.
Use of Reverberation of Sound
Following instruments use this property of sound:
- Megaphones, Shehanais, Trumpets
- Stethoscope
- Curved dome of concert halls
Range of Hearing in humans
From 20 Hz to 20000 Hz
Sounds below 20 Hz are called infrasonic.
Sounds above 20000 Hz are called ultrasonic.
NCERT Solutions For Class 9th Sound
Q1. How does the sound produced by a vibrating object in a medium reach your ear?
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 the vibrating tuning fork travels outward. This energy that reaches the ears makes the eardrums vibrate and thus we hear the sound
Q2. Explain how sound is produced by your school bell.
Air is the commonest material through which sound propagates. When the school bell is rung, it pushes the molecules of the air in front of it.
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 bell moves back, it creates a region of low pressure in the air, commonly called rarefaction.
This region has low pressure, low density, and more volume. As the bell 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 the vibrating bell travels outward. This energy that reaches the ears makes the eardrums vibrate and thus we hear sound.
Q3. Why are sound waves called mechanical waves?
Some mechanical energy is required to make an object vibrate. Sound energy cannot be produced on its own. The mechanical energy Of a vibrating object travels through a medium and finally reaches the ear. Therefore, the sound waves are 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?
No, I will not be able to hear the sound, because the moon has no atmosphere. Therefore, no sound waves can travel to your ears and, no sound is heard.
Q5. Which does wave property determines (a) loudness, (b) Pitch?
- The amplitude of the wave determines the loudness; the more the amplitude of a wave, the more is the loudness produced.
- The pitch is determined by the frequency of the wave. Higher the frequency of a wave more is its pitch and shriller is the sound.
Q6. Guess which sound has a higher pitch; guitar or car horn?
A car horn has a higher pitch than a guitar because the sound produced by the former is shriller than the latter.
Q7. What is the wavelength, frequency, time period, and amplitude of a sound wave?
Frequency- The number of compressions or rarefactions are 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.
Q8. How are the wavelength and frequency of a sound wave related to its speed?
Speed of sound Frequency x Wavelength
Q9. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m s-1 in a given medium.
Frequency = 220 Hz
Speed of sound = 440 m s-1
We know the speed of sound Frequency x Wavelength = 220 x Wavelength
Wavelength =
V ϑ = 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 successive compressions from the source?
Answer:
v= 500Hz
distance=450m
v=1/t
T= 1/v= 1/500; 0.002 seconds.
Q11. Distinguish between loudness and intensity of sound.
The loudness depends on energy per unit area of the wave and 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.
Q12. In which of the three media, air, water, or iron, does sound travel the fastest at a particular temperature?
Sound travels fastest in iron as compared to water and air.
An echo is returned in 3 s. mat is the distance of the reflecting surface from the source, given the speed of sound is 342 m s-1
Q13. Why are the ceilings of concert halls curved?
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.
Q14. What is the audible range of the average human ear?
An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.
Q15. What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?
(a) Infrasound: Sound waves between the Frequencies 1 and 20 Hz.
(b) Ultrasound: Sound waves of frequencies above 20,000 Hz.
Q16. What is sound and how is it produced?
Sound is mechanical energy that produces a sensation of hearing. When an object is set into vibrations, the sound is produced.
Q17. Cite an experiment to show that sound needs a material medium for its propagation.
Take an electric circuit that 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, the 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 that has to carry energy from the bell to the bell jar is removed. It shows that sound needs a material medium for its propagation.
Q18. Why is a sound wave called a longitudinal wave?
The sound wave is called a longitudinal wave because the particles of the medium vibrate in the direction of the propagation of the wave.
Q19. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a darkroom?
The characteristic of sound is quality or timbre.
Q20. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?
The speed of sound is 330 m/sec in air medium at 0°C. Whereas the speed of light is 3 x 108m/sec. When we compare the speed of light with that of the speed of sound, the speed of light is greater than that of the speed of sound. Therefore thunder is heard a few seconds after the flash is seen.
Question 21. 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.
Answer: google docs see dekh le
Q22. Two children are at opposite ends of an aluminum rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and aluminum to reach the second child.
Answer:
Q23. The frequency of a source/ sound is 100 Hz. How many times does it vibrate in a minute?
Answer:
Q24. Does sound follow the same laws of reflection as light does? Explain.
Yes. Sound follows the same laws of reflection as that of light because,
(i) Angle of incidence of sound is always equal to that of the angle of reflection of sound waves.
(ii) The direction in which sound is incident, the direction in which it is reflected and normal all lie in the same plane.
Q25. 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 remain the same. Do you hear echo sounds on a hotter day?
Answer:
Time is inversely proportional to speed. As the temperature increases, the speed increases. Thus on a hot day due to high temperature the speed of sound increases. Hence the time will decrease and we can hear the echo sooner.
Q26. Give two practical applications of reflection of sound waves.
Reflection of sound is used in megaphones, horns, and musical instruments such as trumpets and shehnai. It is used in a stethoscope for hearing a patient’s heartbeat. Ceilings of the concert halls are curved, so that sound after reflection reaches all comers of the hall.
Q27. A stone dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Giving, g = 10 ms-2 and speed of sound = 340 m s-1.
Answer:
Question 14. A sound wave travels at a speed of 339 ms-1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
Answer:
Q28. What is reverberation? How can it be reduced?
The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.
To reduce the undesirable effects due to reverberation, roofs and walls of the auditorium are generally covered with sound-absorbent materials like compressed fiberboard, rough plaster, or draperies. The seat materials are also selected to have sound absorption properties.
Q29. What is the loudness of sound? What factors does it depend on?
The loudness of sound is determined by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate. Loud sound can travel a larger distance as it is associated with higher energy. A sound wave spreads out from its source. As it moves away from the source its amplitude as well as its loudness decreases.
Q30. Explain how bats use ultrasound to catch prey.
Bats search out their prey by emitting and detecting reflections of ultrasonic waves. The high-pitched ultrasonic squeaks of the bat are reflected from the obstacles or prey and return to the bat’s ear. The nature of reflection tells the bat where the obstacle or prey is and what it is like.
Q31. How is ultrasound used for cleaning?
Ultrasound is used to clean parts located in hard-to-reach places (i.e.) spiral tubes, odd-shaped parts, electronic components, etc. Objects to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the dust particles, grease get detached and drop out. The objects thus get thoroughly cleaned.
Q32. Explain the working and application of a sonar.
Working: SONAR Consists of a transmitter and a detector and is installed in a boat or a ship as shown in fig. The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted. The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between the transmission and reception of the ultrasound.
Q33. A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
Time is taken between transmission and reception of signal = 5 sec.
Distance of the object from the submarine = 3625 m.
Q34. Explain how defects in a metal block can be detected using ultrasound.
Ultrasounds can be used to detect cracks and flaws in metal blocks. Metallic components are used in the construction of big structures like buildings, bridges, machines, and scientific equipment.
The cracks or holes inside the metal blocks, which are invisible from the outside, reduce the strength of the structure. Ultrasonic waves are allowed to pass through the metallic block, and detectors detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected, indicating the flaw or defect.
Q35. Explain how the human ear works.
The outer ear is called “pinna. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal, a thin membrane called the eardrum or the tympanic membrane.
When compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches it.
In this way, the eardrum vibrates. The vibrations are amplified several times by three bones (the hammer, anvil, and stirrup) in the middle ear. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear.
In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve and the brain interprets them as sound.
Conclusion
We have penned down the short and detailed summary of class 9th physics chapter Sound along with the ncert solutions and inbuilt questions and answers with the relevant diagrams. Students would get help as to how to solve the back exercises and attempt questions during their examinations.
