# Class 9 Science Chapter 12 Sound

NCERT Class 9 Science Chapter 12 Sound Question Answer to each chapter is provided in the list so that you can easily browse throughout different chapters Class 9 Science Chapter 12 Sound and select need one.

## NCERT Class 9 Science Chapter 12 Sound

Also, you can read the SCERT book online in these sections Solutions by Expert Teachers as per SCERT (CBSE) Book guidelines. These solutions are part of SCERT All Subject Solutions. Here we have given SCERT Class 9 Science Chapter 12 Sound Solutions for All Subjects, You can practice these here.

### Sound

Chapter – 12

GENERAL SCIENCE

Textbook Page No. 162

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

Ans. When an object vibrates, it forces the neighbouring particles of the medium to vibrate. These vibrating particles then force the particles adjacent to them to vibrate. In this way vibrations produced by an object are transferred till it reaches the ear.

Textbook Page No. 163

1. Explain how sound is produced by your school bell.

Ans. When the school bell is struck with a hammer, the hammer hits the gong again and again that causes compressions and rarefactions in the air. These vibrations travel in the air in the form it starts vibrating and as a result sound is produced in the bell.

2. Why are sound waves called mechanical waves?

Ans. Sound waves are classed as mechanical waves because they pass through a physical medium such as air, liquids like water, or metals like silver. A sound wave’s frequency is defined as the number of rarefactions and compressions per unit of time.

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

Ans. Sound waves are mechanical waves. They cannot travel without a medium. The moon has no atmosphere or medium through which sound waves can travel. Therefore, any sound produced by my friend on the moon will not be heard.

Textbook Page No. 166

1. Which wave property determines

(a) Loudness.

Ans. (a) Amplitude determines loudness.

(b) Pitch?

Ans: Frequency determines pitch.

2. Guess which sound has a higher pitch : guitar or car horn?

Ans. Sound of guitar has a higher pitch.

Textbook Page No. 166.

1. What are wavelength, frequency, time period and amplitude of a sound wave?

Ans. Wavelength of a sound wave is the separation between two nearest points which have same status of oscillation. It is equal to the distance between two consecutive compressions or two consecutive rarefactions.

The wavelength is usually denoted by  (lambda) and its S.I. unit is metre (m).

Frequency: The number of complete waves produced per second is called frequency of sound wave.

The frequency is usually denoted by u (nu) and its S.I. unit of frequency is hertz (Hz).

Time period: The time required to produce one complete wave is called time period of sound wave.

The S.I. unit of time period is second (s) and the time period is usually denoted by the letter T.

Amplitude: The maximum displacement suffered by the particles of the medium from their original undisturbed positions during the propagation of the wave through the medium is called amplitude of sound wave.

The amplitude is usually denoted by the letter A.

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

Ans. The wavelength and frequency of a sound wave are related to its speed by the relation,

Speed = wavelength × frequency

or, v = λυ

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

Ans.

4. 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?

Ans. The distance between successive compressions of a sound wave is the wavelength of the wave and the time interval between successive compressions is the time period of the sound wave.

Hence, the time interval between successive compressions from the source is 0.002s.

Textbook Page No. 166.

1. Distinguish between loudness and intensity of sound.

Ans. Intensity of a sound wave is defined as the amount of sound energy passing through a unit area per second.

Loudness is a measure of the response of the ear to the sound. The loudness of a sound is defined by its amplitude.

Textbook Page No. 167

1. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

Ans. Sound travels the fastest in iron at a particular temperature.

Textbook Page No. 168

1. An echo was heard after 3s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m s⁻¹?

Ans.

Textbook Page No. 169

1. Why are the ceilings of concert halls curved?

Ans. The ceilings of concert halls are curved so that the sound after reflection from the curved surfaces reaches all the corners of the hall.

Textbook Page No. 170

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

Ans. The audible range of frequencies of the average human ear is from 20 Hz to 20000 Hz.

2. What is the range of frequencies associated with

(a) Infrasound?

(b) Ultrasound?

Ans. (a) The range of frequencies associated with infrasound is below 20 Hz.

(b) The range of frequencies associated with ultrasound is above 20 kHz.

Textbook Page No. 172

1. 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 m/s, how far away is the cliff?

Ans.

EXERCISES

Textbook Page No. 172-175

1. What is sound and how is it produced?

Ans. Sound is defined as vibrations that travel through the air or another medium as an audible mechanical wave. It is produced from a vibrating body. The vibrating body causes the medium (water, air, etc.) around it to vibrate thus producing sound. The sound is produced when something vibrates.

2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

Ans.

Reflections of Sound Sound bounces off a solid or a liquid like a rubber ball bounces off a wall. Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection as you have studied in earlier classes. The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane. An obstacle of large size which may be polished or rough is needed for the reflection of sound waves.

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

Ans. Sound is produced by vibrating objects. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Sound moves through a medium from the point of generation to the listener. When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from propagates through the medium. Compression is the region of high pressure and rarefaction is the region of low pressure. Pressure is related to the number of particles of a medium in a given volume. More density of the particles in the medium gives more pressure and vice versa. Thus, propagation of sound can be visualised as propagation of density variations or pressure variations in the medium.

4. Why is sound wave called a longitudinal wave?

Ans. A wave in which the particles of the medium vibrate back and forth along the direction of motion of the wave is called a longitudinal wave. When a sound wave passes through a medium such as air, the particles of the medium vibrate back and forth along the direction of the propagation of the sound wave. That is why, sound wave is called a longitudinal wave.

5. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

Ans. The characteristics of the sounds helps us to identify our friends by his voice while sitting with others in a drak room are mentioned below:

(i) Frequency.

(ii) Amplitude

(iii) Speed.

6. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

Ans. The velocity of light is much more than the velocity of sound. Therefore, the flash of lightning is seen first and the sound of thunder is heard after sometime, though both are produced simultaneously.

7. 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 m s⁻¹.

Ans.

8. 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.

Ans.

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

Ans. The frequency of the source of sound is 100 Hz. It means, the source of sound vibrates 100 times in 1s.

∴ Number of vibration made in one minute = 100×60 = 6000

Hence, the source of sound vibrates 6000 times in a minute.

10. Does sound follow the same laws of reflection as light does? Explain.

Ans.

Yes sound follows the same laws of reflection as light does.

The laws of the of reflection as light are mentioned below:

(i) The incident sound wave, the normal to the reflecting surface at the point of incidence and the reflected sound wave, all lie in the same plane.

(ii) The angle of incidence of sound is equal to the angle of reflection of sound.

11. 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?

Ans. The impression of any sound heard by a person remains for 0.1 second in his brain. If two sounds reach within an interval of 0.1 seconds, the human ear cannot distinguish between two sounds and they appear to be just one. Now, the velocity of sound increases with an increase in temperature. Therefore on a hotter day, the velocity of sound increases. As a result, the reflected sound waves from the distant object returns to the source in a time less than 0 -1 second. Therefore, a distinct echo cannot be heard on a hotter day.

12. Give two practical applications of reflection of sound waves.

Ans. Two practical applic`ations of reflections of sounds waves are mentioned below:

(i) Speaking tube or megaphone: A megaphone is a horn-shaped tube. When some sound is produced at the narrow end of the megaphone tube, the sound waves are prevented from spreading out due to several reflections on the inner walls of the tube.

(ii) Stethoscope: Stethoscope is an instrument used by the doctors to hear sounds produced within the body of the patient, particularly in the heart or lungs. Here, the sound of the patient’s heart beats (or lungs) reaches to the doctor’s ear due to the multiple reflections of sound along the tube of the stethoscope.

13. A stone is 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? Given, g = 10 m s⁻² and speed of sound 340 ms⁻¹.

Ans.

14. A sound wave travels at a speed of 339 m s⁻¹. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

Ans.

Since the upper limit of frequency for average human being is 20000 Hz, the sound wave will not be audible.

15. What is reverberation? How can it be reduced?

Ans. If a sound is made in a big hall, it persists for a longer time due to the repeated reflections from the walls, ceiling, etc. of the hall. This persistence of sound due to repeated reflection is called reverberation. Excessive reverberation is not desirable in the big hall as the sound becomes  blurred and distorted.

Excessive reverberation can be reduced by covering the roof and walls of the hall or auditoriums with sound-absorbing materials like compressed fibreboard, rough plaster, etc. reverberation, the sound-absorbing properties of some materials are also used in making the seats in the big hall.

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

Ans. It refers to how loud or soft a sound seems to a distant listener. The loudness of sound is determined by the intensity or amount of energy present in sound waves and is expressed in decibels. As the level of decibel gets higher sound waves have greater intensity and sounds are louder.

The factors it depends on are mentioned below:

(i) Sensitiveness of the ear.

(ii) Amplitude and hence on the intensity of sound. If the amplitude of vibration of the source producing the sound waves increases, the loudness of sound also increases.

17. Explain how bats use ultrasound to catch a prey.

Ans. The bats produce the ultrasonic wave which gets reflected from the prey and returns to the bats’ ears. This makes the bat to find out the distance and position of the prey.

18. How is ultrasound used for cleaning?

Ans. Ultrasound is used to clean parts in located in hard-to-reach parts of objects such as small parts used in watches, odd-shaped parts such as a spiral tube, electronic components, etc. Such objects are placed in cleaning solution and ultrasound waves are passed in the solution. Due to high frequency vibrations, particles of dust, grease, etc. get detached from the object and get into cleaning solution and the object gets thoroughly cleaned.

19. Explain the working and application of a sonar.

Ans. SONAR stands for Sound navigation and Ranging. It is a device used in the ships to locate rocks, icebergs, submarines, etc. It is also used to measure the depth of a sea. Sonar uses the phenomenon of reflection of sound waves.

It consists of two parts are mentioned below:

(a) A transmitter for emitting ultrasound waves.

(b) A receiver for receiving ultrasound waves.

A sonar device is installed to the under- side of a ship as shown in the figure. Suppose, we want to measure the depth of a sea.

The transmitter of the sonar produces and transmits ultrasound waves of very high frequency. The waves are reflected from the sea bed and are received by the receiver of the sonar. The receiver converts the ultrasound waves into electrical signals which are appropriately interpreted. The time interval between the transmission and reception of the waves is noted. Let d be the depth of the sea and v be the speed of ultrasound in sea water. The speed of ultrasound in water is same as that of ordinary sound in water. Let be the time interval between the transmission and reception of the ultrasound waves. Then, time taken by the ultrasound to reach the bottom of sea.

20. 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.

Aas. The time taken by the sound to travel from the submarine to the object, and back to the ship (in the form of an echo) is 5 s. Therefore, the time taken by the sound to travel from the submarine to the object is half of this time, which is

Let the speed of sound in water be v.

Distance of the object from the submarine, d = 3625 m

∴ d = v × time taken by the sound to travel from the submarine to the object.

Hence, the speed of sound in water is 1450 m s⁻¹.

21. Explain how defects in a metal block can be detected using ultrasound.

Ans. Metallic blocks are used in the construction of big structures like buildings, bridges, machines, etc. If there are defects, such as cracks or holes inside the metal block used, the strength of the structure is reduced. Such defects are invisible from outside. Ultrasound waves are used to detect such defects.

Ultrasound waves are passed through one face of the metallic block to be tested and ultrasound detectors are placed on the opposite face to detect the transmitted waves.

If there is no defect (crack or hole) inside the metal block, the ultrasound waves passes through the block without any interruption. However, if there is even a small defect in the path of the ultrasound waves, they will be reflected back as shown in the figure. When this happens, it indicates that the metal block is defective.

22. Explain how the human ear works.

Ans. Ear is an extremely sensitive device for hearing sounds. It allows us to hear by converting pressure variations in air with audible frequencies into electric signals that travel to the brain via the auditory nerve.

The human ear works are mentioned below:

(i) Outer car.

(ii) Middle ear. and

(iii) Inner ear.

These three parts have different organs and different functions are mentioned:

(i) Outer ear: The outer ear is called ‘pinna’. It collects the sound waves from the surroundings and passes the sound waves through the auditory canal. The auditory canal bears a thin membrane called the ear drum or tympani membrane, at the end of the canal. When a compression of the medium (i.e. air) reaches the eardrum, the pressure on the outside of the membrane increases which forces the eardrum inward. When the rarefaction reaches the eardrum, it again moves outward. Thus, vibration of eardrum occurs in various levels.

(ii) Middle ear: The middle ear consists of three bones: the hammer, anvil and stirrup. The vibrations are amplified several times by these three bones in the middle ear. The middle ear transmits the amplified pressure variations received from the sound waves to the inner ear.

(iii) Inner ear: The inner ear consists of cochlea and auditory nerve. Here, the pressure variations are turned into electrical signals by the cochlea, a coil type organ. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound. Thus, by various mechanisms in the three parts of an ear ultimately make sound to be heard by us.

Scroll to Top