12. Sound Class 9 Science Chapter 12 NCERT Solutions (CBSE)

CBSE Class 9 Science Chapter 12: Sound – Full NCERT Textbook Solutions

Chapter 12: Sound

Internal Question and Answer:

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

Answer: When an object vibrates, it sets the particles of the surrounding medium into vibration. These vibrating particles pass on the vibration to nearby particles. In this way, sound travels through the medium as a wave and reaches our ear. The eardrum vibrates, and we hear the sound.

Internal Question and Answer:

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

Answer: When the school bell is struck with a hammer, it starts vibrating rapidly. These vibrations produce sound waves in the surrounding air. The sound waves travel through the air and reach our ears, and we hear the ringing sound of the bell.

2. Why are sound waves called mechanical waves?

Answer:  Sound waves are called mechanical waves because they require a material medium to travel. They cannot travel through a vacuum. The propagation of sound waves is due to the vibration of particles in the medium, which transfer energy from one particle to another. Without a medium, no sound can be produced or heard.

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

Answer: No, you would not be able to hear any sound produced by your friend on the moon. This is because sound waves need a material medium to travel. The moon has no atmosphere and sound cannot travel through a vacuum. Therefore, even if your friend produces sound, it will not reach your ears.

Internal Question and Answer:

1. Which wave property determines (a) loudness, (b) pitch?

Answer: (a) Loudness is determined by the amplitude of the wave.
(b) Pitch is determined by the frequency of the wave.

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

Answer: The guitar has a higher pitch compared to the car horn.

Example 12.1:  A sound wave has a frequency of 2 kHz and wave length 35 cm. How long will it take to travel 1.5 km?

Solution : Given,

Wavelength,

We know that speed, v of the wave = wavelength × frequency

 

The time taken by the wave to travel a distance, d of 1.5 km is

 

Thus sound will take 2.14 s to travel a distance of 1.5 km.

Internal Question and Answer:

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

Answer:  Wavelength :  The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength .

Frequency : The number of complete oscillations per unit time is called the frequency .

 Time period : The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave .

 Amplitude : The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.           

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

Answer: The product of wavelength and frequency of a sound wave is equal to speed.

i.e.,   Speed = wavelength × frequency

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

Solution:  Here,  ,  m/s and  Wavelength,

We know that,

Therefore, the wavelength of a sound wave is 2 m .

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?

Solution : Here, Frequency, , Wavelength,

We know that ,

 

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

Internal Question and Answer:

1. Distinguish between loudness and intensity of sound.

Answer: The distinguishes between loudness and intensity of sound:

Loudness	Intensity
(i) Loudness is a physiological response of the ear to sound.	(i)  Intensity is a physical quantity of sound.
(ii) It depends on the sensitivity of the ears.	(ii) It does not depend on the sensitivity of the ears.
(iii) Loudness is not simply proportional to the square of the amplitude.	(iii) Intensity is proportional to the square of the amplitude of the wave.
(iv) It is measured in decibels (dB).	(iv) It is measured in watts per square meter .

Internal Question and Answer:

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

Answer: Sound travels the fastest in iron at a particular temperature because particles in solids are closely packed, which helps sound vibrations pass more quickly.

Example 12.2: A person clapped his hands near a cliff and heard the echo after 2 s. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 m/s ?

Solution:  Here,  346 m/s ,  2 s

Distance travelled by the sound

In 2 s sound has to travel twice the distance between the cliff and the person.

Hence, the distance between the cliff and the person

Internal Question and Answer:

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

Solution: Here , 342 m/s ,  3 s

Distance travelled by the sound

Therefore, the distance of the reflecting surface from the source

Internal Question and Answer:

1. Why are the ceilings of concert halls curved ?

Answer: The ceilings of concert halls are curved so that sound after reflection from the ceiling reaches all parts of the hall uniformly. This prevents sound from being focused only in one area and ensures that the audience seated in the back rows hears the sound clearly and evenly.

Internal Question and Answer:

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

Answer: The audible range of the average human ear is from 20 Hz to 20,000 Hz (20 kHz).

2. What is the range of frequencies associated with
(a) Infrasound?   (b) Ultrasound?

Answer:  (a) Infrasound: Frequencies less than 20 Hz (below the audible range).

(b) Ultrasound: Frequencies greater than 20,000 Hz (20 kHz) (above the audible range).

Example 12.3: A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound through seawater is 1531 m/s, what is the distance of the seabed from the ship?

Solution: let d is the depth of the sea.

Here,  3.42 s ,  1531 m/s

Distance travelled by the ultrasound

Thus, the distance of the seabed from the ship is 2618 m or 2.62 km.

Internal Question and Answer:

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?

Solution:  let d be the distance between the cliff and submarine .

Here, 1531 m/s

The distance travelled by sonar pulse  speed of sound  time

A/Q,

 

Exercises of Chapter 12: Sound

1. What is sound and how is it produced?

Answer: Sound is a form of energy that produces the sensation of hearing. It is produced by a vibrating object. When an object vibrates, it causes the particles in the surrounding medium (such as air) to vibrate, creating sound waves. If the object stops vibrating, sound production also stops.

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

Answer: When a vibrating object moves forward, it pushes the air particles in front of it together. This forms a region of high pressure called compression (C).

When the object moves backward, the air particles spread apart, forming a region of low pressure called rarefaction (R). These compressions and rarefactions travel through air as sound waves.

 

3. Why is sound wave called a longitudinal wave?

Answer: Sound wave is called a longitudinal wave because the particles of the medium vibrate back and forth along the same direction in which the wave travels. These vibrations create regions of compressions and rarefactions, unlike transverse waves where particles move perpendicular to the wave direction.

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

Answer: The characteristic of sound that helps us identify our friend’s voice is quality or timbre. Different people have different voice qualities, so we can recognise a person by their voice even in a dark room.

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

Answer: Flash and thunder are produced at the same time, but the flash is seen earlier because light travels much faster than sound. Sound travels slowly in air, so thunder is heard a few seconds after the lightning is seen.

6. 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 .

Answer:  We know that ,

For 20 Hz: Here,

Wavelength 

For 20 kHz:

Wavelength

Therefore, the typical wavelengths of sound waves in air corresponding to frequencies of 20 Hz and 20 kHz are approximately 17.2 meters and 17.2 millimeters, respectively.

7. 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.

Solution:  Here, The speed of the sound in air = 346 m/s  and the speed of the sound in Aluminium is = 6420 m/s .

Let  be the length of the aluminium rod .

We know that, 

 

For air  :

For aluminium : 

The sound wave will reach the second child faster in aluminium compared to air. The ratio of the time taken by the sound wave in air to the time taken in aluminium will be less than 1.

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

Answer : The frequency of a source of sound is the number of vibrations or cycles it completes in one second.

Frequency of the source = 100 Hz

Number of vibrations in a minute = Frequency × 60 = 100 Hz × 60 = 6000 vibrations

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

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

Answer:  Yes, sound follows the same laws of reflection as light. The sound wave gets reflected from a surface in such a way that the angle of incidence is equal to the angle of reflection. Also, the incident sound wave, reflected sound wave and the normal all lie in the same plane.

10. 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?

Answer: No, the echo may not be heard clearly on a hotter day. On a hot day, the speed of sound in air increases. As a result, the reflected sound reaches our ears in less time. If the time interval is less than 0.1 second, the ear cannot distinguish the echo separately.

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

Answer: Two practical applications of reflection of sound waves are:

(i) Megaphones and horns : They use reflection of sound to direct sound waves in a particular direction.

(ii) Stethoscope : It uses multiple reflections of sound to carry the heartbeat sound clearly to the doctor’s ears.

12. 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  and speed of sound = 340 m/s.

Solution:  Here, , ,

Using the equation of motion,

 

So, it takes 10 seconds for the stone to fall from the top of the tower to the water surface.

We have,  

 

Therefore, it takes approximately 1.47 seconds for the sound to travel from the water surface to the top of the tower.

Total time = time for stone to fall + time for sound to travel

Total time = 10 seconds + 1.47 seconds Total time = 11.47 seconds

Thus, the splash is heard at the top of the tower approximately 11.47 seconds after the stone is dropped

13. A 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 it be audible?

Answer:  Here, Wavelength, ,  and

We know that , Speed of sound = Frequency × Wavelength

 

 

The frequency of the sound wave is approximately 22,600 Hz.

The audible range for most humans is typically between 20 Hz and 20,000 Hz. Since the frequency of the given sound wave falls within this range, it should be audible to most people.

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

Answer: The persistence of sound in an auditorium due to repeated reflections of sound is called reverberation. It can be reduced by using sound-absorbing materials like curtains, carpets, fibre boards and rough walls in halls or rooms.

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

Answer: Loudness of sound is a physiological response of the ear to the intensity of sound.

Factors loudness depends on: (i) Amplitude of the vibrating body . (ii) Distance from the source . (iii) Surface area of the vibrating body .  (iv) Sensitivity of the ear .

16. How is ultrasound used for cleaning?

Answer: Ultrasound is used for cleaning through a process called ultrasonic cleaning. It involves using high-frequency sound waves, typically above the range of human hearing, to create microscopic bubbles in a liquid cleaning solution. These bubbles collapse rapidly near the objects being cleaned, producing a scrubbing action that removes dirt, grease, and contaminants from the surfaces. Ultrasound is commonly used for cleaning delicate items, jewelry, medical instruments, and industrial parts.

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

Answer: Ultrasound waves are sent through the metal block. If the metal block has no defect (crack or hollow), the waves pass through it and are reflected back from the other side. However, if there is a defect inside the block, the ultrasound waves get reflected early from that defect. By detecting these reflected waves using a detector, the location and size of the defect can be determined.

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Posted 4 months ago