Science NCERT Grade 9, Chapter 12, Sound quenches the curiosity on questions related to sound. The chapter, Sound deals with concepts like production of sound with the help of activity using a tuning fork. The demonstration of the bell jar experiment justifies the fact that sound needs a medium to travel. The properties and characteristics of sound waves are also discussed comprehensively in the chapter, Sound. Differences between longitudinal waves and transverse waves are included in the discussion.
The concept of frequency, amplitude and speed of the sound wave is well covered with mathematical expressions and numericals. Graphical representation of a sound wave showing the regions of compressions and rarefactions finds mention in this chapter. It also discusses the effect of variation of density and pressure on a sound wave.

In addition to this, the formula for the time period of the sound wave along with its SI unit is also given in the chapter, Sound. Characteristics associated with a sound like pitch, loudness and softness, tone and noise are discussed thoroughly in the chapter. Values of the speed of sound in different media are shown in a tabular format. The chapter, Sound also includes concepts like the reflection of sound, echo, reverberations, uses of multiple reflections of sound, ultrasound, sonar etc. The chapter ends with a detailed discussion on the structure of the human ear.

The chapter, Sound covers the following topics:

1. Production of Sound
2. Propagation of Sound
3. Reflection of Sound
4. Range of Hearing
5. Applications of Ultrasound
6. Structure of Human Ear

#### Question 1:

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

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 from one particle to another till it reaches the ear.

##### Video Solution for sound (Page: 162 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 162 , Question 1

#### Question 1:

Explain how sound is produced by your school bell.

When the school bell vibrates, it forces the adjacent particles in air to vibrate. This disturbance gives rise to a wave and when the bell moves forward, it pushes the air in front of it. This creates a region of high pressures known as compression. When the bell moves backwards, it creates a region of low pressure know as rarefaction. As the bell continues to move forward and backward, it produces a series of compressions and rarefactions. This makes the sound of a bell propagate through air.

##### Video Solution for sound (Page: 163 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 163 , Question 1

#### Question 2:

Why are sound waves called mechanical waves?

Sound waves force the medium particles to vibrate. Hence, these waves are known as mechanical waves. Sound waves propagate through a medium because of the interaction of the particles present in that medium.

##### Video Solution for sound (Page: 163 , Q.No.: 2)

NCERT Solution for Class 8 science - sound 163 , Question 2

#### Question 3:

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

Sound needs a medium to propagate. Since the moon is devoid of any atmosphere, you cannot hear any sound on the moon.

##### Video Solution for sound (Page: 163 , Q.No.: 3)

NCERT Solution for Class 8 science - sound 163 , Question 3

#### Question 1:

Distinguish between loudness and intensity of sound.

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. The amplitude of a sound decides its intensity, which in turn is perceived by the ear as loudness.

##### Video Solution for sound (Page: 166 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 166 , Question 1

#### Question 2:

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

Speed, wavelength, and frequency of a sound wave are related by the following equation:

Speed ( ) = Wavelength ( ) × Frequency ( ) ##### Video Solution for sound (Page: 166 , Q.No.: 2)

NCERT Solution for Class 8 science - sound 166 , Question 2

#### Question 3:

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

Frequency of the sound wave, = 220 Hz

Speed of the sound wave, v = 440 m s−1

For a sound wave,

Speed = Wavelength × Frequency Hence, the wavelength of the sound wave is 2 m.

##### Video Solution for sound (Page: 166 , Q.No.: 3)

NCERT Solution for Class 8 science - sound 166 , Question 3

#### Question 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?

The time interval between two successive compressions is equal to the time period of the wave. This time period is reciprocal of the frequency of the wave and is given by the relation: ##### Video Solution for sound (Page: 166 , Q.No.: 4)

NCERT Solution for Class 8 science - sound 166 , Question 4

#### Question 1:

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

The speed of sound depends on the nature of the medium. Sound travels the fastest in solids. Its speed decreases in liquids and it is the slowest in gases.

Therefore, for a given temperature, sound travels fastest in iron.

##### Video Solution for sound (Page: 167 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 167 , Question 1

#### Question 1:

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

Speed of sound, v = 342 m s−1

Echo returns in time, t = 3 s

Distance travelled by sound = v × t = 342 × 3 = 1026 m

In the given time interval, sound has to travel a distance that is twice the distance of the reflecting surface and the source.

Hence, the distance of the reflecting surface from the source ##### Video Solution for sound (Page: 168 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 168 , Question 1

#### Question 1:

Why are the ceilings of concert halls curved?

Ceilings of concert halls are curved so that sound after reflection (from the walls) spreads uniformly in all directions.

##### Video Solution for sound (Page: 169 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 169 , Question 1

#### Question 1:

What is the audible range of the average human ear?

The audible range of an average human ear lies between 20 Hz to 20,000 Hz. Humans cannot hear sounds having frequency less than 20 Hz and greater than 20,000 Hz.

##### Video Solution for sound (Page: 170 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 170 , Question 1

#### Question 2:

What is the range of frequencies associated with

(a) Infrasound?

(b) Ultrasound?

(a) Infrasound has frequencies less than 20 Hz.

(b) Ultrasound has frequencies more than 20,000 Hz.

##### Video Solution for sound (Page: 170 , Q.No.: 2)

NCERT Solution for Class 8 science - sound 170 , Question 2

#### Question 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?

Time taken by the sonar pulse to return, t = 1.02 s

Speed of sound in salt water, v = 1531 m s−1

Total distance covered by the sonar pulse = Speed of sound × Time taken

Total distance covered by the sonar pulse .....(i)

Let d be the distance of the cliff from the submarine.

Total distance covered by the sonar pulse = 2d

##### Video Solution for sound (Page: 172 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 172 , Question 1

#### Question 1:

What is sound and how is it produced?

Sound is produced by vibration. When a body vibrates, it forces the neighbouring particles of the medium to vibrate. This creates a disturbance in the medium, which travels in the form of waves. This disturbance, when reaches the ear, produces sound.

##### Video Solution for sound (Page: 174 , Q.No.: 1)

NCERT Solution for Class 8 science - sound 174 , Question 1

#### Question 2:

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

When a vibrating body moves forward, it creates a region of high pressure in its vicinity. This region of high pressure is known as compressions. When it moves backward, it creates a region of low pressure in its vicinity. This region is known as a rarefaction. As the body continues to move forward and backwards, it produces a series of compressions and rarefactions (as shown in the following figure). ##### Video Solution for sound (Page: 174 , Q.No.: 2)

NCERT Solution for Class 8 science - sound 174 , Question 2

#### Question 3:

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

Take an electric bell and hang this bell inside an empty bell-jar fitted with a vacuum pump (as shown in the following figure). Initially, one can hear the sound of the ringing bell. Now, pump out some air from the bell-jar using the vacuum pump. It will be observed that the sound of the ringing bell decreases. If one keeps on pumping the air out of the bell-jar, then at one point, the glass-jar will be devoid of any air. At this moment, no sound can be heard from the ringing bell although one can see that the prong of the bell is still vibrating. When there is no air present inside, we can say that a vacuum is produced. Sound cannot travel through vacuum. This shows that sound needs a material medium for its propagation.

##### Video Solution for sound (Page: 174 , Q.No.: 3)

NCERT Solution for Class 8 science - sound 174 , Question 3

#### Question 4:

Why is sound wave called a longitudinal wave?

The vibration of the medium that travels along or parallel to the direction of the wave is called a longitudinal wave. In a sound wave, the particles of the medium vibrate in the direction parallel to the direction of the propagation of disturbance. Hence, a sound wave is called a longitudinal wave.

##### Video Solution for sound (Page: 174 , Q.No.: 4)

NCERT Solution for Class 8 science - sound 174 , Question 4

#### Question 5:

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

Quality of sound is that characteristic which helps us identify a particular person. Sound produced by two persons may have the same pitch and loudness, but the quality of the two sounds will be different.

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NCERT Solution for Class 8 science - sound 174 , Question 5

#### Question 6:

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

The speed of sound is less than the speed of light . Sound of thunder takes more time to reach the Earth as compared to light. Hence, a flash is seen before we hear a thunder.

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NCERT Solution for Class 8 science - sound 174 , Question 6

#### Question 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.

For a sound wave,

Speed = Wavelength × Frequency Given that the speed of sound in air = 344 m/s

(i) For, = 20 Hz (ii) For,  Hence, for humans, the wavelength range for hearing is 0.0172 m to 17.2 m.

##### Video Solution for sound (Page: 174 , Q.No.: 7)

NCERT Solution for Class 8 science - sound 174 , Question 7

#### Question 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.

Let the length of the aluminium rod be d.

Speed of sound wave in aluminium at 25°C, Therefore, time taken by the sound wave to reach the other end, Speed of sound wave in air at 25°C, Therefore, time taken by sound wave to reach the other end, The ratio of time taken by the sound wave in air and aluminium: ##### Video Solution for sound (Page: 174 , Q.No.: 8)

NCERT Solution for Class 8 science - sound 174 , Question 8

#### Question 9:

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

Frequency is defined as the number of oscillations per second. It is given by the relation: Number of oscillations = Frequency × Total time

Given, Frequency of sound = 100 Hz

Total time = 1 min = 60 s

Number of oscillations/Vibrations = 100 × 60 = 6000

Hence, the source vibrates 6000 times in a minute, producing a frequency of 100 Hz.

##### Video Solution for sound (Page: 174 , Q.No.: 9)

NCERT Solution for Class 8 science - sound 174 , Question 9

#### Question 10:

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

Sound follows the same laws of reflection as light does. The incident sound wave and the reflected sound wave make the same angle with the normal to the surface at the point of incidence. Also, the incident sound wave, the reflected sound wave, and the normal to the point of incidence all lie in the same plane.

##### Video Solution for sound (Page: 174 , Q.No.: 10)

NCERT Solution for Class 8 science - sound 174 , Question 10

#### Question 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?

An echo is heard when the time interval between the original sound and the reflected sound is at least 0.1 s. The speed of sound in a medium increases with an increase in temperature. Hence, on a hotter day, the time interval between the original sound and the reflected sound will decrease. Therefore, an echo can be heard only if the time interval between the original sound and the reflected sound is greater than 0.1 s.

##### Video Solution for sound (Page: 174 , Q.No.: 11)

NCERT Solution for Class 8 science - sound 174 , Question 11

#### Question 12:

Give two practical applications of reflection of sound waves.

(i) Reflection of sound is used to measure the distance and speed of underwater objects. This method is known as SONAR.

(ii) Working of a stethoscope is also based on reflection of sound. In a stethoscope, the sound of the patient’s heartbeat reaches the doctor’s ear by multiple reflection of sound.

##### Video Solution for sound (Page: 174 , Q.No.: 12)

NCERT Solution for Class 8 science - sound 174 , Question 12

#### Question 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 m s−1.

Height of the tower, s = 500 m

Velocity of sound, v = 340 m s−1

Acceleration due to gravity, g = 10 m s−2

Initial velocity of the stone, u = 0 (since the stone is initially at rest)

Time taken by the stone to fall to the base of the tower, t1

According to the second equation of motion:  Now, time taken by the sound to reach the top from the base of the tower, Therefore, the splash is heard at the top after time, t

Where, ##### Video Solution for sound (Page: 174 , Q.No.: 13)

NCERT Solution for Class 8 science - sound 174 , Question 13

#### Question 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?

Speed of sound, v = 339 m s−1

Wavelength of sound, = 1.5 cm = 0.015 m

Speed of sound = Wavelength × Frequency The frequency range of audible sound for humans lies between 20 Hz to 20,000 Hz. Since the frequency of the given sound is more than 20,000 Hz, it is not audible.

##### Video Solution for sound (Page: 174 , Q.No.: 14)

NCERT Solution for Class 8 science - sound 174 , Question 14

#### Question 15:

What is reverberation? How can it be reduced?

Persistence of sound (after the source stops producing sound) due to repeated reflection is known as reverberation. As the source produces sound, it starts travelling in all directions. Once it reaches the wall of a room, it is partly reflected back from the wall. This reflected sound reaches the other wall and again gets reflected partly. Due to this, sound can be heard even after the source has ceased to produce sound.

To reduce reverberations, sound must be absorbed as it reaches the walls and the ceiling of a room. Sound absorbing materials like fibreboard, rough plastic, heavy curtains, and cushioned seats can be used to reduce reverberation.

##### Video Solution for sound (Page: 175 , Q.No.: 15)

NCERT Solution for Class 8 science - sound 175 , Question 15

#### Question 16:

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

A loud sound has high energy. Loudness depends on the amplitude of vibrations. In fact, loudness is proportional to the square of the amplitude of vibrations.

##### Video Solution for sound (Page: 175 , Q.No.: 16)

NCERT Solution for Class 8 science - sound 175 , Question 16

#### Question 17:

Explain how bats use ultrasound to catch a prey.

Bats produce high-pitched ultrasonic squeaks. These high-pitched squeaks are reflected by objects such as preys and returned to the bat’s ear. This allows a bat to know the distance of his prey.

##### Video Solution for sound (Page: 175 , Q.No.: 17)

NCERT Solution for Class 8 science - sound 175 , Question 17

#### Question 18:

How is ultrasound used for cleaning?

Objects to be cleansed are put in a cleaning solution and ultrasonic sound waves are passed through that solution. The high frequency of these ultrasound waves detaches the dirt from the objects.

##### Video Solution for sound (Page: 175 , Q.No.: 18)

NCERT Solution for Class 8 science - sound 175 , Question 18

#### Question 19:

Explain the working and application of a sonar.

SONAR is an acronym for Sound Navigation And Ranging. It is an acoustic device used to measure the depth, direction, and speed of under-water objects such as submarines and ship wrecks with the help of ultrasounds. It is also used to measure the depth of seas and oceans. A beam of ultrasonic sound is produced and transmitted by the transducer (it is a device that produces ultrasonic sound) of the SONAR, which travels through sea water. The echo produced by the reflection of this ultrasonic sound is detected and recorded by the detector, which is converted into electrical signals. The distance (d) of the under-water object is calculated from the time (t) taken by the echo to return with speed (v) is given by 2d = v × t. This method of measuring distance is also known as ‘echo-ranging’.

##### Video Solution for sound (Page: 175 , Q.No.: 19)

NCERT Solution for Class 8 science - sound 175 , Question 19

#### Question 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.

Time taken to hear the echo, t = 5 s

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

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

Velocity of sound in water, ##### Video Solution for sound (Page: 175 , Q.No.: 20)

NCERT Solution for Class 8 science - sound 175 , Question 20

#### Question 21:

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

Defects in metal blocks do not allow ultrasound to pass through them and they are reflected back. This fact is used to detect defects in metal blocks. Ultrasound is passed through one end of a metal block and detectors are placed on the other end. The defective part of the metal block does not allow ultrasound to pass through it. As a result, it will not be detected by the detector. Hence, defects in metal blocks can be detected using ultrasound. ##### Video Solution for sound (Page: 175 , Q.No.: 21)

NCERT Solution for Class 8 science - sound 175 , Question 21

#### Question 22:

Explain how the human ear works. 