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Updated 6 August 2026

NCERT Solutions

Sound Waves Characteristics and Applications

CBSE · Class 9 · Science

NCERT Solutions for Sound Waves Characteristics and Applications — CBSE Class 9 Science.

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Think It Over

1Two astronauts are repairing the arm of a space station together during a spacewalk. Can they talk to each other and hear the sounds of metal clanking as they do on the Earth?
How do most bats use sound to locate their prey in the dark at night?Show solution

No. In outer space there is a near vacuum, so sound cannot propagate. Therefore astronauts cannot directly hear each other speak or hear metal clanking. They communicate using special devices fitted into their spacesuits.

Most bats locate prey by echolocation: they emit short bursts of ultrasonic waves, and by sensing the reflected echoes they determine the position of obstacles and prey.

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2Which form of energy gets converted to sound energy? How is sound produced and how does it reach our ears?Show solution

Sound is produced by vibrations. Energy from the vibrating source changes into sound energy. Sound then reaches our ears by propagating through a medium such as air, liquid, or solid, in the form of sound waves made of alternate compressions and rarefactions.

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Activity 10.1: Let us explore

1Take a cardboard box with one side open and a rubber band.Show solution

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2Stretch the rubber band across the open side of the box (Fig. 10.2).Show solution

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3Holding the box steady with one hand, pluck the rubber band with a finger. Do you hear any sound?Show solution

Yes, when the stretched rubber band is plucked, it vibrates and produces sound.

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4Pluck the rubber band again and watch it carefully. Is it vibrating?Show solution

Yes, the rubber band is vibrating when plucked.

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5Wait till the rubber band stops vibrating. Do you still hear the sound?Show solution

No. Once the rubber band stops vibrating, the sound also stops.

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6Change the tension in the rubber band by stretching it more or loosening it slightly and plucking it each time. Does the sound change? What changes do you notice?Show solution

Yes. When the tension is increased, the rubber band vibrates faster and the sound becomes higher in pitch. When the tension is loosened slightly, it vibrates slower and the sound becomes lower in pitch. So the change noticed is mainly in the frequency of sound.

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7Remove the rubber band from the box. Stretch it between two fingers and pluck it near your ear. Is the sound still produced? Is it as loud as before?Show solution

Yes, sound is still produced when the rubber band is plucked between two fingers. But it is usually not as loud as before, because the box helps in making the sound louder.

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Threads of Curiosity

1How do humans and animals create sound? While talking or singing, gently touch your throat. Do you feel vibrations anywhere? In humans and some other animals, sound is produced by the vibration of vocal cords, which are tightly stretched muscular flaps located inside the voice box or larynx, in the throat (Fig. 10.3). The tongue, lips, mouth and nasal cavity in humans help in converting sound into speech or music.

Some animals produce sound by striking or rubbing certain body parts. For example, grasshoppers and crickets rub their wings or legs to produce sound.Show solution

Humans and some animals create sound by vibrations. In humans, sound is produced by the vibration of the vocal cords inside the larynx or voice box. The tongue, lips, mouth, and nasal cavity help convert sound into speech or music. Some animals produce sound by striking or rubbing body parts; for example, grasshoppers and crickets rub their wings or legs to produce sound.

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2How do humans and animals create sound? While talking or singing, gently touch your throat. Do you feel vibrations anywhere?Show solution

Yes. While talking or singing, if you gently touch your throat, you can feel vibrations there. These vibrations are from the vocal cords.

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Activity 10.2: Let us explore

1Take a tuning fork and a soft rubber pad.Show solution

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2Hold the tuning fork by its stem.Show solution

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3Strike one of the prongs of the tuning fork gently against the rubber pad (Fig. 10.4b) and bring it close to your ear. Do you hear a sound? (Take care not to strike the tuning fork against a hard surface).Show solution

Yes. A vibrating tuning fork brought near the ear produces sound, so you hear it.

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4Now, gently touch a water surface with one of the vibrating prongs of the tuning fork. Do you see waves forming on the surface of water?Show solution

Yes. The vibrating prongs disturb the water surface, so waves form on the water.

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5Repeat step 3 a few times while bringing the prongs of the tuning fork near your ear in different orientations. Do you hear the sound?Show solution

Yes. You hear the sound in different orientations because the tuning fork is vibrating and sound spreads in different directions.

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Pause and Ponder

1Explore various ways of producing sound.Show solution

Sound can be produced in many ways, such as by plucking strings, striking metal objects, blowing through air columns like a flute, and by vibrating membranes. You can also explore sound from everyday objects and animals that produce sound by rubbing or striking parts of their bodies.

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2Make a list of different types of musical instruments and identify their vibrating parts which produce sound.Show solution

Examples:
- Sitar — vibrating strings
- Tabla — vibrating membrane
- Flute — vibrating air column
- Tanpura — vibrating strings
- Mridangam — vibrating membrane
- Bansuri — vibrating air column

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3Assertion (A): We cannot hear the sound of a bell ringing in a closed jar after most of the air is pumped out.

Reason (R): Sound requires a medium to travel.

Choose the correct statement:

(i) Both A and R are true, but R is not the correct explanation of A.
(ii) Both A and R are true, and R is the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.Show solution

The assertion is true because when most air is removed, sound cannot travel well. The reason is also true because sound needs a material medium to travel. The reason correctly explains the assertion.

So the correct statement is (ii).

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4Assertion (A): Compressions and rarefactions move through the medium.

Reason (R): Individual particles of the medium continuously move forward with the wave.

Choose the correct statement:

(i) Both A and R are true, but R is not the correct explanation of A.
(ii) Both A and R are true, and R is the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.Show solution

The assertion is true: compressions and rarefactions move through the medium. The reason is false because the particles do not move forward with the wave; they only oscillate about their mean positions.

So the correct statement is (iii).

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5When sound travels from a tuning fork to your ear, which of the following actually reaches your ear?
(i) Air particles near the tuning fork
(ii) Energy carried by sound waves
(iii) The tuning fork material
(iv) A continuous stream of compressed airShow solution

What reaches your ear is not the material of the tuning fork or air particles themselves, but the energy carried by sound waves.

So the correct option is (ii).

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6The variation of density of the medium for two sound waves is shown in Fig. 10.17 (a) and (b). Label compression and rarefaction by C and R on it. In the graph given in Fig. 10.17 (c) and (d), label the axes and draw the curves corresponding to Fig. 10.17 (a) and (b).Show solution

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7Conduct Activity 10.1 once again with a thick rubber band and then with a thin rubber band. Does the thin rubber band vibrate faster than the thick rubber band? If yes, how do the frequency and time period of the sound produced by the thin rubber band differ from that of the thick rubber band?Show solution

Yes. The thin rubber band vibrates faster than the thick rubber band. So the sound produced by the thin rubber band has higher frequency and shorter time period than that of the thick rubber band.

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8If the frequency of a sound wave produced by an oscillating piston of a long tube filled with air is

20\mathrm{Hz}

, then how many oscillations does the piston complete per minute?Show solution

Frequency

= 20\,\text{Hz}

means 20 oscillations per second.

In 1 minute = 60 s,

20 \times 60 = 1200

So the piston completes 1200 oscillations per minute.

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9For the sound wave represented by the graph shown in Fig. 10.19, what is half of its wavelength?Show solution

In such a graph, half of the wavelength would be half the distance between two consecutive crests or troughs.

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10Table 10.1 shows the speed of sound in a few media at atmospheric pressure.

Table 10.1: Speed of sound in different media at

15^{\circ}\mathrm{C}

| State | Substance/Medium | Approximate speed |
|---|---|---|
| Solid | Steel | 5000 m s-1 |
| Liquid | Water | 1500 m s-1 |
| Gas | Air | 340 m s-1 |

Compare the speeds in different media by finding the ratio of

(i) the speed of sound in water with respect to the speed in the air.
(ii) the speed of sound in steel with respect to the speed in the water.Show solution

Given speeds:
- Water =

1500\,\mathrm{m\,s^{-1}}

- Air =

340\,\mathrm{m\,s^{-1}}

- Steel =

5000\,\mathrm{m\,s^{-1}}

(i) Ratio of water to air:

1500:340 = 150:34 = 75:17

(ii) Ratio of steel to water:

5000:1500 = 50:15 = 10:3

So the ratios are 75:17 and 10:3.

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11Two friends are standing along a steel fence at a distance of 340 m from each other (Fig. 10.23). Gunjan places her ear over the fence and her friend knocks the fence with a metal object. Using the values of the speed of sound in steel and air given in Table 10.1, calculate the time difference between the sound that reached Gunjan through the airShow solution

Sound reaches through steel faster than through air.

Distance =

340\,\text{m}

Time through air:

t_{air}=\frac{340}{340}=1\,\text{s}

Time through steel:

t_{steel}=\frac{340}{5000}=0.068\,\text{s}

Time difference:

1-0.068 = 0.932\,\text{s}

So the sound through steel reaches about 0.932 s earlier than through air. The difference in arrival times is 0.932 s.

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12An experiment is being set up that requires echoes to arrive at least 0.2 s after the emission of sound. What minimum distance should a reflecting surface be placed at? Assume the speed of sound to be

343 \, \text{m} \, \text{s}^{-1}

.Show solution

For an echo, the sound must travel to the reflecting surface and back in 0.2 s.

So one-way time =

0.2/2 = 0.1\,\text{s}

Distance from reflecting surface:

d = v\times t = 343\times 0.1 = 34.3\,\text{m}

So the reflecting surface should be at least 34.3 m away.

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13Sound travels much farther in water than light, and thus, is used for various underwater applications. A sonar signal sent to find the depth of ocean takes 4 s to return. What is the depth of the ocean at that location if the speed of sound in seawater is

1500\mathrm{ms}^{-1}

?Show solution

The sonar signal takes 4 s for a round trip, so one-way time is

\frac{4}{2}=2\,\text{s}

Depth of ocean:

d = v\times t = 1500\times 2 = 3000\,\text{m}

So the depth is 3000 m.

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Both statements are true. In a closed jar after most air is pumped out, sound cannot be heard because sound requires a medium to travel. So the reason correctly explains the assertion.

Therefore the correct option is (ii).

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A is true because sound travels as alternating compressions and rarefactions through a medium. R is false because the particles of the medium do not move forward with the wave; they only vibrate about their mean positions. So the correct statement is (iii) A is true, but R is false.

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5When sound travels from a tuning fork to your ear, which of the following actually reaches your ear?Show solution

What reaches the ear is not the air particles themselves or the tuning fork material. In sound propagation, energy is transferred through the medium by sound waves. So the correct choice is (ii) Energy carried by sound waves.

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10Table 10.1 shows the speed of sound in a few media at atmospheric pressure.

Table 10.1: Speed of sound in different media at

15^{\circ}\mathrm{C}

Show solution

The table shows the speed of sound in different media at

15^{\circ}\mathrm{C}

. The approximate speeds are: **steel = 5000 m s

^{-1}

, water = 1500 m s

^{-1}

, and air = 340 m s

^{-1}

. The order is solid > liquid > gas**.

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Activity 10.3: Let us investigate

1You and your friend stand on opposite sides of a desk in the classroom. Let your friend gently knock or scratch on the desk. Listen carefully to the sound produced with your ear in the air.Show solution

You can hear the sound of the knock or scratch through the air. This shows that sound is produced by a vibrating or disturbed object and that the sound reaches your ear through the surrounding air.

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2Now, place your ear against the desk, close your other ear and listen again, as shown in Fig. 10.5. Are you able to hear the sound through the table?Show solution

Yes, the sound is heard more clearly through the table. This shows that sound can travel through solids and is usually heard better when the ear is placed against the desk.

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Activity 10.4: Let us investigate

1Take a large tub or bucket of water filled to the brim and two metal spoons.

2Tap the spoons against one another and listen to the sound produced (Fig. 10.6a).

3Now, submerge the two metal spoons in water without touching the sides or bottom of the bucket and tap them against one another again (Fig. 10.6b). Do you again hear the sound produced?

Activity 10.7: Let us experiment (demonstration activity)

1Use a mobile app, such as Phyphox that can identify frequencies of sounds. Use the 'Audio Spectrum' option that displays the frequency graphically or in hertz (Hz).

2Try to sing the musical notes 'Sa, Re, Ga, Ma, Pa, Dha, Ni, Sa' one after another, or use a music or tone generating app on another phone to produce those notes. Observe how the frequency changes as each note is produced.

3Record the approximate frequency values for each musical note.

4Compare the musical notes by taking the ratio of each frequency with respect to the 'Sa'. Do you observe any pattern?

5If both voice and mobile-generated notes are used, compare their frequencies for the same musical notes.

Activity 10.8: Let us experiment (demonstration activity)

1Open a mobile app that can generate sounds.

2Set the frequency to 100 Hz, tap 'play', and listen carefully.

3Increase the frequency in steps of 100 Hz up to 1000 Hz and describe how the sound changes.

4Next, set the frequency to 50 Hz. Reduce the frequency till about 20 Hz or the point where you cannot hear the sound anymore.

Revise, Reflect, Refine

1Which observation best supports the idea that sound is a mechanical wave?

(i) Sound shows reflection
(ii) Sound needs a medium to propagate
(iii) Sound has frequency
(iv) Sound carries energy

2For a sound wave propagating in a medium, increasing its frequency will increase its

(i) wavelength
(ii) speed
(iii) number of compressions per second
(iv) time period

3If 20 compressions pass a point in 4 seconds, the frequency is

(i)

80\mathrm{Hz}

(ii)

5\mathrm{Hz}

(iii)

10\mathrm{Hz}

(iv)

0.2\mathrm{Hz}

4In a room, the reflected sound reaches the ear 0.05 s after its production. Will it produce an echo or reverberation? Justify your answer.

5Graphs representing two sound waves are given in Fig. 10.30. If the scales on the X and Y axes of the two graphs are the same, which of the two sound waves has (i) greater wavelength, and (ii) smaller amplitude?

6The sound waves emitted by three sources A, B and C are represented in Fig. 10.31. If the frequency of A is maximum and C is minimum, identify the corresponding curves, and mark A, B and C on them.

7Draw a graph to represent a sound wave for which the density amplitude is 3 units and wavelength is

4\mathrm{cm}

8In a movie, while showing the explosion of a spacecraft in space, a flash of light is shown along with sound at the same time. What are the errors in this depiction?

9A source produces a sound wave of wavelength

3.44\mathrm{m}

. If the wave travels with a speed of

344\mathrm{m}\mathrm{s}^{-1}

find its time period.

10A ship searching for a sunken ship sent a sonar signal and detected an echo after 5 s. If ultrasonic wave travels at

1525\mathrm{ms}^{-1}

in seawater, approximately how far down in the ocean is the wreckage of the sunken ship located?

11A vehicle is fitted with an ultrasonic distance sensor as part of parking assistance system which provides echolocation, while the driver is reversing the vehicle. It emits ultrasonic wave (about

40\mathrm{kHz}

) which is reflected by the obstacle. When the warning beep starts sounding at a distance of

1.2\mathrm{m}

from the obstacle, how much time is taken by ultrasonic wave to travel to the obstacle and come back? Assume the speed of ultrasonic wave in air to be

345\mathrm{ms}^{-1}

12The speed of sound in air is about

331\mathrm{ms}^{-1}

0^{\circ}\mathrm{C}

and nearly

344\mathrm{ms}^{-1}

22^{\circ}\mathrm{C}

. Roughly how much extra time will the sound of thunder take to travel a distance of

1720\mathrm{m}

, if the air temperature changes from

22^{\circ}\mathrm{C}

0^{\circ}\mathrm{C}

? Assume that all other conditions remain unchanged.

13The variation of density of medium for a sound wave propagating with a speed of

340\mathrm{ms}^{-1}

is shown in Fig. 10.32. Calculate the wavelength and frequency of the sound wave.

14The graphical representation of two sound waves A and B propagating at the same speed of

345\mathrm{m}\mathrm{s}^{-1}

is shown in Fig. 10.33. What is the wavelength of each of them? Also, calculate their frequencies.

15Two identical sound sources are placed at A and B—one in air and one submerged in water (Fig. 10.34). Both produce sounds at the same time, which travel horizontally to the vertical side of the cliff and come back. If the time taken by the sound to return to A is 4.5 times than that of B, what is the ratio between the speeds of sound in air and water?

1Which observation best supports the idea that sound is a mechanical wave?

2For a sound wave propagating in a medium, increasing its frequency will increase its

3If 20 compressions pass a point in 4 seconds, the frequency is

The Journey Beyond

1Many people use earphones extensively these days. Find out the research studies that might have been done to understand the impact of excessive use of earphones on hearing (if any). Also, find out how hearing is tested and what are the decibel ranges for defining mild, moderate and severe hearing loss. What are the government schemes for purchasing or fitting of aids or appliances and free cochlear implants? Write an article on your findings.

2Make a cone using a poster paper or cardboard and adhesive tape. Cover a mobile phone that is playing music with the cone. Compare the loudness of the sound with and without the cone. You can also use another mobile phone with an app to measure the characteristics of the sound in both cases. Try experimenting with different shapes and record your observations. (This activity is to be facilitated by the teacher.)

3How does the curved design of ceilings and walls behind the stage in concert and conference halls improve the quality of sound for the audience compared to flat surfaces? You may consult an architect or search it on the internet.

4Carry out a simple activity to measure the speed of sound, along with a friend in a large open ground of size

200\mathrm{m}

or more. (This activity is to be facilitated by the teacher.)

(i) Your friend stands at one end of the open ground with the balloons, while you stand at the other end with the stopwatch.
(ii) Signal your friend to burst one balloon. When you see the balloon burst, start the stopwatch. As soon as you hear the 'pop' sound of the bursting balloon, stop the timer and note down the reading.
(iii) Repeat this experiment multiple times and take the average value of the times noted.
(iv) Note the approximate distance between you and your friend using a map application on a mobile phone.
(v) Divide the distance measured with the average time to get the average speed of sound. What value of speed did you get from the experiment? Compare it with the speed of sound in air, which is typically about

346\mathrm{ms}^{-1}

25^{\circ}\mathrm{C}

.
(vi) Why did you measure the time between 'seeing' and 'hearing' the balloon burst?

5Explore the internet resources to explore the effect of humidity and temperature on the speed of sound. Some such resources are:

(i) https://phet.colorado.edu/en/simulations/sound-waves/
(ii) https://musiclab.chromeexperiments.com/Experiments
(iii) https://phyphox.org/experiments

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Frequently Asked Questions

What are the important topics in Sound Waves Characteristics and Applications for CBSE Class 9 Science?

Sound Waves Characteristics and Applications covers several key topics that are frequently asked in CBSE Class 9 board exams. Focus on the core concepts listed on this page and practise related questions to build confidence.

How to score full marks in Sound Waves Characteristics and Applications — CBSE Class 9 Science?

Understand the core concepts first, then work through the 77 practice questions available for this chapter. Revise formulas and definitions regularly, and use flashcards for quick recall before the exam.

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Sound Waves Characteristics and Applications

Studying Sound Waves Characteristics and Applications? Get the full interactive chapter.

Think It Over

Activity 10.1: Let us explore

Threads of Curiosity

Activity 10.2: Let us explore

Pause and Ponder

Activity 10.3: Let us investigate

Activity 10.4: Let us investigate

Activity 10.7: Let us experiment (demonstration activity)

Activity 10.8: Let us experiment (demonstration activity)

Revise, Reflect, Refine

The Journey Beyond

Frequently Asked Questions

Sources & Official References

More resources for Sound Waves Characteristics and Applications

Practice Quiz

Important Questions

Revision Notes

Formula Sheet

Chapter Summary

Concept Maps

Study Plan

Flashcards

Syllabus

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