Class 9 · ScienceNCERT 2026 Revised Syllabus
Chapter 10: Sound Waves: Characteristics and Applications (0906CH10)
Exploration · Wave properties, echo, SONAR, ultrasound.
Chapter code: 0906ch10 · Book: Exploration · Class 9 Science
Find complete NCERT solutions for Class 9 Science, Exploration Chapter 10. Every question has a clear, step-by-step answer for chapter code 0906CH10.
Pause and Ponder(13 questions)
1
Explore various ways of producing sound.
School Level Solution
Ans.
Try plucking a string, striking a metal object, rubbing surfaces or blowing across an air column. In each case, sound is associated with vibrations of the source.
2
Make a list of different types of musical instruments and identify their vibrating parts which produce sound.
School Level Solution
Ans.
Examples: guitar — strings; tabla — stretched membrane; flute — air column; harmonium — reeds; cymbal — metal plate.
3
Assertion (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.
School Level Solution
Ans.
A is true because the sound becomes inaudible as the air is removed, and R is true because sound needs a material medium. Thus R correctly explains A.
4
Assertion (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.
School Level Solution
Ans.
Compressions and rarefactions propagate through the medium, but particles of the medium oscillate about their mean positions; they do not continuously travel forward with the wave.
5
When 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 air
School Level Solution
Ans.
The air particles oscillate locally. It is the energy of the sound wave that is transmitted from source to ear.
6
The 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).
School Level Solution
Ans.
Mark C at the crowded/high-density regions and R at the spread-out/low-density regions. In the graph, put distance on the x-axis and density on the y-axis, with corresponding maxima (crests) and minima (troughs).
7
Conduct 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?
School Level Solution
Ans.
A thin, tightly stretched rubber band generally vibrates with a higher frequency than a thick one. Therefore its sound has a higher frequency and a shorter time period (T = 1/ν).
8
If the frequency of a sound wave produced by an oscillating piston of a long tube filled with air is 20 Hz, then how many oscillations does the piston complete per minute?
School Level Solution
Ans.
20 oscillations/s × 60 s = 1200 oscillations/min.
9
For the sound wave represented by the graph shown in Fig. 10.19, what is half of its wavelength?
School Level Solution
Ans.
From the graph, one full wavelength is 3.0 cm. Therefore half the wavelength is 1.5 cm.
10
Table 10.1 shows the speed of sound in a few media at atmospheric pressure. Compare the speeds in different media by finding the ratio of (i) the speed of sound in water with respect to the speed in air; (ii) the speed of sound in steel with respect to the speed in water.
School Level Solution
Ans.
Water:air = 1500:340 = 75:17. Steel:water = 5000:1500 = 10:3.
11
Two 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 air and the steel. Would it have been possible for her to distinguish between the two sounds? (The time interval between two sounds must be at least 0.1 s to be heard separately.)
School Level Solution
Ans.
Through steel: t = 340/5000 = 0.068 s. Through air: t = 340/340 = 1.000 s. Time difference = 1.000−0.068 = 0.932 s. Since 0.932 s > 0.1 s, yes, Gunjan could distinguish the two sounds.
12
An 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 m s⁻¹.
School Level Solution
Ans.
The sound travels to the reflecting surface and back, so 2d = vt = 343×0.2. Thus d = (343×0.2)/2 = 34.3 m.
13
Sound 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 m s⁻¹?
School Level Solution
Ans.
The 4 s is the round-trip time. Depth d = vt/2 = 1500×4/2 = 3000 m.
Activities — How to Do(8 questions)
1
Activity 10.1 (rubber band and sound): pluck a band stretched over an open box; watch it vibrate; stop it and listen; change tension; then pluck it off the box near your ear. What links vibration and sound?
School Level Solution
Ans.
Sound is heard only while the band vibrates — stopping it silences it. More tension → shriller sound (higher frequency). Off the box near the ear it sounds feeble: the box acts as a soundboard that amplifies. Conclusion: vibrating bodies produce sound.
2
Activity 10.2 (tuning fork and water): strike a fork on a rubber pad, hold it near the ear, then touch a prong to water. What do the two observations prove?
School Level Solution
Ans.
Near the ear you hear the hum though prongs look still. Touching water throws up ripples/splashes — the prongs are vibrating, too fast to see. Together: the fork is a vibrating source; water makes its motion visible.
3
Activity 10.3 (sound through solids): a friend knocks on a desk — first listen with ear in air, then with ear pressed to the desk (other ear closed). Which hearing is clearer, and why?
School Level Solution
Ans.
The knock heard through the desk is louder and clearer than through air. Sound travels faster and more efficiently in solids (particles tightly packed) than in air — proof that sound needs a medium and solids conduct best.
4
Activity 10.4 (spoons in water): clash two spoons in air and listen; then clash them submerged (not touching the bucket). Is sound still heard under water?
School Level Solution
Ans.
The clash is clearly heard underwater too (tone slightly different). Conclusion: liquids also carry sound — sound propagates through solids, liquids and gases, only not through vacuum.
5
Activity 10.5 (slinky wave): mark one turn; push-pull one end once, then repeatedly. Does the disturbance travel? Does the mark travel with it? What kind of wave is this?
School Level Solution
Ans.
Each push sends a compression travelling to the friend’s end, yet the marked turn only shuttles back and forth. Since particle motion is parallel to wave travel, this is a longitudinal wave — the model for sound in air.
6
Activity 10.6 (sound shakes grains): stretch a sheet over a bowl, sprinkle grains, and produce loud sounds nearby. What happens on changing volume or sound source?
School Level Solution
Ans.
The grains jitter and jump without being touched — vibrating air pushes the sheet. Louder volume → livelier jumping; different sources/grains respond differently. Proof: sound is a form of energy that can do work on matter.
7
Activity 10.7 (Phyphox audio spectrum): sing Sa–Re–Ga–Ma–Pa–Dha–Ni–Sa into the app’s Audio Spectrum and note each frequency; compare ratios to Sa. What pattern emerges?
School Level Solution
Ans.
Each higher note shows a higher Hz value (e.g., Sa ≈ 240 Hz, higher Sa ≈ 480 Hz — double). Ratios like 9/8, 5/4, 3/2 recur, showing the scale’s mathematical structure. Voice vs app-generated same notes give nearly equal frequencies.
8
Activity 10.8 (frequency steps): play 100 Hz, rise in 100 Hz steps to 1000 Hz, then drop from 50 Hz downwards. How does the sound change, and where does hearing stop?
School Level Solution
Ans.
Each 100 Hz step sounds noticeably shriller — pitch follows frequency. Dropping below ~50 Hz the tone deepens, and near 20 Hz it vanishes: human hearing spans roughly 20 Hz–20 kHz; lower is infrasound.