Sound Waves: Characteristics and Applications – Class 9 Science Chapter 10 Notes & NCERT PDF

Class 9ScienceChapter 10NCERT book: Exploration - Science Class 9

Notes and a simple summary of Chapter 10 of the NCERT Class 9 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 10: Sound Waves: Characteristics and Applications

Think It Over

Two astronauts repairing a space station during a spacewalk — can they hear each other or the clanking of metal as they would on the Earth?

How do most bats use sound to find their prey in the dark?

Introduction

Human voices, birds chirping, waves, mobile phones ringing, thunder — sound is an everyday sensory experience. In Chapter 7 you learnt that sound is a form of energy. This chapter answers: how is sound produced, how does it travel, how do we describe it, and how do we use it?

Key Concepts

10.1 Production of Sound

Sound is produced by vibrations. A vibration is the periodic to-and-fro motion (oscillation) of an object.

In Activity 10.1, a rubber band stretched over a box makes sound only while it vibrates; changing its tension changes the sound.

Sound can come from vibrating strings, membranes, air columns and many other objects. The object producing sound is the source.

In humans, sound is produced by the vocal cords in the voice box (larynx). Grasshoppers and crickets rub wings or legs.

A tuning fork is a U-shaped metal bar (steel or aluminium) with a stem; its prongs (tines) are struck on a rubber pad. A vibrating prong touching water makes ripples — proof it is vibrating.

10.2 Propagation of Sound

Sound travels through solids (ear on the desk), liquids (spoons tapped under water) and gases. The material through which sound travels is a medium.

Vacuum bell jar experiment: as air is pumped out, the ringing bell becomes fainter and almost silent even though it is seen ringing.

Sound needs a medium to propagate. It cannot travel through vacuum.

That is why astronauts on a spacewalk use special communication devices in their spacesuits.

10.3 Sound Waves

A slinky pushed and pulled shows regions where the turns are closer together and more spread out; a marked turn only oscillates about its position.

A piston in an air-filled tube: moving forward creates a compression (C) — higher-than-average density; moving back creates a rarefaction (R) — lower density.

A sound wave is a series of alternating compressions and rarefactions propagating through a medium, without the actual flow of particles of the medium.

Particles vibrate parallel to the direction of propagation → sound is a longitudinal wave.

Waves that need a material medium are mechanical waves. (In a transverse wave particles vibrate perpendicular to the direction of propagation; light is a transverse wave that can travel through vacuum.)

10.4 Energy of Sound Waves

Grains sprinkled on a stretched sheet over a container jump when a loud sound is made nearby — sound carries energy.

In the propagation of a sound wave, it is the energy that is transferred, not the particles of the medium.

A microphone converts sound energy to electrical energy (a vibrating diaphragm); a speaker does the opposite.

10.5 Graphical Representation

A graph of density vs distance at an instant: the highest point is the crest (compression, maximum density), the lowest is the trough (rarefaction, minimum density).

10.6 Characteristics of a Sound Wave

Wavelength (λ): distance between two consecutive crests (or troughs). SI unit: metre (m).

Frequency (ν): number of density oscillations at a fixed point per unit time. SI unit: hertz (Hz = s⁻¹).

Time period (T): time for one complete density oscillation. SI unit: second (s).

ν = 1/T and v = λ × ν (speed = wavelength × frequency)

Amplitude: the maximum change in density in a compression (or rarefaction) compared to the average density. Larger amplitude → more energy.

Intensity: sound energy passing through unit area perpendicular to the direction of propagation per unit time. It decreases with distance as energy spreads over a larger area.

Speed of sound depends on the medium: fastest in solids, slower in liquids, slowest in gases (Table 10.1 at 15 °C: steel ~5000 m s⁻¹, water ~1500 m s⁻¹, air ~340 m s⁻¹). It increases with temperature and humidity — about 331 m s⁻¹ at 0 °C and nearly 344 m s⁻¹ at 22 °C in dry air.

10.6.4 Human Perception of Sound

Pitch is how we perceive frequency: shrill sounds (whistle, siren) have high pitch; thunder has low pitch.

Loudness is how we perceive amplitude; it decreases with distance. Loudness depends on the listener; intensity is measurable.

Audible range: 20 Hz to 20,000 Hz (20 kHz) — varies by person and decreases with age.

- Below 20 Hz: infrasonic (elephants can detect it).

- Above 20 kHz: ultrasonic (dogs, cats, bats, dolphins can detect it).

Loudness is measured in decibels (dB): rustling leaves a few dB, conversation about 60 dB, firecrackers over 100 dB. Unwanted sound is noise.

Timbre makes a flute, ektara and tabla sound different on the same note. An octave is the interval where one note has double the frequency of the other (e.g., 200 Hz and 400 Hz).

Sir C. V. Raman studied the acoustics of the tabla and mridangam; the black syaahi patch gives these drums their rich sound.

10.7 Reflection of Sound

Sound follows the same laws of reflection as light.

Echo: we hear an echo only if the reflected sound arrives at least 0.1 s after the original. With speed 340 m s⁻¹, sound travels 34 m in 0.1 s, so the minimum distance of the reflecting surface is 17 m.

Reverberation: multiple reflections making sound persist, when reflections arrive with a time difference less than 0.05 s. Auditoriums use soft, porous surfaces to control it.

The Whispering Gallery of Gol Gumbaz (Bijapur, Karnataka) lets even a faint whisper be heard multiple times.

10.8 Ultrasonic and Infrasonic Waves

Ultrasound: imaging organs (ultrasonography), breaking kidney stones, ultrasonic welding and cleaning, detecting defects in metal blocks, locating objects.

Infrasound: detecting earthquakes, volcanic eruptions and severe storms.

Echolocation: bats emit ultrasonic bursts and sense the echoes; dolphins, whales and some birds do this too.

SONAR (sound navigation and ranging) sends ultrasonic waves into water and analyses reflections.

Worked Numericals

Ex 10.1: 10 oscillations in 2 s → ν = 10/2 = 5 Hz; T = 2/10 = 0.2 s.

Ex 10.2: λ = v/ν. For 20 Hz: 344/20 = 17.2 m; for 20 kHz: 344/20000 = 0.0172 m = 1.72 cm.

Ex 10.3: Thunder heard 5 s after lightning: 340 × 5 = 1700 m ≈ 1.7 km.

Ex 10.4: In steel, λ = 50 m, v = 5000 m s⁻¹ → ν = 5000/50 = 100 Hz, T = 1/100 = 0.01 s.

Ex 10.5: Echo after 0.5 s: distance = (340 × 0.5)/2 = 85 m.

Ex 10.6: Sonar returns after 0.90 s: one-way time 0.45 s, distance = 1530 × 0.45 = 688.5 m.

Activities in the Chapter

10.1 Rubber band on a box — sound only while vibrating.

10.2 Tuning fork touching water — ripples show vibration.

10.3 / 10.4 Ear on a desk; spoons tapped under water — sound travels in solids and liquids.

10.5 Slinky — compressions and rarefactions travel; particles only oscillate.

10.6 Grains jump on a stretched sheet — sound carries energy.

10.7 / 10.8 Phyphox app — frequencies of Sa Re Ga Ma…; listening from 100 Hz to 1000 Hz and down to about 20 Hz.

Key Terms

Vibration (कंपन), Medium (माध्यम), Compression (संपीडन), Rarefaction (विरलन)

Longitudinal wave (अनुदैर्ध्य तरंग), Mechanical wave (यांत्रिक तरंग)

Wavelength (तरंगदैर्ध्य), Frequency (आवृत्ति), Time period (आवर्तकाल)

Amplitude (आयाम), Intensity (तीव्रता), Pitch (तारत्व), Loudness (प्रबलता)

Echo (प्रतिध्वनि), Reverberation (अनुरणन), Ultrasonic (पराश्रव्य), Infrasonic (अवश्रव्य)

Real-life Connections

Microphones, speakers and hearing aids (microphone + amplifier + speaker).

Ultrasonography in hospitals; parking sensors in cars; sonar on ships.

Audio surveillance detects the low-frequency hum of drones.

Noise pollution above safe limits can damage hearing and sleep.

Common Misconceptions

"Air particles travel from the source to my ear." — No; only the energy (density disturbance) travels; particles vibrate about their mean positions.

"Sound travels fastest in air." — No; it is slowest in gases and fastest in solids.

"A higher frequency sound travels faster." — In a given medium like air the speed does not depend on frequency; a higher frequency means a shorter wavelength.

"The echo distance is v × t." — The sound goes to the wall and back, so distance = v × t ÷ 2.

"Loudness and intensity are the same." — Intensity is measurable; loudness depends on the listener.

Exercise Hints

For any echo or sonar problem, halve the total time (or the total distance).

Use ν = 1/T and v = λν; read the wavelength from a graph as crest-to-crest distance.

For the Gunjan fence problem (340 m): time in air = 1 s, time in steel = 0.068 s, difference 0.932 s — more than 0.1 s, so she hears two separate sounds.

💡 Key Learning Points

✓Sound is produced by vibrating objects and needs a material medium (solid, liquid or gas); it cannot travel through vacuum, as the bell jar experiment shows.
✓A sound wave is a longitudinal mechanical wave of alternating compressions and rarefactions; energy travels, particles only oscillate about their mean positions.
✓Wavelength, frequency, time period, amplitude and intensity describe a sound wave, with ν = 1/T and v = λν.
✓Speed of sound depends on the medium and on temperature/humidity; humans hear 20 Hz–20 kHz, below is infrasonic and above is ultrasonic.
✓Reflection of sound gives echoes (minimum 17 m at 340 m s⁻¹) and reverberation, and is used in echolocation, sonar and ultrasonography.

👨‍🏫 Teaching Tips

→Start with the Think It Over question about astronauts, then run or show the vacuum bell jar to settle why sound needs a medium.
→Use a slinky in pairs (Activity 10.5) and mark one turn with tape so students see that the turn oscillates while the disturbance travels.
→Sprinkle rice or semolina on a stretched balloon sheet (Activity 10.6) and strike a metal plate harder and softer to link amplitude with energy.
→Use the free Phyphox app to show the frequencies of Sa Re Ga Ma and to demonstrate a near single-frequency whistle.
→Drill the 'halve the time' rule with a quick board race on echo and sonar problems (Examples 10.5 and 10.6).
→Draw density–distance graphs on the board and let students label crest, trough, wavelength and amplitude themselves.
→Connect to Indian heritage: Gol Gumbaz whispering gallery, C. V. Raman's tabla research and Kongthong's 'whistling village'.

📋 Assessment Questions

Can the student explain, using the bell jar experiment, why astronauts cannot hear each other directly in space?
Can the student distinguish compression and rarefaction and explain why sound is a longitudinal wave?
Can the student calculate frequency, time period, wavelength and speed using ν = 1/T and v = λν?
Can the student solve echo and sonar distance problems correctly by halving the total time?
Can the student state the audible range and give uses of ultrasonic and infrasonic waves?

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