Chapter 10: Sound Waves: Characteristics and applications Quick Revision notes | Class 9th Science (Exploration) notes

August 1, 2026 • Admin ETeam • Class 9, class 9 Quick revision notes

CLASS 9 · PHYSICS · CHAPTER 10

Sound Waves: Characteristics
and Applications

🔊 detailed handwritten-style notes for quick + deep revision 🎶

Sound Waves chapter - astronaut in space

BASICS

🎻 10.1 Production of Sound

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

Taal musical instrument — Taal 🎶 (sonority)

Some metals show sonority — they produce sound when struck (e.g. Taal, bells, cymbals).

Object that produces sound = the source of sound. Sound can be produced by vibrating strings, membranes, air columns and many other objects.

Vibrating rubber band experiment — Stretched rubber band 🪕

🧪 Rubber Band Activity — key takeaway

A plucked, vibrating rubber band produces sound.
Once vibration stops, sound stops too.
Changing tension (stretch) changes the sound produced.

➡️ Conclusion: Sound is produced only as long as an object vibrates.

🗣️ How humans & animals make sound

Vocal cords in humans — Vocal cords & larynx 🫁

In humans, sound is produced by vibration of vocal cords — stretched muscular flaps inside the larynx (voice box) in the throat. The tongue, lips, mouth & nasal cavity help convert this into speech/music.

Some animals (like grasshoppers, crickets) produce sound by rubbing body parts such as wings or legs together.

🔺 Tuning Fork

A tuning fork = U-shaped metal bar (steel/aluminium) with a stem. The two sides of the “U” are prongs, struck on a rubber pad to make them vibrate.

Tuning fork on water — Vibrating prong touches water → ripples!

💡 Ripples on the water surface when a vibrating prong touches it prove that the tuning fork is vibrating — hence proving sound comes from vibration.

MEDIUM

🌬️ 10.2 Propagation of Sound

Sound propagates (travels) through solids, liquids and gases. The material through which sound travels is called a medium.

Sound through solids - desk experiment — Ear on desk → sound travels through solid 🪵

Proof sound travels through solids

Knock gently on a desk. A friend with their ear on the desk (other ear closed) can hear it clearly — even better than through air. ✅ Sound travels through solids.

Tapping spoons underwater — Spoons tapped underwater 🥄

Proof sound travels through liquids

Two metal spoons tapped together underwater (without touching the tub) can still be heard. ✅ Sound travels through liquids too.

🔔 10.2.1 Sound needs a medium — Vacuum Bell Jar Experiment

An electric bell rings inside a jar. As air is pumped out (vacuum pump), the sound becomes fainter — near vacuum, almost no sound is heard even though the bell is seen ringing. When air re-enters, sound returns.

Sound CANNOT travel through vacuum — it needs a material medium (solid, liquid or gas) to propagate.

Why astronauts can’t talk directly in space: Outer space is a near vacuum, so sound cannot propagate. Astronauts use radio devices fitted in their spacesuits to communicate — not direct sound.

CORE CONCEPT

💨 10.3 Sound Waves — Compression & Rarefaction

Slinky analogy: Push-pull one end of a stretched slinky quickly. A disturbance (region where turns bunch up / spread out) travels along it — but the marked turn only oscillates back and forth, it does NOT travel with the disturbance.

Piston pushed forward → Compression (C, high density); pulled back → Rarefaction (R, low density)

Term	Meaning
Compression (C)	Region of higher density / pressure than average
Rarefaction (R)	Region of lower density / pressure than average

A sound wave = a disturbance of alternating compressions & rarefactions travelling through a medium, without the actual flow of medium particles.Only the disturbance (energy) travels — particles just vibrate about their mean position.

📝 Sudden loud sounds (firecrackers, thunder): rapid heating makes gas expand suddenly → sudden disturbance = loud pulse. A sonic boom is a similar (bigger) disturbance made by a supersonic aircraft flying faster than sound.

WAVE TYPES

↔️ Longitudinal vs Transverse Waves

Longitudinal: particles vibrate parallel to wave direction. Transverse: particles vibrate perpendicular to wave direction.

Longitudinal Wave	Transverse Wave
Particles vibrate parallel to propagation direction	Particles vibrate perpendicular to propagation direction
Made of compressions & rarefactions	Made of crests & troughs
Example: Sound	Example: Light, seismic S-waves

🌍 A small source emits sound in all directions → spreads as spherical waves through the surrounding medium (like ripples but in 3-D).

Waves that need a material medium to travel = Mechanical waves.Sound is a mechanical (longitudinal) wave — it cannot travel through vacuum. Light is NOT mechanical — it travels through vacuum too.

ENERGY

⚡ 10.4 Energy of Sound Waves

Activity: Stretch a rubber sheet over a bowl, sprinkle grains on it. Produce a loud sound nearby (without touching) — the grains jump! This shows sound carries energy that makes the sheet vibrate.

Sound is a form of energy. A vibrating source transfers energy to the medium; this energy travels as the sound wave (NOT the particles themselves).

🎤 Real-life devices

A microphone’s diaphragm vibrates with incoming sound and converts it to an electrical signal. A speaker’s diaphragm/cone does the reverse — vibrating to recreate the sound from an electrical signal.

💡 Particles of a medium are always randomly vibrating (thermal energy). A sound wave temporarily increases this vibration as it passes — then particles return to normal.

GRAPH

📈 10.5 Graphical Representation of a Sound Wave

We plot density of medium (y-axis) vs distance (x-axis) at a given instant. Average density is shown as a dashed line.

Compression → density above average → Crest (highest point). Rarefaction → density below average → Trough (lowest point).

Compression = Crest (maximum density) | Rarefaction = Trough (minimum density)

10.6.1

📏 Wavelength, Frequency & Time Period

Quantity	Meaning	Symbol	SI Unit
Wavelength	Distance between 2 consecutive crests (or troughs)	λ (lambda)	metre (m)
Frequency	No. of density oscillations at a fixed point per unit time	ν (nu)	hertz (Hz)
Time Period	Time for ONE complete density oscillation	T	second (s)

ν = 1 ⁄ T (Eq. 10.1)Frequency and time period are inversely related — shorter time period ⇒ higher frequency.

✏️ Worked Example — using ν = 1/T If there are 10 density oscillations in 2 seconds:
Frequency = oscillations ÷ time = 10 ÷ 2 s = 5 Hz
Time period = 1 ÷ 5 Hz = 0.2 s

Everyday sounds are usually a mixture of many frequencies. Nearly single-frequency sounds come from a tuning fork or a whistle.

🎵 In musical notes ‘Sa Re Ga Ma Pa Dha Ni Sa’, frequency is lowest for ‘Sa’ and increases for later notes — each note has a distinct frequency.

10.6.2 / 10.6.3

📢 Amplitude, Intensity & Speed of Sound

Amplitude

Amplitude = maximum change in density (in a compression/rarefaction) compared to average density.Larger amplitude ⇒ more energy carried by the wave (struck harder → grains jump higher in the activity).

Intensity

Intensity = sound energy passing per unit area (⊥ to propagation direction) per unit time.Intensity decreases as we move away from the source, since the same energy spreads over a larger area.

Speed of Sound

Speed = distance travelled by a wave point (like a crest) in unit time.

v = λ × ν (Eq. 10.2)speed = wavelength × frequency

Medium	Approx. speed of sound (15°C)
Solid (Steel)	≈ 5000 m/s (fastest)
Liquid (Water)	≈ 1500 m/s
Gas (Air)	≈ 340 m/s (slowest)

🌡️ Speed of sound in air increases with temperature & humidity. e.g. ≈331 m/s at 0°C, ≈344 m/s at 22°C. Speed depends on the medium, not on the source/frequency — if frequency changes, wavelength changes but speed stays constant (in a given medium).

✏️ Worked Example — Lightning & Thunder Time gap between seeing lightning & hearing thunder = 5 s. Speed of sound in air = 340 m/s (light reaches ~instantly).
Distance = v × t = 340 × 5 = 1700 m ≈ 1.7 km away

10.6.4

👂 Human Perception of Sound

Physical properties (T, λ, ν, amplitude, speed) are measurable. But how we experience sound is subjective — described using pitch and loudness.

Term	Depends mainly on	Details
Pitch	Frequency	High frequency = shrill/high pitch (whistle); Low frequency = low pitch (thunder)
Loudness	Amplitude	Larger amplitude = louder; decreases with distance from source

⚠️ Loudness ≠ Intensity. Intensity is measurable/objective; loudness depends on the listener’s own hearing ability.

Audible Range

Human hearing range: 20 Hz – 20,000 Hz (20 kHz)Below 20 Hz = infrasonic waves | Above 20 kHz = ultrasonic waves. Range narrows with age.

Infrasound: detected by elephants
Ultrasound: detected by dogs, cats, bats, dolphins

🔊 Loudness is measured in decibels (dB). Rustling leaves ≈ few dB, conversation ≈ 60 dB, firecrackers > 100 dB. Prolonged loud sound (noise pollution) can damage hearing.

Ear Anatomy (brief)

Sound → vibrates the eardrum → tiny bones amplify the vibration → cochlea converts it to electrical signals → brain perceives sound. Two ears help the brain pinpoint the sound’s direction using the tiny time gap between the ears.

🎼 Tone, Note, Timbre & Octave

Tone = single-frequency sound (tuning fork, whistling).
Musical note = fundamental frequency + overtones (richer sound), e.g. tanpura, singing.
Timbre = the unique quality that lets us tell apart a flute, tabla, and sitar playing the same note at the same loudness.
Octave = interval between two notes where one has double the frequency of the other.

Tabla set — Tabla — the syaahi patch gives rich, controlled tone

Sir C. V. Raman

Won India’s first Nobel Prize in Science (for the Raman Effect in light). He also made key contributions to acoustics, studying how Indian percussion instruments like the tabla and mridangam produce rich, nuanced sound.

10.7

🔁 Reflection of Sound

Sound bounces off solids/liquids — follows the same laws of reflection as light (angle of incidence = angle of reflection; incident ray, reflected ray, normal all lie in the same plane).

10.7.1 Echo

Echo = the reflected sound heard again after the original sound.Heard only if the reflecting surface is far enough that the gap between direct & reflected sound ≥ 0.1 s (else the brain can’t separate them).

📐 Minimum distance for an echo (speed of sound = 340 m/s): distance travelled in 0.1 s = 340 × 0.1 = 34 m (there and back) → minimum reflecting-surface distance ≈ 17 m.

Echoes are stronger from hard, smooth surfaces (reflect well). Soft surfaces (curtains) absorb sound; rough surfaces scatter it — echoes aren’t clear from these.

Sound travelling through a steel fence — Sound via steel fence vs air — reaches at different times!

✏️ Worked Example — Echo distance Clap in an empty corridor, hear echo after 0.5 s; speed of sound = 340 m/s.
Distance from wall = (v × t) ÷ 2 = (340 × 0.5) ÷ 2 = 85 m

10.7.2 Reverberation

Reverberation = persistence of sound due to multiple reflections in a hall, when reflected sounds arrive with a time gap < 0.05 s.

Auditoriums are designed for a desirable amount of reverberation using sound-absorbing panels, curtains and upholstered chairs — too much reverberation garbles sound.

🕌 The Whispering Gallery of Gol Gumbaz (Bijapur) is famous for its acoustic design — even a faint whisper can be heard multiple times across the dome!

10.8

🦇 Ultrasonic & Infrasonic Waves — Applications

Type	Frequency
Infrasonic waves	< 20 Hz
Audible range	20 Hz – 20 kHz
Ultrasonic waves	> 20 kHz

Infrasonic: Detecting earthquakes, volcanic eruptions & storms (travel very long distances)

Ultrasonography: Imaging internal organs without surgery

Medical: Breaking kidney stones into smaller pieces

Industry: Ultrasonic welding & cleaning delicate parts

Testing: Detecting defects inside metal blocks

Navigation: Locating objects using reflected sound waves

🦇 Echolocation

Bats emit short bursts of ultrasonic waves; by sensing the echoes reflected from objects/prey, they determine position without seeing. This is called echolocation. Also used by dolphins, whales, and some birds.

🚢 SONAR (SOund NAvigation and Ranging)

Humans use the same principle underwater: ultrasonic waves are sent into water, and reflected waves are analysed to find the distance, direction & speed of objects like submarines or shipwrecks.

✏️ Worked Example — SONAR distance A sonar signal returns after 0.90 s; speed of sound in seawater = 1530 m/s.
Time to reach object = 0.90 ÷ 2 = 0.45 s
Distance = speed × time = 1530 × 0.45 = 688.5 m

🧮 Quick Formula Sheet

ν = 1 / T frequency = 1 ÷ time period

v = λ × ν speed = wavelength × frequency

distance = speed × time for direct-path sound travel

echo distance = (v × t) / 2 sound travels to surface AND back

min. echo distance ≈ 17 m using 0.1 s gap & v = 340 m/s

Audible range: 20 Hz – 20 kHz <20 Hz infrasonic, >20 kHz ultrasonic

🌟 At a Glance — Full Chapter Revision

Sound is produced by vibrating objects and is a form of energy.
Sound is a longitudinal mechanical wave — needs a medium, cannot travel in vacuum.
Sound travels through solids, liquids & gases — fastest in solids, slowest in gases.
It’s the disturbance (density change) that travels, not the medium’s particles.
Made of alternating compressions (high density) & rarefactions (low density).
Wavelength (λ): distance between 2 consecutive crests/troughs.
Frequency (ν): oscillations per second; Time period (T): time for 1 oscillation; ν = 1/T.
Amplitude: max. density change → relates to loudness & energy carried.
Intensity: sound energy per unit area per unit time — decreases with distance.
Speed of sound: v = λ × ν; depends on medium, temperature & humidity.
Pitch ~ frequency; Loudness ~ amplitude (but loudness is subjective).
Audible range: 20 Hz – 20 kHz; below = infrasonic, above = ultrasonic.
Echo = reflected sound heard separately (gap ≥ 0.1 s); Reverberation = persistence of sound (gap < 0.05 s).
Echolocation (bats) and SONAR (ships) use reflected ultrasonic waves to locate objects.

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