Physics · Class 9 · CBSE Chapter 12

Sound

Production, propagation, reflection and properties of sound waves

343 m/s Speed in air at 25°C

20–20k Human hearing range (Hz)

1482 m/s Speed in water

5120 m/s Speed in steel

What is Sound?

Sound is a form of energy that is produced by vibrating objects and travels through a medium as a mechanical longitudinal wave, causing compressions and rarefactions.

Analogy

Think of sound like ripples on water — but instead of water moving up and down, air molecules push and pull against each other in the same direction the wave travels.

Sound CANNOT travel through vacuum. This is why space is completely silent — there are no molecules to carry the vibrations.

Sound requires a material medium (solid, liquid or gas) to travel
Sound is produced by vibrating bodies — vocal cords, guitar strings, tuning forks
In a longitudinal wave, particle displacement is parallel to wave propagation direction
Regions of high pressure are called compressions; low pressure regions are rarefactions

Wave Speed Formula

\[ v = f \lambda \]

v

Wave speed — how fast the sound travels through the medium (m/s)

f

Frequency — number of compressions or rarefactions per second (Hz)

lambda

Wavelength — distance between two consecutive compressions or rarefactions (m)

Note

For a given medium at constant temperature, wave speed v is fixed. So if frequency increases, wavelength decreases proportionally.

v

Wave speed

m/s (metres per second)

f

Frequency

Hz (hertz) = cycles per second

T

Time period

s (seconds) — T = 1/f

lambda

Wavelength

m (metres)

Explore: Wave Speed Calculator

Drag the sliders to explore how frequency and wavelength relate to wave speed. Notice that for air at 25°C, speed stays at 343 m/s.

\( v = f \times \lambda \)

f

343 Hz

Frequency (Hz)

lambda

1 m

Wavelength (m)

Wave speed

— m/s

How Sound is Produced and Travels

1

Vibration begins Source

An object vibrates — for example, a guitar string is plucked or a tuning fork is struck. The object moves back and forth rapidly.

2

Compression formed Medium

When the vibrating surface moves forward, it pushes air molecules together, creating a region of high pressure called a compression.

3

Rarefaction formed Medium

When the surface moves backward, air molecules spread apart, creating a region of low pressure called a rarefaction.

4

Wave propagates Propagation

Alternating compressions and rarefactions travel outward through the medium. The medium itself does not travel — only energy is transferred.

5

Ear detects sound Receiver

When compressions and rarefactions reach the eardrum, it vibrates at the same frequency. The brain interprets these vibrations as sound.

Longitudinal Wave — Compressions and Rarefactions

A longitudinal wave showing alternating compressions (C) and rarefactions (R). Wavelength is the distance between two consecutive compressions.

Speed of Sound in Different Media

343 m/s

Air (25°C)

Increases ~0.6 m/s per °C rise

1482 m/s

Water (25°C)

~4x faster than in air

5120 m/s

Iron / Steel

~15x faster than in air

3560 m/s

Aluminium

Solids conduct sound best

0 m/s

Vacuum

Sound cannot travel — no medium

1530 m/s

Seawater

Whales communicate over 100s of km

Compressions vs Rarefactions

Property	Compression	Rarefaction
Particle density	High — particles crowded together	Low — particles spread apart
Air pressure	Higher than normal	Lower than normal
Particle displacement	Particles pushed toward each other	Particles pulled away from each other
Analogy	Traffic jam on a highway	Empty stretch of highway
Symbol	C	R

💡

Did you know?

Sound travels about 4 times faster in water than in air. This is why submarines use SONAR — sound waves travel efficiently underwater and can detect objects kilometres away.

✗

Common Mistake

Sound is NOT a transverse wave. It is a LONGITUDINAL wave. Particle vibration is parallel (not perpendicular) to the direction of wave propagation. Light is transverse; sound is longitudinal.

✓

Exam Tip

Remember: Speed increases as medium changes from gas → liquid → solid because intermolecular forces are stronger in denser media. For gases, speed also increases with temperature.

⚠

Misconception

The medium does NOT travel with the sound wave. Only energy is transferred. Air molecules just vibrate back and forth around their mean position — they do not move from source to ear.

Characteristics of Sound

Characteristic	Depends On	Unit	Description
Loudness	Amplitude of vibration	Decibel (dB)	How loud or soft a sound is. More amplitude = louder sound.
Pitch	Frequency of vibration	Hertz (Hz)	How high or low a sound feels. Higher frequency = higher pitch.
Quality/Timbre	Waveform / harmonics	—	What makes a violin sound different from a flute at same pitch and loudness.
Intensity	Amplitude + distance	W/m²	Energy carried per unit area per second. Falls with square of distance.
Frequency	Source vibration rate	Hz	Number of complete oscillations per second.

Echo and Reflection of Sound

When sound waves strike a hard surface (like a wall, cliff or building), they bounce back. If the reflected sound reaches the listener at least 0.1 seconds after the original, it is heard as a distinct echo.

Analogy

Think of echo like a bouncy ball — throw it at a wall and it comes back. Sound does the same: it hits a hard surface and bounces back toward the source.

The human ear can distinguish two sounds as separate only if there is a gap of at least 0.1 seconds between them. At 343 m/s, this means the reflecting surface must be at least 17.2 m away.

Minimum distance for echo = (speed of sound × 0.1) / 2 = 17.15 m from the reflecting surface
Reverberation is the persistence of sound due to multiple reflections in an enclosed space
Auditoriums and concert halls are designed with sound-absorbing materials to reduce reverberation
SONAR uses echo to measure depth of oceans and detect underwater objects
Bats use echolocation — they emit ultrasound and use the echo to navigate

Echo Distance Formula

\[ d = \frac{v \times t}{2} \]

d

Distance to the reflecting surface (m)

v

Speed of sound in the medium (343 m/s in air at 25°C)

t

Time between producing the sound and hearing the echo (s)

÷ 2

Divided by 2 because sound travels to the wall AND back — total distance is 2d

d

Distance to reflector

metres (m)

v

Speed of sound

m/s

t

Time of echo

seconds (s)

Test Your Understanding

Q1 A sound wave travels 1715 m in 5 seconds. What is its speed?

A. 343 m/s

B. 300 m/s

C. 8575 m/s

D. 171 m/s

Speed = Distance / Time = 1715 / 5 = 343 m/s. This is the standard speed of sound in air at 25°C.

Q2 Sound is which type of wave?

A. Transverse wave

B. Electromagnetic wave

C. Longitudinal mechanical wave

D. Light wave

Sound is a longitudinal mechanical wave — particle vibration is parallel to wave propagation, and it requires a physical medium (cannot travel in vacuum).

Q3 A person claps near a cliff and hears the echo after 2 seconds. How far is the cliff? (Speed of sound = 340 m/s)

A. 680 m

B. 340 m

C. 170 m

D. 1360 m

d = (v × t) / 2 = (340 × 2) / 2 = 340 m. The sound travels to the cliff and back, so we divide total distance by 2.

Q4 Which of the following cannot transmit sound?

A. Steel rod

B. Water

C. Vacuum

D. Air

Sound requires a material medium for propagation. Vacuum has no molecules, so there is nothing to vibrate and carry the sound energy.

Q5 The frequency of a sound wave is 500 Hz and its wavelength is 0.686 m. What is its speed?

A. 343 m/s

B. 500 m/s

C. 0.686 m/s

D. 729 m/s

v = f × lambda = 500 × 0.686 = 343 m/s. This confirms the standard speed of sound in air at 25°C.

Key Vocabulary

Compression

Latin: comprimere (to press together)

Region in a longitudinal wave where particles are pushed closer together — high pressure zone.

In a sound wave, the speaker cone moving forward creates a compression.

Rarefaction

Latin: rarefacere (to make thin)

Region in a longitudinal wave where particles are spread apart — low pressure zone.

The speaker cone moving backward creates a rarefaction.

Frequency

Latin: frequens (often)

Number of complete oscillations (compressions + rarefactions) per second. Determines pitch.

A 440 Hz sound (note A) makes 440 complete waves per second.

Amplitude

Latin: amplitudo (largeness)

Maximum displacement of a particle from its rest position. Determines loudness of sound.

Striking a drum harder increases amplitude — louder sound.

Wavelength

Old English: waeg (wave) + length

Distance between two consecutive compressions (or rarefactions) in a sound wave.

At 343 m/s and 343 Hz, wavelength = 1 metre exactly.

Reverberation

Latin: reverberare (to beat back)

Persistence of sound in a closed space due to multiple rapid reflections from walls and surfaces.

Clapping in an empty hall vs a furnished room — hall has more reverberation.

Ultrasound

Latin: ultra (beyond) + sound

Sound with frequency above 20,000 Hz — beyond the upper limit of human hearing.

Medical USG scans, SONAR, bat echolocation all use ultrasound.

Infrasound

Latin: infra (below) + sound

Sound with frequency below 20 Hz — below the lower limit of human hearing.

Elephants communicate using infrasound over distances of several kilometres.

History of Sound Science

~350 BC

Aristotle

Proposed that sound travels through air as a disturbance, and that higher pitch means faster motion

1638

Galileo Galilei

First quantitative study of vibrating strings; related tension and length to pitch

1660

Robert Boyle

Famous bell-in-vacuum experiment proved sound cannot travel without a medium

1738

Bianconi & Branca

First accurate experimental measurement of speed of sound in air (~332 m/s)

1877

Lord Rayleigh

Published 'The Theory of Sound' — the definitive mathematical treatment of acoustics

1915

Paul Langevin

Developed SONAR using ultrasound to detect submarines — first practical use of ultrasound

1942

Karl Dussik

First medical use of ultrasound — attempted to image the brain using sound waves

Amazing Sound Facts

01

The loudest natural sound on Earth was the 1883 Krakatoa volcanic eruption — heard 4,800 km away and measuring ~180 dB.

02

A blue whale's call can reach 188 decibels — louder than a jet engine — and can be heard by other whales over 1,600 km away.

03

Sound travels about 1 million times slower than light. That is why you see lightning before you hear thunder.

04

The human cochlea (inner ear) can detect sounds as quiet as 0 dB — that is a pressure variation of just 20 micropascals, or 0.00000002% of normal air pressure.

05

Astronauts on a spacewalk cannot hear each other because space is a vacuum. They communicate using radio waves (electromagnetic, not sound).

06

Bats emit ultrasound pulses up to 100,000 Hz — 5 times higher than the human hearing limit — and can detect objects as thin as a human hair using echo.

Sound in steel travels fast enough to cross the length of a cricket pitch in under 0.00005 seconds. The bat hits the ball before you even hear the previous shot.

— Physics perspective

If you could hear in vacuum, a supernova explosion would be about 10 octillion times louder than the loudest sound possible on Earth. Good thing space is silent.

— Astrophysics calculation

A mosquito's wings beat at ~600 Hz — right in the middle of the frequency range humans find most irritating. Evolution really did design it to annoy us.

— Entomological acoustics

Experiment: Verifying That Sound Needs a Medium

Aim: To demonstrate that sound cannot travel through a vacuum (medium is necessary for sound propagation)

Materials

Electric bell or buzzer with connecting wires
Bell jar with a rubber stopper
Vacuum pump
Battery / power supply
Thick rubber sheet or foam (to isolate vibrations)

Procedure

Place the electric bell inside the bell jar. Connect wires through the rubber stopper (airtight seal).
Connect the bell to the battery. Confirm you can hear the bell ringing clearly through the glass.
Start the vacuum pump. Slowly remove air from inside the bell jar.
Observe and note the change in loudness of the bell as air is removed.
Allow air back in. Note how loudness returns when air is restored.
Record observations at different stages: full air, partial vacuum, near-complete vacuum.

Observation

As air is progressively removed from the bell jar, the sound of the bell becomes fainter and fainter. In a near-complete vacuum, almost no sound is heard — even though the bell is still ringing (visible vibration). When air is let back in, sound is heard again immediately.

Conclusion

Sound requires a material medium for propagation. In the absence of air (vacuum), sound waves cannot be transmitted even though the source continues to vibrate. This proves that sound is a mechanical wave that needs a medium.

Applications of Sound and Ultrasound

1

SONAR (Sound Navigation and Ranging)

Uses ultrasound pulses to measure ocean depth and detect submarines. Pulse sent down; echo time measured; depth = (v × t) / 2.
2

Medical Ultrasonography

High-frequency ultrasound waves (1–18 MHz) reflect differently from different tissues. Used for pregnancy scans, organ imaging, detecting tumours — no radiation, non-invasive.
3

Echolocation in Bats

Bats emit 20–100 kHz ultrasound pulses up to 100 times per second. Echo analysis reveals size, distance and texture of objects — allows navigation in complete darkness.
4

Industrial Ultrasonic Testing

Detects cracks, flaws and defects in metal components (aircraft parts, pipelines) without destroying the material — called Non-Destructive Testing (NDT).
5

Seismology

Infrasound waves produced by earthquakes travel through the Earth's interior. Seismographs detect these waves and help map the Earth's internal structure.
6

Noise Pollution Control

Understanding sound frequency and intensity is crucial for designing quiet machinery, noise barriers on highways, and soundproofed buildings and studios.

Common Doubts

Why does sound travel faster in solids than in liquids or gases?

Speed of sound depends on the elasticity (ability to restore shape) and density of the medium. Solids have much stronger intermolecular bonds, so they transmit vibrations much more quickly. The formula is v = sqrt(E/rho) where E is elastic modulus and rho is density.

What is the difference between loudness and intensity?

Intensity is the physical quantity — energy per unit area per second (W/m²). Loudness is the physiological sensation — how our brain perceives that intensity. Loudness is measured in decibels (dB) on a logarithmic scale. A 10 dB increase sounds roughly twice as loud to human ears.

Can sound be reflected, refracted and diffracted like light?

Yes! Sound undergoes all wave phenomena. Reflection gives us echo. Refraction occurs when sound passes between media of different densities (sound bends toward denser medium). Diffraction allows sound to bend around corners — which is why you can hear someone talking in the next room even without line of sight.

What is the audible range and why does it change with age?

Humans can typically hear from 20 Hz to 20,000 Hz. As we age, the hair cells in the cochlea that detect high frequencies gradually die and are not replaced. This is why older people often lose the ability to hear high-pitched sounds — the upper limit drops to around 12,000–15,000 Hz by age 60.

How is an echo different from reverberation?

An echo is a distinct, separate repetition of a sound heard at least 0.1 seconds after the original — requires a reflecting surface at least 17.15 m away. Reverberation is the persistence of sound due to many rapid reflections arriving less than 0.1 seconds after the original — they blend with the original sound, making it seem prolonged.

Chapter Summary — Sound

Class 9 Physics · CBSE Chapter 12 · Everything you need for exams

Key Takeaways

Sound is a longitudinal mechanical wave — requires a medium, cannot travel in vacuum
Wave speed formula: v = f × lambda (speed = frequency × wavelength)
Speed in air ~343 m/s; increases in liquids; fastest in solids
Characteristics: Loudness (amplitude), Pitch (frequency), Quality (waveform)
Echo condition: reflecting surface must be at least 17.15 m away (0.1 s gap needed)
Echo distance: d = (v × t) / 2
Ultrasound (>20,000 Hz): SONAR, medical imaging, bat echolocation
Infrasound (<20 Hz): earthquakes, elephant communication
Human hearing range: 20 Hz to 20,000 Hz

Exam Tips

Always write the formula, substitute values with units, then calculate — show all steps
Draw and label the longitudinal wave diagram — compressions marked C, rarefactions R
Echo formula d = vt/2 is most common calculation question — practice with different speeds
Distinguish clearly: echo (distinct repetition) vs reverberation (prolonged blending)
Know 3 applications each of ultrasound and infrasound with brief explanation

Sound Class 9 Physics CBSE Waves Chapter 12 Longitudinal Wave Echo Ultrasound