ICSE Class 10 Physics Chapter 7 Sound Notes | PDF Download

Introduction 

Preparing for the ICSE Class 10 Physics examination? These Chapter 7: Sound Notes are designed according to the latest ICSE syllabus and cover every important concept in a simple and student-friendly language. Whether you are revising before the examination or studying the chapter for the first time, these notes will help you understand the topic quickly and score better marks.
These notes include all important definitions, formulas, diagrams, numerical concepts, resonance, echo, SONAR, musical instruments, previous year questions, and board examination tips. You can also download the PDF for offline study and quick revision.

Why These Notes Are Useful

• Based on the latest ICSE syllabus
• Easy and simple explanations
• Exam-oriented content
• Important formulas and definitions
• Board-level numerical concepts
• Quick revision before exams
• Useful for Selina Concise Physics solutions

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Study materials related to chapter 7:  Sound

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Section (A) : Reflection of Sound Waves and Echoes

What is Sound?

Sound is a type of mechanical wave produced when an object vibrates. These vibrations are transferred through the particles of a medium such as air, water, or solids.

***Note:
When the medium particles vibrate, there is a change of kinetic energy into the potential energy and vice versa.

Important Points

• Sound requires a material medium for propagation.
• It cannot travel through a vacuum.
• Sound travels as longitudinal waves in gases and liquids.
• In solids, sound can travel as both longitudinal and transverse waves.

Audible and Inaudible Sound

The normal human ear can hear sounds having frequencies between 20 Hz and 20,000 Hz (20 kHz).

Examples:

• Elephants communicate using infrasonic waves.
• Bats and dolphins use ultrasonic waves.

Basic Terms of Sound Waves

1. Amplitude (A)

• The maximum displacement of the particle from its mean position is called amplitude.
• SI unit: metre (m)
• Determines the loudness of sound
• Effect: Greater amplitude ⇒ Louder sound

2. Time Period (T)

• The time taken to complete one vibration is called time period.
• SI unit: second (s)

3. Frequency (f)

• The number of vibrations produced in one second is called frequency.
• SI unit: hertz (Hz) or s^–1

4. Wavelength (λ)

• The distance travelled by a wave during one complete vibration is called wavelength.
• SI Unit: metre (m)

5. Wave Velocity (V)

• Distance travelled by sound in one second is called Wave Velocity.
• SI Unit: m s^–1

Relation between time-period and frequency

$\mathrm{f}=\frac{1}{\mathrm{T}}$

Where,

• f = frequency
• t = time period

Relationship Between Velocity, Frequency and Wavelength of a Wave

$\mathrm{Velocity}=\frac{\mathrm{Distance}\ \mathrm{travelled}}{\mathrm{Time}\ \mathrm{taken}}$

⇒ $\mathrm{f}=\frac{1}{\mathrm{T}}$

⇒ V = f × λ   $\left[\because f=\frac{1}{T}\right]$

Where:

• v = Velocity of the wave (m s^–1)
• f = Frequency of the wave (Hz)
• λ (lambda) = Wavelength of the wave (m)

Mechanical Waves

Mechanical waves require a material medium for propagation.

They are of two types:

1. Longitudinal Waves

• Particles vibrate parallel to the direction of propagation.
• Consist of compressions and rarefactions.
• Example: Sound in air.

2. Transverse Waves

• Particles vibrate perpendicular to the direction of propagation.
• Consist of crests and troughs.
• Example: Waves on a stretched string.

Speed of Sound

Sound travels fastest in solids and slowest in gases.

• Iron: 5000 m/s
• Water: 1500 m/s
• Air: 330 m/s

Factors Affecting the Speed of Sound

1. Temperature
As the temperature increases, the particles of the medium move faster, so sound travels more quickly.
Temperature ↑ ⇒ Speed of sound ↑

2. Humidity
Moist air is less dense than dry air. Therefore, sound travels faster in humid air.
Humidity ↑ ⇒ Speed of sound ↑

3. Density of the Medium
The speed of sound depends on the density of the medium. Generally, sound travels faster in denser media like solids because their particles are closely packed.

3. Elasticity of the Medium
The greater the elasticity of a medium, the faster sound travels through it because the particles return to their original positions more quickly.Elasticity ↑ ⇒ Speed of sound ↑

Does Pressure Affect Speed?
No. The speed of sound in air is almost independent of pressure.

Difference Between Light and Sound Waves

Reflection of Sound Waves

Definition: Reflection of sound is the bouncing back of sound waves after striking a surface.

Laws of Reflection:

1. Angle of incidence = Angle of reflection.
2. Incident wave, reflected wave and normal lie in the same plane.

• Condition: The reflecting surface should be larger than the wavelength of the sound.

• Applications: Megaphone, speaking tube, soundboards, and ear trumpet.

Echo

An echo is the repetition of sound caused due to reflection from a distant obstacle.
The reflected sound should be heard separately after the original sound.

Persistence of Hearing: 
Human ears retain the sensation of a sound for 0.1 second (1/10th of a second).

Conditions for Hearing an Echo

1. Distance between listener and reflector should be at least 17 m in air.
2. Reflecting surface should be large.
3. Reflected sound must be sufficiently loud.

Reverberation

When sound undergoes repeated reflections, it persists for some time even after the source stops.
This effect is called reverberation.

Examples

• Large halls
• Temples
• Auditoriums

Determination of Speed of Sound by Echo Method

The speed of sound can be determined using the echo produced by a distant reflector.

Formula: $V=\frac{2d}{t}$

Where

• V = speed of sound
• d = distance from reflector
• t = total time taken by echo

Ultrasonic Waves & Practical Applications

Why Ultrasonic Waves are Used (Properties):

1. Travel undeviated over long distances.
2. Can be confined into a narrow beam.
3. Not easily absorbed by the medium.

Uses of Echo

1. Animals (Sound Ranging):

• Bats: Emit and detect high-frequency ultrasound (up to 100 kHz) to locate obstacles and fly safely in the dark.
• Dolphins & Fishermen: Emit ultrasonic waves to detect obstacles/prey and shoals of fish.

2. SONAR (Sound Navigation and Ranging):
It is a device that uses ultrasonic waves. It is used on ships to measure the depth of the ocean (echo depth sounding) and to detect or locate underwater objects such as submarines, icebergs, and shipwrecks.

3. Medical Field:

• Ultrasonography: Used to obtain images of  internal human organs (liver, gallbladder, uterus, foetus etc.).
• Echocardiography: Used to study the structure and functioning of the human heart.

Section (B) : Natural, Damped and Forced Vibrations; Resonance

Natural Vibrations (Free Vibrations)

The periodic vibrations of a body in the absence of any external force on it are called natural (or free) vibrations.

Characteristics of Natural Vibrations

• No external periodic force acts on the body.
• The body vibrates with its own natural frequency.
• Frequency depends on the size, shape and material of the body.
• In vacuum, the amplitude remains constant.
• In air, the amplitude gradually decreases because of air resistance.

Natural Frequency

The frequency with which a body vibrates naturally after being disturbed is called its natural frequency.
Different bodies have different natural frequencies.

Examples of Natural Vibrations

1. Simple Pendulum
The frequency of a simple pendulum is:
$f=\frac{1}{2\pi }\sqrt{\frac{g}{l}}$

Where

• f = frequency
• g = acceleration due to gravity
• l = length of the pendulum

***Note:

• Longer pendulum ⇒ Lower frequency
• Shorter pendulum ⇒ Higher frequency

2. Spring-Mass System
A stretched spring vibrates with its natural frequency.
$f=\frac{1}{2\pi }\sqrt{\frac{k}{m}}$

Where

• K = force constant of spring
• m = mass attached

***Note

• Larger K ⇒ Higher frequency
• Larger mass ⇒ Lower frequency

3. Tuning Fork
A tuning fork produces vibrations of only one frequency, giving a pure note.

4. Piano
Each string vibrates with its own natural frequency.

5. Air Column in Flute

The frequency depends upon the effective length of the air column.

• Smaller air column → Higher frequency
• Longer air column → Lower frequency

6. Stringed Musical Instruments

Examples:

• Guitar
• Sitar
• Violin

Frequency depends upon:

• Length of string (l)
• Radius (thickness) (r)
• Tension (T)

Formula: $f=\frac{1}{2l}\sqrt{\frac{T}{\pi r^{2}d}}$

where d is the density of the material.

Frequency Increases When:

• Length decreases
• Radius decreases
• Tension increases

7. Modes of Vibration in a Stretched String

A stretched string can vibrate in different modes.

• Frequency Ratio =1 : 2 : 3
• Wavelength Ratio = 3 : 2: 1
  • Principal note = 2l
  • First subsidiary = l
  • Second subsidiary = 2l/3

Nature of Natural Vibrations

Natural vibrations are simple harmonic vibrations.

In an ideal vacuum:

• Amplitude remains constant.
• Frequency remains constant.
• No loss of energy occurs.

In practical conditions, air resistance gradually reduces the amplitude.

Damped Vibrations

When a vibrating body loses energy due to friction or air resistance, its amplitude gradually decreases with time. Such vibrations are called damped vibrations.

Characteristics of Damped Vibrations

• Amplitude continuously decreases.
• Energy is gradually lost.
• Motion finally stops.
• Frequency is slightly lower than natural frequency.

Causes of Damping

• Air resistance
• Friction
• Viscosity of medium

Examples of Damped Vibrations

• ​A slim branch of a tree pulled and released.
• ​A tuning fork vibrating in the air.
• ​A simple pendulum oscillating in air or liquid.
• ​A loaded spring vibrating in air.

Difference Between Natural and Damped Vibrations

Forced Vibrations

The vibrations of a body produced under the influence of an external periodic force are called forced vibrations.

Characteristics:

• External periodic force acts continuously.
• Frequency becomes equal to the frequency of the applied force.

• Amplitude depends upon the applied force.
• Energy supplied compensates for energy lost.

Forces Acting on the Body:

1. ​Restoring force
2. ​Frictional/resistive force
3. ​External periodic force (Driving force)

Key Characteristics:

• The body does not vibrate with its natural frequency; it is forced to vibrate at the frequency of the applied external force.
• The amplitude depends on the difference between the applied frequency and the body’s natural frequency:
  • If frequencies are far apart ⇒ Small amplitudeIf frequencies are close/equal ⇒ Large amplitude.

Examples of Forced Vibrations

• Table vibrating due to a tuning fork
• Guitar strings while playing
• Loudspeaker diaphragm
• Sound box of musical instruments

Difference Between Natural and Forced Vibrations

Resonance (Special Case of Forced Vibrations)

Definition:
Resonance is a special case of forced vibrations in which the frequency of the applied periodic force becomes exactly equal to the natural frequency of the vibrating body, causing it to vibrate with a very large amplitude.

Conditions for Resonance

• External periodic force should act.
• Frequency of applied force must equal the natural frequency.
• Energy transfer should be continuous.

Characteristics of Resonance

• Maximum amplitude
• Loud sound produced
• Maximum transfer of energy
• Vibrations remain in phase

Demonstration of Resonance

1. Two Tuning Forks

Observation:
When one tuning fork vibrates, the other also starts vibrating if both have the same natural frequency.

Reason:
Resonance occurs because both tuning forks have identical natural frequencies.

2. Pendulums

Observation:

• Pendulum B (same length as A) vibrates with maximum amplitude.
• Pendulums C and D vibrate with very small amplitude.

Reason:
Only pendulum B has the same natural frequency as pendulum A.

3. Resonance in Air Column

Observation:
A loud sound is heard when the air column length becomes suitable.

Reason:
Natural frequency of air column becomes equal to the tuning fork frequency.

Difference Between Forced and Resonant Vibrations

Applications of Resonance

1. Pendulums
Pendulums of equal length resonate with each other.

2. Musical Instruments
Sound boxes in guitars, violins and sitars increase the loudness of sound due to resonance.

3. Air Column
Flutes and organ pipes produce loud sounds because of resonance.

4. Troops Marching Across a Suspension Bridge: Soldiers are ordered to break step while crossing a bridge. If the rhythmic marching frequency matches the natural frequency of the bridge, resonance could cause heavy oscillations and structural failure.

5. Rattling of Vehicle Parts
Sometimes, at a particular speed, a car or bike produces a rattling sound. This happens because the engine’s vibration frequency matches the natural frequency of a loose part, causing it to vibrate strongly due to resonance.

Solution to Stop the Sound:

• Change the speed of the vehicle so that resonance does not occur.
• Tighten any loose parts or screws.
• Use rubber pads or other damping materials to reduce vibrations.

6. Radio and Television Receivers
Radio and TV receivers are tuned using resonance. By changing the capacitance or inductance of the tuning circuit, the receiver selects the desired frequency while rejecting others.

Section (C) : Characteristics of Sound and Their Subjective and Objective Nature

Characteristics of Sound

The three important characteristics of sound are:

1. Loudness
2. Pitch (Shrillness)
3. Quality (Timbre)

These characteristics can be studied using a Microphone and a Cathode Ray Oscilloscope (CRO).

1. Loudness

Definition:
Loudness is the characteristic of sound that enables us to distinguish between a loud sound and a faint (soft) sound.

Factors Affecting Loudness

1. Amplitude of Vibration
Loudness is directly proportional to the square of the amplitude.
Loudness ∝ Amplitude²

• Larger amplitude ⇒ Louder sound
• Smaller amplitude ⇒ Softer sound
• Double amplitude ⇒ Four times louder.
• Half amplitude ⇒ One-fourth loudness.

2. Distance from the Source
Loudness decreases as the distance from the source increases.

$\mathrm{Loudness}\propto \frac{1}{{\left(\mathrm{Distance}\right)}^{2}}$

• Closer to the source ⇒ Louder sound

3. Surface Area of the Vibrating Body
A larger vibrating surface sets more air particles into vibration and produces a louder sound.

Examples:

• School bell
• Temple bell
• Sound box of guitar

4. Density of the Medium
Denser media transmit sound more effectively, making it appear louder.

5. Presence of Resonant Bodies
Resonant bodies increase the intensity of sound and make it louder.
Example: Sound box of a guitar.

Intensity of Sound

• Definition:
  The intensity of sound is the amount of sound energy passing normally through unit area per second.
• SI Unit: Watt per square metre (W m^–2)
• Minimum audible intensity: I₀ = 10^–12 W m^–2

Loudness vs Intensity

Relationship Between Loudness and Intensity

According to Weber–Fechner Law,

$L={Klog}_{10}I$ dB

Where

• L = Loudness
• I = Intensity
• K = Constant

Sound Level Formula

$\mathrm{L}={\mathrm{Klog}}_{10}\left(\frac{{\mathrm{I}}_{1}}{{\mathrm{I}}_{\mathrm{o}}}\right)$

where

• I = Intensity of sound
• I₀ = 10^–12 W m^–2

***Note
Units: Phon (unit of loudness) and Decibel (dB) (unit of sound level).

Important Fact

A change of 1 dB corresponds to nearly 26% change in intensity.

Decibel (dB)
The sound level is measured in decibel (dB).

Important Facts

• 0 dB ⇒ Minimum audible sound (pin drop sound)
• Soft whisper ⇒ 30 dB
• Safe hearing range ⇒ 0 – 80 dB
• Above 120 dB ⇒ Harmful to ears

Noise Pollution

Noise pollution is caused by unwanted, unpleasant and excessively loud sounds.

Sources

• Loudspeakers
• Vehicle horns
• Factories
• Construction sites
• Aircraft
• Industrial machines

Harmful Effects

• Hearing loss
• Headache
• High blood pressure
• Stress
• Lack of concentration
• Sleep disturbance

2. Pitch (Shrillness)

Definition:
Pitch is the characteristic of sound by which an acute (shrill) sound can be distinguished from a grave (flat or deep) sound.

Pitch depends directly on Frequency (f):

• Higher Frequency ⇒ Higher Pitch ⇒ Shrill sound (e.g., flute, whistle, female voice).
• Lower Frequency ⇒ Lower Pitch ⇒ Flat/Grave sound (e.g., bass drum, male voice).

Examples

• Women’s voice has higher pitch than men’s voice.
• Thin strings produce higher pitch.
• Short strings produce higher pitch.
• Greater tension in strings produces higher pitch.
• A shorter air column in a flute gives a higher pitch.

Change of Pitch in Musical Instruments

(i) Stringed Instruments

Pitch depends on:

• Thickness of string (thinner → higher pitch)
• Tension in string (more tension → higher pitch)
• Length of vibrating string (shorter → higher pitch)
• Examples: Guitar, Sitar, Piano

(ii) Wind Instruments

Pitch depends on the length of vibrating air column.

• Shorter air column → higher pitch
• Longer air column → lower pitch
• Examples: Flute, Shehnai, Bugle

(iii) Membrane Instruments

Pitch depends on:

• Size of membrane (small size → higher pitch)
• Tension of membrane (more tight → higher pitch)
• Examples: Tabla, Drum, Dholak

Subjective Nature of Pitch

• Pitch depends on the sensation experienced by the listener.
• Different people may perceive the same sound differently.

Objective Nature of Frequency

• Frequency is measurable.
• It depends only on the source and not on the listener.

Difference Between Pitch and Frequency

3. Quality (Timbre)

Definition:
Quality or timbre is the characteristic of sound that distinguishes two sounds having the same loudness and pitch but produced by different sources.

Quality Depends Upon

• Quality depends on the waveform.
• Different waveforms produce different sound qualities.

Why Different Instruments Sound Different?

Although a piano and a flute may produce the same frequency and loudness, they have different waveforms.
Therefore, they sound different.

Principal and Subsidiary Vibrations

Musical instruments generally produce a combination of vibrations.

Principal Vibration:

• Lowest frequency
• Maximum amplitude

Subsidiary Vibrations:

• Higher frequencies
• Smaller amplitudes
• Multiples of the principal frequency

These combine to produce a unique waveform.

Recognition of Sound Sources

Every source has its own characteristic waveform.
Therefore, we can recognize

• Different musical instruments
• Different people’s voices

even if they have the same pitch and loudness.

Factors Determining Sound Characteristics Characteristic

Music

Music is a pleasant sound produced by regular, periodic vibrations.

Characteristics

• Pleasant
• Continuous
• Periodic
• Regular waveform
• Sound level usually between 10 dB and 30 dB

Examples

• Piano
• Violin
• Guitar
• Flute
• Tuning fork

Noise

Noise is an unpleasant sound produced by irregular vibrations.

Characteristics

• Harsh
• Discontinuous
• Irregular waveform
• Non-periodic
• Usually above 120 dB

Examples

• Aircraft
• Road roller
• Factory machines
• Loud traffic

Difference Between Music and Noise

Important Formulae – Sound

ICSE Class 10 Physics Chapter 7 Sound Notes PDF Download

Students can download the ICSE Class 10 Physics Chapter 7 Sound Notes PDF for offline study and quick revision. The PDF is useful for last-minute preparation and helps you revise the complete chapter anytime.

Conclusion

The ICSE Class 10 Physics Chapter 7 Sound Notes provide a complete and easy-to-understand revision resource for students. Study these notes thoroughly, practice previous year questions, and revise the formulas regularly to perform well in the ICSE Board Examination. Download the PDF and start your preparation today.

You can also visit:

ICSE Class 10 Physics Notes

Official Website:

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• Council for the Indian School Certificate Examinations (CISCE)