Sound – ICSE Class 10 Physics – PYQ Junction Notes

        Sound is honestly one of the most interesting chapters in ICSE Physics — vibrations, echoes, resonance, it's all around us. The numericals are straightforward (mostly v = fλ and echo problems) and the theory is actually fun once you connect it to real life. Let's get into it!
    

1) Production and Propagation of Sound

Sound is produced when a body vibrates. The vibration disturbs the particles of the surrounding medium, and this disturbance travels as a wave to your ear.

Key Points

Sound needs a material medium — solid, liquid, or gas. It cannot travel through vacuum. That's why astronauts use radios in space.
Sound travels fastest in solids, slower in liquids, slowest in gases. (Speed in air ≈ 340 m/s, in water ≈ 1500 m/s, in steel ≈ 5000 m/s)
Sound is a mechanical wave — it needs particles to transfer energy.

Longitudinal Nature of Sound

In air, sound is a longitudinal wave. The particles of air vibrate parallel to the direction of wave travel.

Compression: Region where particles are close together (high pressure, high density)
Rarefaction: Region where particles are spread apart (low pressure, low density)

One complete wave = one compression + one rarefaction

2) Wave Parameters — The Language of Sound

Quantity	Symbol	Definition	SI Unit
Amplitude	A	Maximum displacement from mean position	metre (m)
Frequency	f	Number of complete vibrations per second	hertz (Hz)
Time Period	T	Time for one complete vibration	second (s)
Wavelength	λ	Distance between two successive compressions or rarefactions	metre (m)
Velocity	v	Distance travelled by the wave per second	m/s

v = f × λ T = 1 / f f = 1 / T

This v = fλ formula is your best friend. It connects everything. If you know any two, you can find the third.

Quick trick: Frequency and time period are reciprocals. If frequency is 50 Hz, T = 1/50 = 0.02 s. If T = 0.005 s, f = 1/0.005 = 200 Hz. ICSE loves asking this conversion.

3) Reflection of Sound

Sound, like light, reflects from surfaces. The laws of reflection are the same:

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

Echo

An echo is the distinct repetition of sound caused by reflection from a distant obstacle. The reflected sound reaches your ear after the original sound has stopped.

2d = v × t d = (v × t) / 2 Condition for echo: t ≥ 0.1 s Minimum distance for echo in air ≈ 17 m (since v ≈ 340 m/s, 340 × 0.1 / 2 = 17 m)

The "2d" in the formula is because sound travels to the obstacle AND back. Don't forget to divide by 2 when finding distance!

Reverberation

Reverberation is the persistence of sound in an enclosed space due to multiple reflections — the sound seems to hang in the air even after the source stops.

Echo	Reverberation
Distinct repetition of sound	Prolongation / overlapping of sound
Requires t ≥ 0.1 s	Sound returns in less than 0.1 s
Single reflection is enough	Multiple reflections
Heard separately from original	Mixes with original sound

How to reduce reverberation: Use curtains, carpets, cushioned seats, acoustic panels, false ceilings with absorbent materials. This improves sound clarity in auditoriums.

Applications of Reflection of Sound

Device	How it uses reflection
Megaphone / Loudspeaker	Directs sound in one direction, preventing spreading
Stethoscope	Multiple reflections carry sound from chest to ear
Soundboard	Reflects sound toward the audience in a hall
Ear trumpet	Collects and directs sound into the ear
Sonar	Uses reflection of sound (ultrasound) to detect objects underwater

4) Natural Vibrations

Every object has a natural frequency — the frequency at which it vibrates when disturbed and left free. It depends on size, shape, material, tension, etc.

A tuning fork vibrates at its natural frequency when struck
A pendulum swings at its natural frequency depending on its length
A stretched string has a natural frequency depending on length, tension, and mass per unit length

The amplitude of natural vibrations gradually decreases due to air resistance — this leads us to damped vibrations.

5) Damped Vibrations

In real life, vibrations don't go on forever. The amplitude decreases with time because energy is lost to friction and air resistance. This is called damped vibration.

Damped vibration — amplitude decreases gradually until the body comes to rest.

Examples of Damped Vibration

A swinging pendulum in air gradually slows down and stops
A tuning fork stops vibrating after some time
A plucked guitar string eventually becomes silent
A bell stops ringing after being struck

In damped vibration, the frequency remains the same but the amplitude keeps decreasing. The energy is dissipated as heat due to friction. This is a common 2-mark question.

6) Forced Vibrations

When a body vibrates because of an external periodic force applied to it, it's called forced vibration. The body vibrates with the frequency of the external force, not its own natural frequency.

A table top vibrating because a loudspeaker is placed on it
A window pane rattling because of loud music outside
Your eardrum vibrating when sound waves hit it

In forced vibration, the external force keeps supplying energy to the body, so it continues vibrating as long as the force is applied.

7) Resonance — The Most Interesting One

Resonance is a special case of forced vibration. It occurs when the frequency of the external force equals the natural frequency of the body. At resonance, the amplitude becomes maximum.

Resonance: f_external = f_natural At resonance → Amplitude becomes maximum

Examples of Resonance

Swing: When you push a child on a swing at the right moment (matching the swing's natural frequency), it goes higher and higher
Tuning forks: Strike one tuning fork and bring it near another of the same frequency — the second one starts vibrating on its own
Musical instruments: The air column in a flute or organ pipe resonates to produce a loud, clear sound
Glass shattering: An opera singer can shatter a glass by singing at its resonant frequency
Radio tuning: You adjust the frequency of your radio to match the broadcast frequency — that's resonance too!

Harmful Resonance

Resonance isn't always good. If not controlled, it can cause destruction:

Soldiers break step while crossing a bridge — if their marching frequency matches the bridge's natural frequency, the bridge could collapse (this actually happened in 1831 in England)
Wind can cause a bridge to oscillate at its resonant frequency and collapse (Tacoma Narrows Bridge, 1940)

The swing example is ICSE's favourite. "Why does a swing go higher when pushed at regular intervals?" Because when the push frequency matches the swing's natural frequency, resonance occurs and amplitude increases. This exact question has appeared many times.

8) Characteristics of Sound

There are three characteristics that distinguish one sound from another:

a) Pitch (Shrillness or Depth)

Depends on frequency
High frequency → high pitch (shrill sound) — like a baby crying or a whistle
Low frequency → low pitch (deep sound) — like a drum beat or a lion's roar

b) Loudness

Depends mainly on amplitude
Large amplitude → loud sound
Small amplitude → soft sound
Also depends on: distance from source, area of vibrating surface, sensitivity of ear, density of medium

c) Quality (Timbre)

Depends on waveform
Helps distinguish the same note played on different instruments (e.g., a guitar vs a piano playing the same note sound different because of waveform)

Intensity of Sound

Intensity is the amount of sound energy passing per unit area per second. It is a physical quantity that can be measured.

I ∝ A² Intensity ∝ (Amplitude)² SI unit: watt per square metre (W/m²)

Pitch	Loudness	Quality
Depends on frequency	Depends on amplitude	Depends on waveform
Determines how shrill or deep	Determines how loud or soft	Determines the "flavour" of the sound
Unit: hertz (Hz)	Unit: decibel (dB)	No unit — it's a perception

Don't confuse loudness with intensity! Loudness is a sensation (subjective, depends on the ear), while intensity is a physical quantity (objective, can be measured). Loudness ∝ log I — human ears respond logarithmically, not linearly.

9) Audible Range

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

Infrasonic (below 20 Hz): Not audible to humans. Produced by earthquakes, volcanic eruptions. Some animals (elephants) can hear these.
Audible (20 Hz - 20,000 Hz): Normal human hearing range. Decreases with age (older people lose high frequencies).
Ultrasonic (above 20,000 Hz): Not audible to humans. Used in sonar, ultrasound imaging, cleaning, and by bats and dolphins for navigation.

10) Simple Musical Instruments

Type	Examples	How Pitch is Changed
String instruments	Sitar, violin, guitar, veena	By changing length / tension / thickness of string
Wind instruments	Flute, organ pipe, trumpet	By changing length of air column (opening/closing holes)
Percussion instruments	Drum, tabla, dholak	By changing tightness of membrane

A note is a sound of a single frequency. Music is a pleasant sound with a regular pattern of frequencies. Noise is an unpleasant, irregular sound.

11) Solved Numericals

Numerical 1 — Wave Relation (v = fλ)

A sound wave has a frequency of 500 Hz and a wavelength of 0.68 m. Calculate its speed.

v = fλ = 500 × 0.68 = 340 m/s

Numerical 2 — Frequency from Time Period

The time period of a wave is 0.02 s. Find its frequency.

f = 1/T = 1 / 0.02 = 50 Hz

Numerical 3 — Echo Distance

A person claps in front of a cliff and hears the echo after 1.2 s. If the speed of sound is 340 m/s, find the distance of the cliff from the person.

2d = v × t 2d = 340 × 1.2 = 408 d = 408 / 2 = 204 m

Numerical 4 — Minimum Distance for Echo

Calculate the minimum distance from a reflecting surface to hear an echo. (Speed of sound = 340 m/s, human ear persistence = 0.1 s)

2d = v × t = 340 × 0.1 = 34 d = 34 / 2 = 17 m

This is why you don't hear echoes in a normal room — the walls are too close (less than 17 m away).

Numerical 5 — Speed of Sound in Different Media

A sound wave of frequency 500 Hz has a wavelength of 0.68 m in air and 3 m in water. Find the speed of sound in air and water.

In air: v = fλ = 500 × 0.68 = 340 m/s In water: v = 500 × 3 = 1500 m/s

Numerical 6 — Resonance

A tuning fork of frequency 384 Hz is vibrated above a resonance tube. The first resonance occurs when the length of the air column is 22 cm. Find the speed of sound.

For first resonance: λ/4 = 22 cm = 0.22 m λ = 4 × 0.22 = 0.88 m v = fλ = 384 × 0.88 = 337.92 m/s ≈ 338 m/s

12) Quick Revision — Night Before the Exam

Sound is a longitudinal mechanical wave — needs a medium, can't travel in vacuum
v = fλ and T = 1/f — the two most important formulas
Reflection of sound: same laws as light. Echo needs t ≥ 0.1 s. Minimum distance = 17 m
2d = vt — for echo problems (don't forget the 2!)
Reverberation: multiple reflections, sound persists. Reduced by curtains, carpets, acoustic tiles
Natural vibration: body vibrates at its own natural frequency
Damped vibration: amplitude decreases with time due to energy loss
Forced vibration: body vibrates at external force's frequency
Resonance: external frequency = natural frequency → max amplitude
Pitch ↔ frequency, Loudness ↔ amplitude, Quality ↔ waveform
Intensity ∝ A², Loudness ∝ log I
Audible range: 20 Hz to 20,000 Hz. Below = infrasonic, above = ultrasonic

Common Mistakes That Cost Marks

❌ Forgetting the "2" in 2d = vt for echo problems
❌ Confusing loudness with intensity — intensity is physical, loudness is a sensation
❌ Saying "sound travels fastest in air" — no, fastest in solids, slowest in gases
❌ Mixing up natural, damped, forced, and resonance — make a quick table to compare
❌ Writing "amplitude" when asked about pitch — pitch depends on frequency, not amplitude
❌ Forgetting units: Hz for frequency, m/s for velocity, m for wavelength, s for time period

        Final tip: Sound is a chapter where the theory is as important as the numericals. ICSE asks a lot of 2-mark definitions and differences (pitch vs loudness, echo vs reverberation, natural vs forced vibration). If you know your definitions well and can do v = fλ and echo problems, you're looking at full marks in this chapter. You've got this! 🎯
    

13) All Formulas at One Glance

v = f × λ T = 1 / f f = 1 / T 2d = v × t (echo) d = v × t / 2 Minimum distance for echo = v × 0.1 / 2 = 17 m (in air) I ∝ A² Loudness ∝ log I