Sound Class 8 Notes Class 8 Science Revision Notes

Sound is a form of energy that travels as waves through a medium. When you speak, sing, or clap your hands, vibrations create disturbances in the air around you. These disturbances travel outward in all directions, carrying sound energy to your ears.

What is a Wave?

A wave is a disturbance (or oscillation) that travels through space with time, accompanied by energy transfer.

Point: Waves transfer energy, NOT matter. When ripples move across water, the water itself doesn't travel forward—only the energy does.

Simple Experiment: Water Ripples

When you drop a stone in still water:

• Circular ripples spread outward
• A floating leaf bobs up and down but doesn't move sideways
• This shows waves carry energy, not the medium itself

Understanding Oscillations and Vibrations

Oscillation

Repetitive motion around a central position (mean position).

Example: A pendulum swinging back and forth.

Vibration

The to-and-fro or back-and-forth motion of an object.

Fact: Oscillations and vibrations are the same phenomenon.

Simple Pendulum

A simple pendulum consists of:

• A bob (mass m) suspended from a rigid support
• An unstretchable, massless string
• Length 'l' measured from the point of suspension to the center of gravity of the bob

Motion: The pendulum oscillates between two extreme positions (A and B) with a constant amplitude.

Classification of Waves

Based on Medium of Propagation

1. Mechanical Waves

Require a medium to propagate.

Examples:

• Sound waves
• Water waves
• Waves on a stretched string

2. Non-Mechanical Waves

Do NOT require a medium; can travel through vacuum.

Examples:

• Light waves
• Radio waves
• X-rays
• Gamma rays

Based on Particle Motion

1. Transverse Waves

Particles vibrate perpendicular to the direction of wave propagation.

Features:

• Propagate as crests (maximum upward displacement) and troughs (maximum downward displacement)
• All electromagnetic waves are transverse

Example: Waves on a string

2. Longitudinal Waves

Particles vibrate parallel to the direction of wave propagation.

Features:

• Propagate as compressions and rarefactions
• Also called pressure waves

Example: Sound waves in air

Compression: Region where particles are closer together than normal (momentary volume decrease).

Rarefaction: Region where particles are farther apart than normal (momentary volume increase).

Important Terms Related to Wave Motion

1. Crest

Point of maximum upward displacement in a transverse wave.

2. Trough

Point of maximum downward displacement in a transverse wave.

3. Amplitude (A)

Maximum displacement of particles from their mean position.

Impact: Greater amplitude = louder sound

4. Wavelength (λ)

Distance between two successive crests or troughs (or two successive compressions or rarefactions).

Unit: meters (m)

5. Wave Number

Reciprocal of wavelength: 1/λ

6. Time Period (T)

Time required for one complete oscillation.

Unit: seconds (s)

7. Frequency (ν)

Number of oscillations or vibrations per second.

Formula: ν = 1/T

Unit: Hertz (Hz)

• 1 Hz = 1 s⁻¹

8. Wave Velocity (v)

Speed at which the wave propagates through the medium.

Formulas:

• v = λ/T
• v = νλ

Important: Wave velocity is different from particle velocity.

• Wave velocity: constant for a given medium
• Particle velocity: varies with time for each particle

What is Sound?

Sound is a type of energy produced by vibrations. When an object vibrates, it creates disturbances in surrounding air particles, which travel as waves.

1. Sound is Energy: Requires energy to start vibrations
2. Sound is a Wave: Carries energy through space
3. Sound is Longitudinal: Particles vibrate parallel to wave direction

Production of Sound

All sounds are produced by vibrating objects:

• Musical instruments: Vibrating strings, membranes, air columns
• Human voice: Vibrating vocal cords in the larynx
• Clapping: Vibrating hands

Rule: When vibration stops, sound stops.

How We Hear Sound

Sound waves cause our eardrum to vibrate, which our brain interprets as sound.

Propagation of Sound

Critical Fact: Sound CANNOT travel through vacuum. It requires a material medium (solid, liquid, or gas).

Sound Through Solids

• Example: Placing your ear on a railway track to detect a distant train
• Speed: Fastest in solids

Sound Through Liquids

• Example: Hearing a squeaking toy inside water
• Speed: Faster than in gases

Sound Through Gases

• Example: Hearing through air
• Speed: Slowest in gases

Speed Comparison

V_solid > V_liquid > V_gas

Factors Affecting Speed of Sound

Sound velocity depends on:

• Temperature
• Pressure
• Humidity
• Density of the medium

Sound velocity is INDEPENDENT of:

• Amplitude
• Frequency
• Wavelength

Special Case: If temperature is constant, speed of sound becomes independent of pressure too.

Standard Conditions: Speed of sound in air ≈ 330 m/s at 273.15 K (0°C) and 100 kPa (0.986 atm).

Characteristics of Sound

1. Loudness

The degree of sensation of sound produced in the ear.

Factors Affecting Loudness:

• Amplitude: Greater amplitude = louder sound
• Area of vibrating body: Larger area = louder sound (large drum vs small drum)
• Distance from source: Loudness decreases with distance
• Sensitivity of the ear: Varies from person to person

2. Pitch

The characteristic that determines the shrillness of sound.

Factors Affecting Pitch:

• Frequency: Higher frequency = higher pitch
• Relative motion:
  • Source approaching listener = pitch appears higher
  • Source receding = pitch appears lower

Important Distinction:

• Pitch is subjective (cannot be measured)
• Frequency is objective (can be measured in Hz)
• Same frequency may sound different in pitch to different people

3. Quality (Timbre)

The characteristic that enables us to distinguish between two sounds of the same pitch and loudness produced by different sources.

Example: A piano and a guitar playing the same note at the same volume sound different due to quality.

Audible and Inaudible Sounds

Human Hearing Range

Audible Range: 20 Hz to 20,000 Hz (20 kHz)

Infrasonic Sound

• Frequency < 20 Hz
• Cannot be heard by humans
• Examples: Earthquakes, ocean waves

Ultrasonic Sound

• Frequency > 20 kHz
• Cannot be heard by humans
• Examples: Dog whistles, ultrasound machines, SONAR

Animal Hearing

Some animals can hear ultrasonic sounds:

• Dogs: Up to 50 kHz
• Bats: Up to 120 kHz
• Dolphins: Up to 150 kHz

Application: Police use high-frequency whistles that dogs can hear but humans cannot.

Noise vs Music

Musical Sound

Pleasant to the ear; produced by regular vibrations.

Examples:

• Harmonium
• Sitar
• Flute

Noise

Unpleasant to the ear; produced by irregular vibrations.

Examples:

• Bus/truck horns
• Construction machinery
• Traffic sounds

Noise Pollution

Unwanted, loud sounds that cause harmful effects on humans and the environment.

Harmful Effects

1. Hearing impairment (temporary or permanent)
2. Sleep disturbance
3. Hypertension (high blood pressure)
4. Anxiety and stress
5. Reduced concentration
6. Cardiovascular problems

Measures to Control Noise Pollution

1. Location planning: Set up noisy industries away from residential areas
2. Natural barriers: Plant trees along roads and around buildings
3. Infrastructure: Build airports away from residential zones
4. Technology: Use quality silencers in vehicles and aircraft
5. Volume control: Run TVs, music systems at low volumes
6. Awareness: Educate public about harmful effects
7. Minimize horn use: Reduce unnecessary honking

Reflection of Sound

Sound waves reflect when they encounter a surface, following the laws of reflection (similar to light).

Echoes

Echo: Phenomenon of hearing reflected sound from a distant surface.

Minimum Distance for Echo

For an echo to be heard distinctly:

• Minimum time gap required: 1/10th second (0.1 s)
• Speed of sound in air: 330 m/s

Calculation:

Distance traveled by sound = Speed × Time = 330 × 0.1 = 33 m

This is the round-trip distance (sound goes to the wall and comes back).

Therefore, minimum distance from the reflecting surface = 33/2 = 16.5 meters

Memory Trick: "Sweet Sixteen and a Half" — minimum distance for echo is 16.5 m.

Applications of Reflection of Sound

1. SONAR (Sound Navigation and Ranging)

Purpose: Detect underwater objects, measure ocean depth, locate submarines.

Working Principle:

1. Transmitter sends ultrasonic waves into water
2. Waves reflect from the ocean floor or objects
3. Receiver detects the reflected waves (echo)
4. Electronic system measures time interval

Formula for Depth Calculation:

If time interval between transmission and reception = t Speed of sound in water = v

Then depth (h) = vt/2

(Divided by 2 because sound travels down and back up)

Why Ultrasonic Waves?

• High frequency (> 20 kHz)
• Greater penetrating power
• More accurate detection

2. Echolocation in Bats

Bats use ultrasonic sounds (up to 120 kHz) to navigate and hunt:

• Emit ultrasonic waves
• Listen to reflected echoes
• Determine location and distance of objects
• Hunt prey in complete darkness

Sonic Booms

Definition

A loud explosive sound created when an object travels faster than the speed of sound (supersonic speed).

Examples of Supersonic Objects

• Fighter jets
• Bullets
• Spacecraft during re-entry

How Sonic Booms Form

1. Object moves faster than sound waves it creates
2. Sound waves pile up and form shock waves
3. Shock waves consist of:
   • High-pressure layer (compression)
   • Low-pressure layer (rarefaction)
4. Shock waves travel at the speed of sound
5. Create a burst or cracking sound when they reach our ears

Impact: Can be loud enough to break windows or damage buildings.

Enhanced Study Notes & Quick Revision

Key Definitions Summary

Important Formulas

1. v = λ/T 2. v = νλ 3. ν = 1/T 4. Echo distance: d = vt/2 5. SONAR depth: h = vt/2

Memory Tricks & Mnemonics

1. Wave Types: "TL" — Transverse needs Less Medium (can go through vacuum), Longitudinal needs Tangible Medium
2. Speed in Media: "Solids are Speedy, Gases are Gentler" (V_solid > V_liquid > V_gas)
3. Audible Range: "Twenty-Twenty vision for sound" (20 Hz to 20,000 Hz)
4. Echo Distance: "Sweet Sixteen and a Half" (16.5 m minimum)
5. Sound Characteristics: "Loud People Quit" — Loudness, Pitch, Quality
6. SONAR Formula: "Half Time Voyage" (h = vt/2 — use half the time)

Quick Revision Table

Class 8 Science Sound Chapter Solved Examples of Sound

Question: A simple pendulum completes 10 oscillations in 20 seconds. Find its time period and frequency.

Solution:

Time period (T) = Total time / Number of oscillations T = 20/10 = 2 seconds

Frequency (ν) = 1/T = 1/2 = 0.5 Hz

Answer: Time period = 2 s, Frequency = 0.5 Hz

Question: The distance between the 1st compression and the rarefaction next to the 2nd compression in a longitudinal wave is 15 m. If the frequency of the wave is 200 Hz, find the speed of the wave.

Solution:

Distance between consecutive compressions = λ (one wavelength) Distance between neighboring compression and rarefaction = λ/2

Given distance = λ + λ/2 = 3λ/2 = 15 m

Therefore: λ = (15 × 2)/3 = 10 m

Speed (v) = νλ = 200 × 10 = 2000 m/s

Answer: 2000 m/s

Question: Find the frequency and wavelength of a wave whose time period is 0.001 sec and speed is 200 m/s.

Solution:

Time period T = 0.001 s

Frequency ν = 1/T = 1/0.001 = 1000 Hz

Wavelength λ = speed/frequency = 200/1000 = 0.2 m

Answer: Frequency = 1000 Hz, Wavelength = 0.2 m

Question: A source produces 12 waves in 4 seconds. The distance between a crest and consecutive trough is 6 m. Find: (a) Frequency (b) Wavelength (c) Velocity of wave

Solution:

(a) Number of waves per second = 12/4 = 3 Therefore, frequency = 3 Hz

(b) Distance between crest and consecutive trough = λ/2 = 6 m Therefore, wavelength λ = 12 m

(c) Velocity v = νλ = 3 × 12 = 36 m/s

Answer: (a) 3 Hz, (b) 12 m, (c) 36 m/s

Question: A wave graph shows time period of 4 milliseconds and distance between trough and crest is 2 m. Find wave velocity, wavelength, and frequency.

Solution:

Time period T = 4 ms = 4 × 10⁻³ s

Frequency ν = 1/T = 1/(4 × 10⁻³) = 250 Hz

Distance between trough and crest = λ/2 = 2 m Therefore, wavelength λ = 4 m

Wave speed v = νλ = 250 × 4 = 1000 m/s

Answer: Velocity = 1000 m/s, Wavelength = 4 m, Frequency = 250 Hz

Question: A person stands between two walls and claps. He hears two successive echoes at an interval of 0.25 seconds. If the distance between the walls is d and speed of sound is v, find the position of the person.

Solution:

Let distance from wall A = x Distance from wall B = (d - x)

Time for echo from wall A: t₁ = 2x/v Time for echo from wall B: t₂ = 2(d - x)/v

Time difference: t₂ - t₁ = [2(d - x) - 2x]/v = 2(d - 2x)/v

Given: t₂ - t₁ = 0.25 = 1/4 s

Therefore: 2(d - 2x)/v = 1/4

Solving: x = d/2 - v/8

Answer: x = d/2 - v/8 (from wall A)

Question: A ship sends a SONAR signal towards the ocean bottom at depth 12 km. After how long will the reflected signal reach the ship? (Speed of sound in seawater = 1200 m/s)

Solution:

Depth h = 12 km = 12,000 m Total distance traveled = 2h = 24,000 m Speed v = 1200 m/s

Time t = Distance/Speed = 24,000/1200 = 20 seconds

Answer: 20 seconds

Question: Why must an obstacle be at least 17 m away to hear an echo clearly? Explain with calculation.

Solution:

Human ear can distinguish two sounds only if time gap ≥ 0.1 s Speed of sound in air = 330 m/s (approximately)

For echo, sound must travel to obstacle and back: Total distance = v × t = 330 × 0.1 = 33 m

Since this is round trip: Minimum distance to obstacle = 33/2 = 16.5 m ≈ 17 m

Answer: 16.5 m (approximately 17 m)

Question: Two sound waves have frequencies 256 Hz and 512 Hz traveling through air at 340 m/s. Compare their wavelengths.

Solution:

For wave 1: λ₁ = v/ν₁ = 340/256 = 1.328 m For wave 2: λ₂ = v/ν₂ = 340/512 = 0.664 m

Ratio: λ₁/λ₂ = 1.328/0.664 = 2:1

Answer: Wave 1 has double the wavelength of wave 2. Higher frequency = shorter wavelength.

Question: A wave has a time period of 20 milliseconds. Express this in seconds and find the frequency.

Solution:

Time period T = 20 ms = 20 × 10⁻³ s = 0.02 s

Frequency ν = 1/T = 1/0.02 = 50 Hz

Answer: Time period = 0.02 s, Frequency = 50 Hz

Question:

Assertion (A): Sound cannot travel through vacuum.

Reason (R): Sound requires a medium with particles to propagate.

(a) Both A and R are true, and R is the correct explanation of A

(b) Both A and R are true, but R is not the correct explanation of A

(c) A is true but R is false

(d) A is false but R is true

Solution:

Sound is a mechanical wave that propagates through the vibration of particles in a medium. In a vacuum, there are no particles to vibrate, so sound cannot travel.

Both statements are true, and R correctly explains why A is true.

Answer: (a)

Context: During a thunderstorm, you see lightning before hearing thunder.

Question: Why does this happen? Calculate the approximate distance of lightning if you hear thunder 3 seconds after seeing the flash. (Speed of sound = 330 m/s, speed of light = 3 × 10⁸ m/s)

Solution:

Light travels much faster than sound (practically instantaneous for short distances).

Distance = Speed of sound × Time Distance = 330 × 3 = 990 m ≈ 1 km

Answer: Lightning is approximately 1 km away. We see it first because light travels much faster than sound.

Question: A bat emits an ultrasonic sound of frequency 50 kHz. Can a human hear it? If the sound travels in air at 340 m/s, what is its wavelength?

Solution:

Frequency = 50 kHz = 50,000 Hz

Human hearing range: 20 Hz to 20,000 Hz Since 50,000 Hz > 20,000 Hz, humans cannot hear it.

Wavelength λ = v/ν = 340/50,000 = 0.0068 m = 6.8 mm

Answer: No, humans cannot hear it. Wavelength = 6.8 mm

Question: Why does sound travel fastest in solids and slowest in gases?

Solution:

Sound propagates through particle vibrations.

• Solids: Particles are closest together, vibrations transfer quickly
• Liquids: Particles are farther apart than solids
• Gases: Particles are farthest apart, vibrations transfer slowly

Answer: Closer particles = faster sound transmission. Solid particles are most closely packed, so sound travels fastest in solids.

Question: Distinguish between frequency and pitch with an example.

Solution:

Example: A tuning fork vibrating at 440 Hz has that frequency for everyone, but a person with hearing loss might perceive its pitch differently than someone with normal hearing.

Answer: Frequency is measurable (Hz); pitch is how we perceive that frequency.

Question: List three factors that affect the loudness of sound. Give one example for each.

Solution:

1. Amplitude: Striking a drum harder (more amplitude) produces louder sound
2. Surface area: Large drum produces louder sound than small drum (more area vibrating)
3. Distance: Sound from a speaker is louder nearby than far away

Answer: Amplitude, surface area, and distance all affect loudness.

Question: A SONAR device on a ship detects a submarine. The echo returns after 4 seconds. If the speed of sound in seawater is 1500 m/s, how deep is the submarine?

Solution:

Total distance = Speed × Time = 1500 × 4 = 6000 m This is the round trip (down and up)

Depth h = 6000/2 = 3000 m = 3 km

Answer: Submarine is at 3 km depth.

Question: Classify the following as transverse or longitudinal waves: (a) Light waves (b) Sound in air (c) Waves on water surface (d) Seismic P-waves

Solution:

(a) Light waves Transverse (electromagnetic)

(b) Sound in air Longitudinal (compression/rarefaction)

(c) Waves on water surface Transverse (up and down motion)

(d) Seismic P-waves Longitudinal (compression waves)

Answer: (a) T, (b) L, (c) T, (d) L

Question: Give three harmful effects of noise pollution and three measures to control it.

Solution:

Harmful Effects:

1. Hearing loss (temporary or permanent)
2. Sleep disturbance and insomnia
3. Increased stress and anxiety

Control Measures:

1. Plant trees as sound barriers
2. Use silencers in vehicles
3. Locate industries away from residential areas

Answer: See above

Question: Why are concert halls designed with specific wall materials and shapes?

Solution:

Concert halls use acoustic design principles:

1. Soft materials (curtains, carpets) absorb sound, reducing echoes
2. Curved walls distribute sound evenly to all areas
3. Proper spacing prevents echo overlap
4. Sound reflectors direct sound to the audience

This ensures clear music without distortion from excessive echoes or dead spots.

Answer: To control sound reflection, prevent echoes, and ensure even sound distribution.

Question: If the frequency of a sound wave is doubled while traveling through the same medium, what happens to its: (i) Wavelength (ii) Speed

(a) Both double

(b) Wavelength halves, speed remains constant

(c) Wavelength doubles, speed halves

(d) Both remain constant

Solution:

Speed v = νλ

In the same medium, speed remains constant.

If frequency doubles (ν → 2ν): v = (2ν)λ_new Since v is constant: λ_new = v/(2ν) = λ/2

Wavelength halves, speed constant.

Answer: (b)

Question: Explain the working of SONAR with a labeled diagram (textual description). Write the formula used and give one application.

Solution:

SONAR (Sound Navigation and Ranging)

Components:

1. Transmitter: Sends ultrasonic waves into water
2. Receiver: Detects reflected waves (echo)

Working:

1. Transmitter emits ultrasonic pulse downward into ocean
2. Sound waves travel through water at speed v
3. Waves hit ocean floor or object and reflect back
4. Receiver detects the echo
5. Electronic timer measures time interval t between transmission and reception

Formula: Depth h = vt/2

(Divided by 2 because sound makes a round trip)

Applications:

• Measuring ocean depth
• Detecting submarines
• Locating shipwrecks
• Finding schools of fish
• Underwater navigation

Why Ultrasonic?

• High frequency (> 20 kHz)
• Better penetration
• More accurate

Answer: See detailed explanation above.

Conclusion

Understanding sound is crucial not just for exams but for comprehending the world around us. From music to medicine (ultrasound), from ocean exploration (SONAR) to environmental protection (noise control), sound physics has countless applications.

• Sound is a longitudinal mechanical wave
• Requires medium to propagate
• Speed varies with medium: fastest in solids
• Human hearing: 20 Hz to 20 kHz
• Echo minimum distance: 16.5 m
• SONAR uses ultrasonic reflection

Learn these concepts through regular practice, solve numerical problems daily, and relate them to real-life observations. This comprehensive guide is designed for Class 8 students following NCERT curriculum. Content is aligned with latest exam patterns and includes CBSE board question types.