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Sound – CBSE NCERT Study Resources

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Sound

Study Materials

8th

8th - Science

Sound

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Overview of the Chapter

This chapter explores the concept of sound, its production, propagation, and characteristics. Students will learn how sound travels through different mediums, the factors affecting its speed, and how humans perceive sound. The chapter also covers topics like amplitude, frequency, and the distinction between noise and music.

Sound: A form of energy produced by vibrating objects that travels through a medium as longitudinal waves.

Production of Sound

Sound is produced when an object vibrates. These vibrations create pressure waves in the surrounding medium, which our ears detect as sound. Examples include plucking a guitar string or striking a drum.

Vibration: A rapid back-and-forth movement of an object about its mean position.

Propagation of Sound

Sound requires a medium (solid, liquid, or gas) to travel. It propagates as longitudinal waves, where particles of the medium oscillate parallel to the direction of wave motion.

Longitudinal Wave: A wave where particles move in the same direction as the wave's energy transfer.

Characteristics of Sound

Sound has three main characteristics:

  • Amplitude: Determines the loudness of sound.
  • Frequency: Determines the pitch (higher frequency means higher pitch).
  • Timbre: Helps distinguish between sounds of the same pitch and loudness.

Frequency: The number of vibrations per second, measured in Hertz (Hz).

Speed of Sound

The speed of sound depends on the medium and its properties. It travels fastest in solids, slower in liquids, and slowest in gases. Temperature also affects the speed of sound in air.

Human Ear and Hearing

The human ear captures sound waves and converts them into electrical signals for the brain to interpret. The audible range for humans is typically 20 Hz to 20,000 Hz.

Audible Range: The range of frequencies that the human ear can detect.

Noise vs. Music

Noise is an unpleasant, irregular sound, while music is a pleasant, organized combination of sounds. Excessive noise can lead to noise pollution, which is harmful to health.

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
What is sound?
Answer:

Sound is a form of energy produced by vibrations.

Question 2:
Name the medium through which sound travels fastest.
Answer:

Solids are the fastest medium for sound.

Question 3:
What is the frequency of sound?
Answer:

Frequency is the number of vibrations per second.

Question 4:
Which part of the human ear vibrates when sound enters?
Answer:

The eardrum vibrates when sound enters.

Question 5:
What is the range of audible sound for humans?
Answer:

20 Hz to 20,000 Hz is the audible range.

Question 6:
What is an echo?
Answer:

An echo is a reflected sound wave.

Question 7:
Name a musical instrument that produces sound by vibration of strings.
Answer:

A guitar produces sound by vibrating strings.

Question 8:
What is the pitch of sound?
Answer:

Pitch depends on the frequency of sound.

Question 9:
Which gas is present in the larynx to produce sound?
Answer:

Air in the larynx helps produce sound.

Question 10:
What is the speed of sound in air at 20°C?
Answer:

343 m/s is the speed of sound.

Question 11:
Name a device used to measure sound intensity.
Answer:

A decibel meter measures sound intensity.

Question 12:
What is noise pollution?
Answer:

Noise pollution is unwanted harmful sound.

Question 13:
Which animal uses ultrasound for navigation?
Answer:

Bats use ultrasound for navigation.

Question 14:
What is the amplitude of a sound wave?
Answer:

Amplitude is the height of a sound wave.

Question 15:
What is the range of audible sound for humans?
Answer:

The range of audible sound for humans is from 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz are called infrasonic, and those above 20,000 Hz are called ultrasonic.

Question 16:
Define amplitude of a sound wave.
Answer:

The amplitude of a sound wave is the maximum displacement of particles in the medium from their mean position when the sound passes through it. It determines the loudness of the sound.

Question 17:
What is the unit of frequency?
Answer:

The unit of frequency is Hertz (Hz), which represents the number of vibrations per second.

Question 18:
Why can't sound travel through a vacuum?
Answer:

Sound cannot travel through a vacuum because it requires a medium (solid, liquid, or gas) for propagation. In a vacuum, there are no particles to vibrate and transmit the sound wave.

Question 19:
What is the relation between time period and frequency of a sound wave?
Answer:

The time period (T) and frequency (f) of a sound wave are inversely related:
T = 1/f
or
f = 1/T

Question 20:
Name the characteristic of sound that distinguishes a shrill sound from a flat sound.
Answer:

The characteristic is pitch, which depends on the frequency of the sound wave. A higher frequency produces a shrill sound, while a lower frequency produces a flat sound.

Question 21:
Why are ceilings of concert halls curved?
Answer:

Ceilings of concert halls are curved to reflect sound waves evenly across the audience, ensuring uniform distribution of sound and preventing echoes or dead spots.

Question 22:
What is the speed of sound in air at room temperature (20°C)?
Answer:

The speed of sound in air at 20°C is approximately 343 meters per second (m/s). It increases with temperature.

Question 23:
How does the sound produced by a drum differ from that of a flute?
Answer:

A drum produces a sound with low frequency (low pitch) due to its large vibrating surface, while a flute produces a sound with high frequency (high pitch) due to the rapid vibration of air columns.

Question 24:
Why do we hear thunder after seeing lightning?
Answer:

We hear thunder after seeing lightning because light travels faster (3 × 108 m/s) than sound (343 m/s). The time difference helps estimate the distance of the lightning.

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
Define sound and state its SI unit.
Answer:

Sound is a form of energy produced by vibrating objects that travels as a longitudinal wave through a medium.
Its SI unit is the decibel (dB).

Question 2:
What is the frequency of a sound wave? How is it related to pitch?
Answer:

Frequency is the number of vibrations per second, measured in Hertz (Hz).
Higher frequency results in a higher pitch, while lower frequency produces a lower pitch.

Question 3:
Why can't sound travel in a vacuum?
Answer:

Sound requires a medium (solid, liquid, or gas) to propagate because it travels by vibrating particles.
In a vacuum, there are no particles to vibrate, so sound cannot travel.

Question 4:
Differentiate between loudness and pitch of sound.
Answer:
  • Loudness depends on the amplitude of the sound wave and is measured in decibels (dB).
  • Pitch depends on the frequency of the wave and determines if the sound is high or low.
Question 5:
What is an echo? State the minimum distance required to hear an echo clearly.
Answer:

An echo is the reflection of sound that reaches the listener after the original sound.
The minimum distance required is 17.2 meters (at 22°C) for clear perception due to the persistence of hearing (0.1 sec delay).

Question 6:
How does the speed of sound vary in solids, liquids, and gases?
Answer:

Speed is fastest in solids (due to tightly packed particles), slower in liquids, and slowest in gases (like air) because particles are farther apart.

Question 7:
Name the part of the human ear that vibrates when sound enters it.
Answer:

The eardrum (tympanic membrane) vibrates when sound waves strike it, transmitting the vibrations to the inner ear.

Question 8:
What is noise pollution? Give one harmful effect of it.
Answer:

Noise pollution is excessive or unwanted sound that disrupts the environment.
Harmful effect: Hearing loss or increased stress levels in humans.

Question 9:
Explain why thunder is heard after the lightning is seen.
Answer:

Light travels faster (3 × 10⁸ m/s) than sound (343 m/s in air), so lightning (light) is seen before thunder (sound) is heard.

Question 10:
What is the audible range of sound for humans?
Answer:

The audible range for humans is 20 Hz to 20,000 Hz. Sounds below 20 Hz are infrasonic, and above 20,000 Hz are ultrasonic.

Question 11:
How do sonar devices use sound waves?
Answer:

Sonar (Sound Navigation and Ranging) uses ultrasonic waves to detect underwater objects by measuring the time taken for echoes to return.
Used in submarines, ships, and oceanography.

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Explain how sound travels through different mediums with examples.
Answer:

Sound travels as a longitudinal wave and requires a medium (solid, liquid, or gas) to propagate.

1. In solids, sound travels fastest because particles are closely packed (e.g., hearing a train through railway tracks).
2. In liquids, it travels slower than solids but faster than gases (e.g., dolphins communicating underwater).
3. In gases, sound travels slowest due to widely spaced particles (e.g., hearing someone speak in air).

Vacuum has no medium, so sound cannot travel through it.

Question 2:
Describe the role of the eardrum in hearing.
Answer:

The eardrum (or tympanic membrane) is a thin, cone-shaped membrane in the ear.

1. It vibrates when sound waves hit it.
2. These vibrations are transferred to the ossicles (tiny bones) in the middle ear.
3. The ossicles amplify the sound and send it to the cochlea, which converts it into electrical signals for the brain.

Without the eardrum, sound waves cannot be efficiently transmitted, leading to hearing loss.

Question 3:
What is the range of audible frequencies for humans? How does it differ for other animals?
Answer:

The audible frequency range for humans is 20 Hz to 20,000 Hz.

1. Dogs can hear up to 45,000 Hz (ultrasound).
2. Bats use frequencies beyond 100,000 Hz for echolocation.
3. Elephants communicate using infrasound (below 20 Hz).

This variation is due to differences in ear structure and evolutionary needs.

Question 4:
Why does sound travel faster in summer than in winter? Explain.
Answer:

Sound travels faster in summer because:

1. Higher temperature increases the kinetic energy of air molecules, allowing them to vibrate and transmit sound faster.
2. In winter, colder air is denser, slowing down molecular movement and reducing sound speed.

For example, thunder is heard clearer on a hot day compared to a cold one.

Question 5:
How does noise pollution affect human health? Suggest two preventive measures.
Answer:

Noise pollution harms health in several ways:

1. Hearing loss due to prolonged exposure to loud sounds.
2. Sleep disturbances and increased stress levels.
3. Hypertension and heart-related issues.

Preventive measures:
1. Use earplugs in noisy areas.
2. Plant trees to absorb sound waves.

Question 6:
Explain the working of a stethoscope with a labeled diagram.
Answer:

A stethoscope amplifies body sounds (e.g., heartbeat) using:

1. Chestpiece: Captures sound vibrations.
2. Tubes: Transmits sound waves to the ears.
3. Earpieces: Directs sound into the doctor's ears.

Diagram:
[Chestpiece] → [Tubes] → [Earpieces]

The airtight design prevents sound loss, making faint noises audible.

Question 7:
What is the difference between noise and music? Provide examples.
Answer:

Noise is an unpleasant, irregular, and discordant sound that causes discomfort (e.g., traffic honking, construction noise).
Music is a pleasant, organized, and harmonious sound with a fixed pattern (e.g., piano notes, flute melody).
Key differences:

  • Noise lacks a fixed frequency, while music has a definite pitch.
  • Noise is unwanted, whereas music is intentionally created for enjoyment.
  • Prolonged exposure to noise can harm hearing, but music is therapeutic.

Question 8:
How does the frequency of sound affect its pitch? Explain with an example.
Answer:

Frequency (measured in Hertz, Hz) determines the pitch of a sound.
Higher frequency means higher pitch (e.g., a whistle or baby's cry).
Lower frequency means lower pitch (e.g., a drum or thunder).
Example: A guitar string tightened more vibrates faster, producing a higher frequency and thus a shriller sound.
Human ears can typically hear frequencies between 20 Hz to 20,000 Hz.

Question 9:
Why do we hear echoes? State the minimum distance required for an echo to be heard clearly.
Answer:

An echo is heard when sound waves reflect off a hard surface (e.g., cliffs, walls) and reach our ears after the original sound.
For a clear echo, the minimum distance between the sound source and the reflecting surface must be 17.2 meters.
This is because the human ear can distinguish sounds only if they are at least 0.1 seconds apart.
Calculation:
Speed of sound in air = 344 m/s
Distance = (Speed × Time)/2 = (344 × 0.1)/2 = 17.2 m.

Question 10:
Explain how a stethoscope helps doctors hear a patient's heartbeat clearly.
Answer:

A stethoscope amplifies faint body sounds (e.g., heartbeat) using three parts:

  • Chestpiece: Captures sound vibrations from the body.
  • Tubes: Transmits these vibrations without loss.
  • Earpieces: Directs sound waves into the doctor's ears.

The stethoscope blocks external noise and focuses on internal sounds, making it easier to detect irregularities.
It works on the principle of multiple reflections of sound waves within the tubes.

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Explain how sound waves propagate through air and why they cannot travel in a vacuum.
Answer:
Concept Overview

Sound is a longitudinal wave that requires a medium like air, water, or solids to travel. It propagates as compressions and rarefactions.

Process Explanation

When an object vibrates, it pushes air molecules, creating high-pressure (compression) and low-pressure (rarefaction) regions. These alternating regions form sound waves.

Real-world Application

Our textbook shows that astronauts use radios in space because sound cannot travel in a vacuum. Similarly, we hear thunder after lightning due to air vibrations.

Question 2:
Describe how the human ear detects sound and processes it for the brain.
Answer:
Concept Overview

The ear has three parts: outer, middle, and inner. Sound waves are converted into electrical signals for the brain.

Process Explanation
  • Outer ear collects sound.
  • Middle ear amplifies vibrations via ossicles.
  • Inner ear's cochlea converts vibrations into nerve signals.
Real-world Application

Our textbook explains hearing loss due to damaged eardrums. Loud noises can harm hair cells in the cochlea, affecting hearing permanently.

Question 3:
Compare loudness and pitch of sound with examples.
Answer:
Concept Overview

Loudness depends on amplitude, while pitch depends on frequency.

Process Explanation
  • High amplitude = louder sound (e.g., shouting).
  • High frequency = higher pitch (e.g., bird chirping).
Real-world Application

Our textbook shows a tabla (low pitch) and whistle (high pitch). In concerts, amplifiers increase loudness without altering pitch.

Question 4:
Explain how echo is produced and list two conditions required for hearing it clearly.
Answer:
Concept Overview

An echo is a reflected sound wave heard after the original sound.

Process Explanation

When sound hits a hard surface like a cliff, it bounces back. The time delay must be ≥0.1s for distinct hearing.

Real-world Application

Our textbook mentions echoes in mountains. Auditoriums avoid echoes by using sound-absorbing materials.

Question 5:
Describe an experiment to show that sound needs a medium to travel, using a bell jar setup.
Answer:
Concept Overview

Sound cannot travel in a vacuum, as demonstrated by a bell jar experiment.

Process Explanation
  1. Place a ringing bell inside a sealed jar.
  2. Pump out air; sound fades as vacuum increases.
Diagram
[Diagram: Bell jar with vacuum pump]
Real-world Application

Our textbook links this to space, where no medium exists for sound propagation.

Question 6:
Explain how sound waves propagate through different mediums with examples.
Answer:
Concept Overview

Sound waves are longitudinal waves requiring a medium (solid, liquid, or gas) to travel. They propagate by compressions and rarefactions.

Process Explanation
  • In air, particles vibrate parallel to wave direction.
  • Solids transmit sound fastest due to tightly packed molecules.
Real-world Application

Our textbook shows whales communicating underwater (liquid medium). Similarly, we hear footsteps clearer through walls (solid) than air.

[Diagram: Compression-rarefaction wave in air]
Question 7:
Describe the human ear structure and its role in hearing.
Answer:
Concept Overview

The ear has three parts: outer, middle, and inner ear, converting sound to electrical signals.

Process Explanation
  • Outer ear collects sound waves.
  • Eardrum vibrates, ossicles amplify it in middle ear.
  • Cochlea (inner ear) sends signals to brain.
Real-world Application

NCERT mentions loud sounds damaging eardrums. Earplugs protect workers in noisy factories.

[Diagram: Labeled ear structure]
Question 8:
How does amplitude affect sound? Compare two sounds with different amplitudes.
Answer:
Concept Overview

Amplitude determines loudness. Higher amplitude means louder sound.

Process Explanation
  • Measured in decibels (dB).
  • Example: Whisper (low amplitude) vs. shout (high amplitude).
Real-world Application

NCERT shows ambulance sirens have high amplitude for urgency. We use volume knobs to adjust amplitude in speakers.

[Diagram: Waveforms comparing high/low amplitude]
Question 9:
Explain echo formation and conditions required for hearing it.
Answer:
Concept Overview

Echo is reflected sound heard after original sound. It occurs due to sound bouncing off hard surfaces.

Process Explanation
  • Minimum 17m distance needed (as per NCERT).
  • Soft materials absorb sound, preventing echo.
Real-world Application

We experience echoes in mountains or empty halls. Architects design theaters to reduce echoes.

[Diagram: Sound reflection path]
Question 10:
What is noise pollution? Suggest two measures to reduce it in cities.
Answer:
Concept Overview

Noise pollution is harmful or unwanted sound exceeding safe levels (above 80dB).

Process Explanation
  • Causes include traffic, industries, loudspeakers.
  • NCERT states it causes hearing loss and stress.
Real-world Application

Planting trees near roads absorbs sound. Using silencers in vehicles reduces noise.

[Diagram: Decibel scale with examples]
Question 11:
Explain how sound is produced and travels through different mediums. Support your answer with examples and a labeled diagram.
Answer:

Sound is produced due to vibrations of an object. When an object vibrates, it causes the air particles around it to move back and forth, creating a longitudinal wave that travels through a medium.

Production of Sound:
For example, when a guitar string is plucked, it vibrates and produces sound. Similarly, our vocal cords vibrate when we speak, producing sound waves.

Travel of Sound:
Sound requires a medium (solid, liquid, or gas) to travel. It cannot travel in a vacuum.

  • Solids: Sound travels fastest in solids because particles are closely packed (e.g., hearing a train through railway tracks).
  • Liquids: Slower than solids but faster than gases (e.g., dolphins communicating underwater).
  • Gases: Slowest due to widely spaced particles (e.g., hearing someone speak through air).

Diagram:
(Draw a labeled diagram showing a vibrating tuning fork creating compressions and rarefactions in air.)

Question 12:
Describe the structure and function of the human ear in detail, explaining how it helps us hear sounds.
Answer:

The human ear is divided into three main parts: the outer ear, middle ear, and inner ear.

1. Outer Ear:
Includes the pinna (collects sound waves) and the ear canal (directs sound to the eardrum).

2. Middle Ear:
Contains the eardrum (vibrates when sound hits it) and three tiny bones (hammer, anvil, stirrup) that amplify vibrations.

3. Inner Ear:
Consists of the cochlea (a fluid-filled, spiral-shaped structure with hair cells that convert vibrations into electrical signals) and the auditory nerve (sends signals to the brain).

Process of Hearing:
Sound waves enter the ear canal → Eardrum vibrates → Bones amplify vibrations → Cochlea converts vibrations into signals → Brain interprets signals as sound.

Additional Info:
The ear also helps maintain balance due to the semicircular canals in the inner ear.

Question 13:
Explain how sound travels through different mediums (solids, liquids, and gases) with examples. Also, discuss why sound cannot travel in a vacuum.
Answer:

Sound is a form of mechanical wave that requires a medium (solid, liquid, or gas) to travel. Here’s how it propagates:

  • Solids: Sound travels fastest in solids because the particles are closely packed, allowing vibrations to transfer quickly. Example: When you tap a table, the sound reaches your ears through the solid wood.
  • Liquids: In liquids, particles are less tightly packed than in solids, so sound travels slower but still effectively. Example: Whales communicate underwater using sound waves.
  • Gases: Sound travels slowest in gases due to widely spaced particles. Example: We hear voices because sound waves travel through air (a gas).

Sound cannot travel in a vacuum (like outer space) because there are no particles to vibrate and transmit the wave. This is why astronauts use radios to communicate in space.

Question 14:
Describe the structure and function of the human ear in detecting sound. Include a labeled diagram of the ear and explain how each part contributes to hearing.
Answer:

The human ear is divided into three main parts: the outer ear, middle ear, and inner ear, each playing a crucial role in hearing.

  • Outer Ear: Consists of the pinna (visible part) and the ear canal. The pinna collects sound waves and directs them into the ear canal, where they travel to the eardrum.
  • Middle Ear: Contains the eardrum and three tiny bones (ossicles: malleus, incus, and stapes). The eardrum vibrates when sound hits it, and the ossicles amplify these vibrations, transmitting them to the inner ear.
  • Inner Ear: Includes the cochlea, a spiral-shaped organ filled with fluid and hair cells. Vibrations from the middle ear create waves in the cochlear fluid, stimulating hair cells that convert sound into electrical signals sent to the brain via the auditory nerve.

Here’s a simple labeled diagram of the ear:

[Diagram: Outer Ear (Pinna, Ear Canal) → Middle Ear (Eardrum, Ossicles) → Inner Ear (Cochlea, Auditory Nerve)]

This process ensures we can hear and interpret sounds accurately.

Question 15:
Describe the structure and function of the human ear in detecting sound. Explain how damage to any part can affect hearing.
Answer:

The human ear has three main parts for detecting sound:

  • Outer Ear: Includes the pinna and ear canal, which collect and direct sound waves to the eardrum.
  • Middle Ear: Contains the eardrum and three tiny bones (ossicles) that amplify vibrations and send them to the inner ear.
  • Inner Ear: The cochlea (a spiral-shaped organ) converts vibrations into electrical signals sent to the brain via the auditory nerve.

Damage to any part can impair hearing:
1. A ruptured eardrum reduces sound transmission.
2. Damaged ossicles weaken vibrations.
3. Cochlea or nerve damage may cause permanent hearing loss.

Protecting ears from loud noises and infections is crucial for maintaining healthy hearing.

Question 16:
Explain how sound travels through different mediums (solid, liquid, gas) with examples. Also, discuss why sound cannot travel in a vacuum.
Answer:

Sound is a form of mechanical wave that requires a medium (solid, liquid, or gas) to travel. The speed and efficiency of sound depend on the medium's properties:

  • Solids: Sound travels fastest in solids because the particles are closely packed, allowing vibrations to transfer quickly.
    Example: When you tap a table, the sound travels through the wood to your ears.
  • Liquids: Sound travels slower in liquids than in solids but faster than in gases because particles are less tightly packed.
    Example: Whales communicate underwater using sound waves.
  • Gases: Sound travels slowest in gases due to widely spaced particles.
    Example: We hear voices through the air (a gas).

Sound cannot travel in a vacuum because there are no particles to vibrate and transmit the wave. This is why space is silent despite explosions or other events.

Question 17:
Describe the structure and function of the human ear in detecting sound. Include a labeled diagram (imaginary) of the ear's main parts and explain how each part contributes to hearing.
Answer:

The human ear has three main parts that work together to detect sound:

  • Outer Ear: Consists of the pinna (collects sound waves) and the ear canal (channels sound to the eardrum).
  • Middle Ear: Contains the eardrum (vibrates when sound hits it) and three tiny bones (hammer, anvil, stirrup) that amplify vibrations.
  • Inner Ear: Includes the cochlea (filled with fluid and hair cells that convert vibrations into electrical signals) and the auditory nerve (sends signals to the brain).

Here’s how hearing works:
1. Sound waves enter the ear canal and hit the eardrum, making it vibrate.
2. The vibrations pass through the three bones, which amplify them.
3. The cochlea converts these vibrations into electrical signals.
4. The auditory nerve carries these signals to the brain, which interprets them as sound.

(Diagram description: A simple labeled sketch would show the outer, middle, and inner ear with arrows indicating the path of sound waves.)

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
A group of students observed that sound travels faster in solids than in air. They tested this by tapping a table and listening through the air and then placing their ears on the table.
Case Summary: Explain why sound travels faster in solids using particle vibration and medium density.
Answer:
Scientific Principle:

Sound travels as vibrations through particles. In solids, particles are closely packed, allowing faster energy transfer compared to gases like air.

Solution Approach:
  • Our textbook shows that solids have higher density, enabling quicker vibrations.
  • Real-world example: Railway tracks transmit sound faster than air, helping detect train arrivals early.
Question 2:
Riya noticed that her voice echoed in an empty hall but not in a furnished room.
Case Summary: Relate this to reflection of sound and absorption with NCERT examples.
Answer:
Scientific Principle:

Echoes occur when sound reflects off hard surfaces. Soft materials absorb sound, reducing echoes.

Solution Approach:
  • NCERT example: Auditoriums use curtains to minimize echoes by absorption.
  • Real-world application: Carpets in rooms reduce noise by absorbing sound waves.
Question 3:
An experiment measured frequency and pitch using a tuning fork (320 Hz) and a drum.
Case Summary: Tabulate observations and explain the link between frequency and pitch.
Answer:
Scientific Principle:

Higher frequency means higher pitch. Tuning forks produce pure tones, while drums create mixed frequencies.

Solution Approach:
ObjectFrequency (Hz)Pitch
Tuning Fork320High
DrumMixedLow

NCERT states pitch depends on frequency, as seen in musical instruments.

Question 4:
A submarine uses sonar to detect objects underwater.
Case Summary: Describe how ultrasonic waves and echo-ranging help in navigation, citing NCERT.
Answer:
Scientific Principle:

Sonar emits ultrasonic waves that reflect off objects, calculating distance using echo time.

Solution Approach:
  • NCERT example: Bats use echolocation similarly to navigate.
  • Real-world use: Submarines measure ocean depth by analyzing reflected sound waves.
Question 5:
A group of students observed that sound travels faster in solids than in air. They tested this by tapping a table and listening through it.
Case Summary: Explain why sound travels faster in solids using particle vibration and medium density.
Answer:
Scientific Principle:

Sound travels as vibrations through particles. In solids, particles are closely packed, allowing faster energy transfer.

Solution Approach:
  • Our textbook shows metals conduct sound quicker due to dense particle arrangement.
  • Real-world example: Railway tracks transmit sound faster than air, helping detect trains early.
Question 6:
Riya noticed her voice echoes in an empty hall but not in a furnished room.
Case Summary: Relate this to reflection of sound and absorption with NCERT examples.
Answer:
Scientific Principle:

Echoes occur when sound reflects off hard surfaces. Soft materials absorb sound, reducing echoes.

Solution Approach:
  • NCERT mentions auditoriums use curtains to minimize echoes.
  • Real-world: Carpets in rooms absorb sound, preventing reverberation.
Question 7:
A tuning fork vibrates at 512 Hz but humans hear only above 20 Hz.
Case Summary: Define frequency and audible range using this example.
Answer:
Scientific Principle:

Frequency is vibrations per second. Humans hear sounds between 20 Hz–20,000 Hz.

Solution Approach:
  • NCERT states dog whistles have frequencies beyond human range.
  • Real-world: Bats use ultrasonic waves (>20,000 Hz) for navigation.
Question 8:
In an experiment, a bell jar’s sound faded when air was pumped out.
Case Summary: Analyze the role of medium in sound propagation with NCERT reference.
Answer:
Scientific Principle:

Sound needs a medium (solid, liquid, gas) to travel. Vacuum lacks particles for vibration.

Solution Approach:
  • NCERT demonstrates this with a bell jar experiment.
  • Real-world: Space is silent due to vacuum absence.
Question 9:
A group of students observed that sound travels faster in solids than in air. They tested this by tapping a table and listening through it versus air.
Case Summary: Students compared sound speed in solids and air.
Q1: Explain why sound travels faster in solids using particle arrangement.
Answer:
Scientific Principle:

Sound travels as vibrations through particles. In solids, particles are closely packed, allowing faster energy transfer. In gases like air, particles are far apart, slowing sound.


Solution Approach:
  • Our textbook shows iron transmits sound faster than air due to dense particles.
  • Real-world example: Doctors use stethoscopes (solid tubing) to hear heartbeats clearly.
Question 10:
Rahul noticed echoes in an empty hall but not in a furnished room.
Case Summary: Echoes depend on room conditions.
Q2: Identify the conditions required for an echo and how furnishings affect it.
Answer:
Scientific Principle:

Echoes occur when sound reflects off hard surfaces. The minimum distance for an echo is 17.2m (NCERT example).


Solution Approach:
  • Furnishings absorb sound, reducing reflections.
  • Real-world application: Concert halls use curtains to control echoes.
Question 11:
A tuning fork vibrating at 512 Hz produces sound waves.
Case Summary: Tuning fork demonstrates wave properties.
Q3: Relate frequency to pitch and mention how humans perceive it.
Answer:
Scientific Principle:

Frequency is vibrations per second (Hz). Higher frequency means higher pitch, as per NCERT.


Solution Approach:
  • Humans hear 20Hz-20,000Hz. 512Hz is a common tuning fork frequency.
  • Application: Musicians use tuning forks to calibrate instruments.
Question 12:
An experiment measured sound intensity at different distances from a speaker. Results:
Distance (m)Intensity (dB)
180
274

Q4: Analyze the data and state the relationship between distance and loudness.
Answer:
Scientific Principle:

Loudness decreases with distance as sound energy spreads out (inverse square law in NCERT).


Solution Approach:
  • Table shows 6dB drop when distance doubles.
  • Application: Public speakers place amplifiers evenly for uniform sound.
Question 13:

Rahul observed that when he struck a tuning fork and placed it near a table tennis ball suspended by a thread, the ball started moving. He repeated the experiment with a tuning fork of higher frequency and noticed the ball moved faster.

Based on this observation:

  • Why did the ball move when the tuning fork was struck?
  • How does the frequency of the tuning fork affect the movement of the ball?
Answer:

1. Why the ball moved: When the tuning fork was struck, it vibrated and produced sound waves. These waves caused the air particles near the fork to vibrate, creating compressions and rarefactions. The vibrating air particles exerted force on the table tennis ball, making it move.


2. Effect of frequency: A higher frequency tuning fork vibrates faster, producing sound waves with more energy and higher pitch. This increases the speed of air particle vibrations, resulting in stronger pushes on the ball, making it move faster.

Question 14:

Priya conducted an experiment where she clapped her hands near two different walls—one made of wood and the other of concrete. She noticed that the echo from the concrete wall was clearer than from the wooden wall.

Explain:

  • What causes an echo?
  • Why was the echo clearer from the concrete wall compared to the wooden wall?
Answer:

1. Cause of echo: An echo is produced when sound waves reflect off a hard surface and return to the listener after a short delay. For an echo to be heard, the reflecting surface must be at least 17 meters away (as sound travels ~340 m/s, and the human ear needs a 0.1-second delay to distinguish the echo).


2. Difference in materials: Concrete is a hard and dense material that reflects sound waves effectively with minimal absorption. Wood, being softer and less dense, absorbs more sound energy, reducing the intensity of the reflected sound and making the echo less clear.

Question 15:
Rahul observed that when he plucked a guitar string gently, it produced a soft sound, but when he plucked it harder, the sound was louder. Based on this observation, answer the following:
(a) What property of sound determines its loudness?
(b) How does the amplitude of vibration relate to the loudness of sound?
Answer:

(a) The property of sound that determines its loudness is the amplitude of the vibrating object. Amplitude refers to the maximum displacement of particles from their mean position during vibration.


(b) The loudness of sound is directly proportional to the amplitude of vibration. When Rahul plucks the string harder, the amplitude increases, causing the sound to be louder. Conversely, a gentle pluck results in smaller amplitude and softer sound. Loudness is measured in decibels (dB), and higher amplitude corresponds to higher energy in the sound wave.

Question 16:
Priya noticed that her voice echoed when she shouted inside an empty hall but not in her living room. Explain:
(a) Why does an echo occur in the hall but not in the living room?
(b) What is the minimum distance required between the source of sound and the reflecting surface for an echo to be heard?
Answer:

(a) An echo occurs when sound waves reflect off a hard surface (like walls in an empty hall) and reach the listener after a short delay. In Priya's living room, the distance is too small, and sound gets absorbed by furniture, carpets, or other soft materials, preventing a clear reflection.


(b) The minimum distance required between the source and the reflecting surface for an echo to be heard is 17.2 meters. This is because the human ear can distinguish two sounds only if they are at least 0.1 seconds apart. Since sound travels at 344 m/s in air, the total distance covered must be at least 34.4 m (to and fro), making the minimum distance 17.2 m.

Question 17:
Rahul observed that when he struck a tuning fork and placed it near a table tennis ball suspended by a thread, the ball started moving. He repeated the experiment with a louder sound and noticed the ball moved farther. Explain the phenomenon involved and how the loudness of sound affects it.
Answer:

The phenomenon observed by Rahul is due to the vibration caused by sound waves. When the tuning fork is struck, it vibrates and produces sound waves that travel through the air. These waves hit the table tennis ball, transferring energy and causing it to move.


The loudness of sound is determined by its amplitude. A louder sound has a higher amplitude, which means the sound waves carry more energy. This increased energy causes the ball to move farther because the force exerted by the waves is greater.


Key points to remember:

  • Sound is produced by vibrating objects.
  • Loudness depends on the amplitude of the sound wave.
  • Higher amplitude means more energy transfer, resulting in greater movement of the ball.

Question 18:
Priya noticed that her voice echoed when she shouted inside an empty hall but not when the hall was crowded. She wondered why this happens. Explain the scientific reason behind her observation and the conditions necessary for an echo to occur.
Answer:

Priya observed the phenomenon of echo, which is the reflection of sound waves. An echo occurs when sound waves bounce off a hard surface and return to the listener after a short delay.


In an empty hall, the sound waves reflect off the walls, ceiling, and floor without much obstruction, creating a clear echo. However, in a crowded hall, the people and furniture absorb most of the sound waves, reducing the reflection and preventing an echo.


Conditions necessary for an echo:

  • The distance between the sound source and the reflecting surface must be at least 17 meters (for the human ear to distinguish the echo).
  • The reflecting surface should be large and hard, like a wall or mountain.
  • The medium (air) should be uniform to allow clear reflection.


This explains why echoes are heard in empty spaces but not in crowded areas.

Question 19:
Rahul observed that when he plucked a guitar string gently, it produced a soft sound, but when he plucked it harder, the sound was louder. Based on this observation, answer the following:
(a) What property of sound determines its loudness?
(b) How does the amplitude of vibration affect the sound produced?
Answer:

(a) The loudness of a sound is determined by its amplitude. Amplitude refers to the maximum displacement of vibrating particles from their mean position. Higher amplitude results in louder sound.

(b) The amplitude of vibration directly affects the sound produced in the following ways:

  • Greater amplitude means the vibrating object displaces more air particles, creating stronger sound waves.
  • This leads to higher energy transfer, making the sound louder.
  • Conversely, smaller amplitude produces softer sounds as less energy is transmitted.

For example, plucking a guitar string harder increases its vibration amplitude, producing a louder sound.

Question 20:
Priya noticed that her friend's voice sounded different when speaking through a long cardboard tube. She wondered why this happened. Explain the phenomenon involved and how the tube affects the sound.
Answer:

The phenomenon observed is due to the reflection of sound waves. Here's how the tube affects the sound:

  • The cardboard tube acts as a medium that reflects sound waves multiple times before they reach the listener's ear.
  • This repeated reflection can amplify certain frequencies, making the voice sound deeper or hollow.
  • The tube also prevents sound waves from spreading out, maintaining their intensity over distance.

Additionally, the length and shape of the tube can create resonance, enhancing specific pitches. This is similar to how musical instruments like flutes work, where air columns amplify sound at particular frequencies.

Question 21:
Rahul observed that when he plucked a guitar string softly, the sound produced was faint, but when he plucked it harder, the sound was louder. Based on this observation, answer the following:
(a) What property of sound determines its loudness?
(b) How does the amplitude of vibration affect the sound produced?
Answer:

(a) The property of sound that determines its loudness is the amplitude of the vibrating object. When Rahul plucked the string harder, the amplitude increased, resulting in a louder sound.

(b) The amplitude of vibration directly affects the sound produced in the following ways:

  • Higher amplitude means the air particles vibrate more intensely, creating a louder sound.
  • Lower amplitude results in softer sound as the vibrations are weaker.

This is because loudness is proportional to the square of the amplitude of vibration.

Question 22:
Priya noticed that her friend's voice sounded different when speaking through a long pipe compared to speaking normally. Answer the following based on this scenario:
(a) Why does the voice sound different when speaking through a pipe?
(b) Name the phenomenon responsible for this change and explain how it occurs.
Answer:

(a) The voice sounds different when speaking through a pipe because the sound waves get reflected multiple times inside the pipe, altering their frequency and timbre.

(b) The phenomenon responsible is resonance. Here's how it occurs:

  • The pipe acts as a closed or open air column, depending on its length.
  • Sound waves reflect back and forth, reinforcing certain frequencies.
  • This amplification of specific frequencies changes the quality (timbre) of the voice.

This is why musical instruments like flutes produce distinct sounds based on their length and shape.

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