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Sound – CBSE NCERT Study Resources

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Sound

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9th

9th - Science

Sound

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Overview of the Chapter: Sound

This chapter introduces students to the fundamental concepts of sound, its production, propagation, and characteristics. It covers topics such as the nature of sound waves, their properties, and how humans perceive sound. The chapter also explains the importance of sound in daily life and its various applications.

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 disturbances in the surrounding medium, which propagate as sound waves. For example, when a guitar string is plucked, it vibrates and produces sound.

Propagation of Sound

Sound requires a medium (solid, liquid, or gas) to travel. It cannot propagate in a vacuum. The particles of the medium vibrate back and forth in the same direction as the wave, forming longitudinal waves.

Longitudinal Wave: A wave in which the particles of the medium vibrate parallel to the direction of wave propagation.

Characteristics of Sound Waves

  • Amplitude: The maximum displacement of a vibrating particle from its mean position. It determines the loudness of sound.
  • Frequency: The number of vibrations per second. It determines the pitch of sound.
  • Time Period: The time taken to complete one vibration.
  • Wavelength: The distance between two consecutive compressions or rarefactions.

Speed of Sound

The speed of sound depends on the properties of the medium, such as density and temperature. It travels fastest in solids, slower in liquids, and slowest in gases.

Reflection of Sound

Sound waves can reflect off surfaces, similar to light. Echoes are produced due to the reflection of sound. The persistence of sound due to repeated reflections is called reverberation.

Echo: A repetition of sound caused by the reflection of sound waves from a surface.

Human Ear and Hearing

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

Applications of Sound

  • Ultrasound: Sound waves with frequencies above 20,000 Hz, used in medical imaging and cleaning.
  • Sonar: A technique that uses sound waves to detect underwater objects.

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 the speed of sound in air at 20°C?
Answer:

343 m/s

Question 2:
Name the part of the ear that vibrates when sound enters.
Answer:

Eardrum

Question 3:
What is the range of human hearing?
Answer:

20 Hz to 20,000 Hz

Question 4:
Which property of sound determines its loudness?
Answer:

Amplitude

Question 5:
What is the unit of frequency?
Answer:

Hertz (Hz)

Question 6:
Name the device used to measure sound intensity.
Answer:

Decibel meter

Question 7:
What is echo?
Answer:

Reflection of sound heard distinctly.

Question 8:
Which gas increases the speed of sound compared to air?
Answer:

Hydrogen

Question 9:
What is the pitch of sound determined by?
Answer:

Frequency

Question 10:
Name the wave type of sound.
Answer:

Longitudinal wave

Question 11:
What is the use of ultrasound in medicine?
Answer:

Imaging internal organs.

Question 12:
Which animal uses ultrasonic waves for navigation?
Answer:

Bat

Question 13:
Define sound.
Answer:

Sound is a form of energy produced by vibrations that travel through a medium (like air, water, or solids) as longitudinal waves and can be heard by the ear.

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

Frequency is the number of vibrations per second, measured in Hertz (Hz). It determines the pitch of the sound.

Question 15:
Name the audible range of sound for humans.
Answer:

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

Question 16:
What is the speed of sound in air at room temperature?
Answer:

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

Question 17:
How does sound travel in different media?
Answer:

Sound travels fastest in solids (due to closely packed particles), slower in liquids, and slowest in gases (like air).

Question 18:
What is an echo?
Answer:

An echo is the reflection of sound that reaches the listener after a delay, caused by obstacles like walls or mountains.

Question 19:
State the principle behind SONAR.
Answer:

SONAR (Sound Navigation and Ranging) uses ultrasonic waves to detect underwater objects by measuring the time taken for echoes to return.

Question 20:
What is the pitch of a sound?
Answer:

Pitch is how high or low a sound appears, determined by its frequency. Higher frequency means higher pitch.

Question 21:
Why can't sound travel in a vacuum?
Answer:

Sound needs a medium (like air or water) to travel because it relies on particle vibrations. A vacuum has no particles to transmit sound.

Question 22:
What is the amplitude of a sound wave?
Answer:

Amplitude is the maximum displacement of particles in a medium from their rest position. It determines the loudness of sound.

Question 23:
How does a stethoscope work?
Answer:

A stethoscope amplifies body sounds (like heartbeat) by capturing vibrations through a chest piece and transmitting them via tubes to the ears.

Question 24:
What is noise pollution?
Answer:

Noise pollution is excessive or harmful sound levels (above 85 dB) that cause discomfort, hearing loss, or environmental harm.

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.
The SI unit of sound is the decibel (dB).

Question 2:
What is the frequency of a sound wave? How is it measured?
Answer:

Frequency is the number of vibrations per second.
It is measured in hertz (Hz) using a device called a frequency counter or calculated from the wave's time period.

Question 3:
Explain why sound cannot travel through a vacuum.
Answer:

Sound requires a medium (solid, liquid, or gas) to propagate because it travels as a mechanical wave.
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 sound 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 arrives at the listener after a delay.
The minimum distance required is 17.2 meters (for a 0.1-second delay at 20°C).

Question 6:
How does the speed of sound change in different media? Arrange solids, liquids, and gases in increasing order of sound speed.
Answer:

The speed of sound is highest in solids, followed by liquids, and slowest in gases due to particle density.
Order: Gases < Liquids < Solids.

Question 7:
Define ultrasound and give one practical application.
Answer:

Ultrasound refers to sound waves with frequencies above 20,000 Hz (inaudible to humans).
Application: Medical imaging (sonography) to examine internal organs.

Question 8:
What is the time period of a sound wave? How is it related to frequency?
Answer:

The time period is the time taken for one complete vibration.
It is inversely proportional to frequency: T = 1/f (where T is time period, f is frequency).

Question 9:
Why do thunder and lightning not occur simultaneously as observed from the ground?
Answer:

Light travels faster (3 × 108 m/s) than sound (343 m/s in air).
Thus, lightning is seen before thunder is heard, even though both occur at the same time.

Question 10:
What is the range of audible frequencies for humans? Name an animal that can hear beyond this range.
Answer:

The human audible range is 20 Hz to 20,000 Hz.
Example: Dogs can hear ultrasound (up to 45,000 Hz).

Question 11:
Describe how the human ear detects sound waves.
Answer:

  • Sound waves enter the ear canal and vibrate the eardrum.
  • Tiny bones (ossicles) amplify vibrations.
  • The cochlea converts vibrations into electrical signals sent to the brain.

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:
Define sound and explain how it is produced.
Answer:

Sound is a form of energy that produces the sensation of hearing in our ears. It is produced by the vibration of objects. When an object vibrates, it causes the air particles around it to vibrate, creating a series of compressions and rarefactions that travel as a longitudinal wave.

For example, when a guitar string is plucked, it vibrates and produces sound waves that travel through the air to our ears.

Question 2:
What is the sonar technique? Explain its working principle.
Answer:

Sonar (Sound Navigation and Ranging) is a technique used to detect objects underwater using sound waves. It works on the principle of echo:


1. A sound wave is transmitted into the water.
2. The wave reflects off underwater objects and returns as an echo.
3. The time taken for the echo to return is measured.
4. The distance to the object is calculated using the formula: Distance = (Speed of sound in water × Time) / 2.

Sonar is widely used in submarines, ships, and underwater exploration.

Question 3:
Explain why sound waves are called mechanical waves.
Answer:

Sound waves are called mechanical waves because they require a material medium (like air, water, or solids) to travel. Unlike electromagnetic waves (e.g., light), sound cannot propagate through a vacuum. The particles of the medium vibrate to transfer energy, making it a mechanical disturbance.

For example, when a bell rings, air molecules vibrate and transfer energy to neighboring molecules, creating a wave.

Question 4:
How does the frequency of a sound wave relate to its pitch?
Answer:

The frequency of a sound wave determines its pitch. Higher frequency means higher pitch (e.g., a whistle), while lower frequency means lower pitch (e.g., a drum).

Human hearing range is 20 Hz to 20,000 Hz. Sounds below 20 Hz are infrasonic, and above 20,000 Hz are ultrasonic.

Question 5:
Describe how echo is produced and state one condition necessary for hearing it clearly.
Answer:

An echo is produced when sound reflects off a hard surface (like a cliff or wall) and reaches the listener after a delay.

Condition for clear echo: The reflecting surface must be at least 17 meters away (for normal human speech) so that the reflected sound arrives after 0.1 second, allowing the ear to distinguish it from the original sound.

Question 6:
Why are sound waves classified as longitudinal waves?
Answer:

Sound waves are longitudinal waves because the particles of the medium vibrate parallel to the direction of wave propagation. This creates regions of compression (high pressure) and rarefaction (low pressure).

For example, when a tuning fork vibrates, it pushes air molecules together and apart in the same direction as the wave travels.

Question 7:
What is the audible range of sound for humans? How does it differ from that of a dog?
Answer:

The audible range for humans is 20 Hz to 20,000 Hz. Dogs can hear up to 45,000 Hz, making them sensitive to ultrasonic sounds. This is why dog whistles (high-frequency) are inaudible to humans.

Question 8:
Explain why sound travels faster in solids than in gases.
Answer:

Sound travels faster in solids because the particles are closely packed, allowing quicker energy transfer via vibrations. In gases, particles are far apart, leading to slower propagation.

Example: Speed of sound in steel (~5000 m/s) is much higher than in air (~343 m/s).

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 a medium. Why can't sound travel in a vacuum?
Answer:
Concept Overview

Sound waves are longitudinal waves that require a medium (solid, liquid, or gas) to travel. They propagate by compressing and rarefying particles in the medium.

Process Explanation

When an object vibrates, it creates pressure variations in the medium. These variations travel as waves, transferring energy. Our textbook shows a tuning fork vibrating in air, creating compressions and rarefactions.

Real-world Application

In space, sound cannot travel because it's a vacuum. This is why astronauts use radio waves to communicate.

Question 2:
Describe the human ear and how it detects sound. Include the role of the eardrum.
Answer:
Concept Overview

The human ear has three parts: outer, middle, and inner ear. It converts sound waves into electrical signals for the brain.

Process Explanation

Sound enters the ear canal and vibrates the eardrum. These vibrations pass through ossicles to the cochlea, where hair cells convert them into signals. Our textbook shows how loud sounds can damage these hair cells.

Real-world Application

Earplugs protect the eardrum from loud noises at concerts, preventing hearing loss.

Question 3:
What determines the pitch and loudness of sound? How are they different?
Answer:
Concept Overview

Pitch depends on frequency (high/low), while loudness depends on amplitude (wave height). Both are characteristics of sound waves.

Process Explanation

A girl's voice has higher pitch (frequency) than a man's. Loudness increases with amplitude, like when we shout. Our textbook shows graphs comparing different sound waves.

Real-world Application

Guitar strings produce different pitches when tightened, while striking harder makes them louder.

Question 4:
Explain echo with an example. What conditions are needed to hear an echo clearly?
Answer:
Concept Overview

Echo is the reflection of sound that arrives after the original sound. It occurs when sound bounces off hard surfaces.

Process Explanation

For clear echo, the reflecting surface must be at least 17m away (as per our textbook). This ensures 0.1s delay between original sound and echo.

Real-world Application

In mountains, we hear echoes because sound reflects off distant cliffs. Architects design concert halls to prevent unwanted echoes.

Question 5:
Compare ultrasonic and infrasonic sounds. Give one use of each in daily life.
Answer:
Concept Overview

Ultrasonic sounds (>20,000Hz) are above human hearing range, while infrasonic (<20Hz) are below it.

Process Explanation

Bats use ultrasonic waves for navigation (echolocation). Elephants communicate using infrasonic sounds that travel long distances, as shown in our textbook.

Real-world Application

Doctors use ultrasound for medical imaging. Scientists monitor infrasound to predict natural disasters like earthquakes.

Question 6:
Explain how sound waves propagate through a medium. Include an example from NCERT and a real-world application.
Answer:
Concept Overview

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

Process Explanation

When an object vibrates, it disturbs nearby particles, creating high-pressure (compression) and low-pressure (rarefaction) regions. Our textbook shows a tuning fork vibrating in air as an example.

Real-world Application

Doctors use stethoscopes to hear heartbeats, where sound travels through the tube's air medium.

Question 7:
Describe the human ear structure and its role in hearing. Use an NCERT diagram reference.
Answer:
Concept Overview

The ear has three parts: outer, middle, and inner. It converts sound waves into electrical signals for the brain.

Process Explanation

Sound enters the ear canal, vibrates the eardrum, and ossicles amplify it. The cochlea (shown in NCERT) converts vibrations into neural signals.

Real-world Application

Hearing aids amplify sound for people with damaged eardrums or ossicles.

Question 8:
What determines the loudness and pitch of sound? Give one NCERT activity and a musical instrument example.
Answer:
Concept Overview

Loudness depends on amplitude, while pitch depends on frequency. NCERT Activity 12.8 demonstrates this using a ruler.

Process Explanation

Higher amplitude means louder sound (like a drum hit hard). Higher frequency means sharper pitch (flute's high notes).

Real-world Application

Guitar strings produce different pitches based on thickness and tension.

Question 9:
Explain echo formation with the minimum distance required. Cite the NCERT example and a practical use.
Answer:
Concept Overview

An echo is reflected sound heard after the original sound. Minimum distance is 17.2m (NCERT) at 20°C.

Process Explanation

Sound reflects off hard surfaces like cliffs. Our textbook shows this with a shout in mountains.

Real-world Application

Sonar uses echoes to measure ocean depth by timing sound reflections.

Question 10:
Compare ultrasonic and infrasonic sounds with examples from NCERT and medical field.
Answer:
Concept Overview

Ultrasonic (>20kHz) and infrasonic (<20Hz) are inaudible to humans. Bats use ultrasound (NCERT example).

Process Explanation

Ultrasound machines image babies by reflecting high-frequency waves. Infrasound from earthquakes can be detected by animals.

Real-world Application

Doctors use ultrasonic scans for tumor detection without surgery.

Question 11:
Explain the production of sound with an example. Describe how sound travels through different media and why it cannot travel in a vacuum.
Answer:

Sound is produced due to vibrations of an object. For example, when a guitar string is plucked, it vibrates and creates sound waves in the surrounding air.

Sound travels as a longitudinal wave and requires a medium (solid, liquid, or gas) to propagate. Here's how it travels:

  • Solids: Sound travels fastest in solids because particles are closely packed, allowing quick energy transfer.
  • Liquids: It travels slower than in solids but faster than in gases due to moderate particle spacing.
  • Gases: Sound travels slowest in gases because particles are far apart, reducing energy transfer efficiency.

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

Question 12:
Describe the human ear and explain how it detects sound. Include the role of the eardrum, cochlea, and auditory nerve.
Answer:

The human ear has three main parts: the outer ear, middle ear, and inner ear, which work together to detect sound.

  • Outer Ear: The pinna collects sound waves and directs them into the ear canal.
  • Middle Ear: Sound waves hit the eardrum, causing it to vibrate. These vibrations are amplified by three tiny bones (hammer, anvil, stirrup).
  • Inner Ear: The cochlea, a spiral-shaped organ, converts vibrations into electrical signals using hair cells.

The auditory nerve carries these signals to the brain, which interprets them as sound. Damage to any part can lead to hearing loss.

Question 13:
What is echo? State the conditions required for an echo to be heard clearly. Explain how this principle is used in sonar technology.
Answer:

An echo is a reflected sound wave that reaches the listener after the original sound. For a clear echo:

  • The minimum distance between the source and reflector must be 17.2 meters (as sound takes ~0.1 sec to travel this distance at 20°C).
  • The reflecting surface should be large and hard (e.g., cliffs, walls).

Sonar technology uses echoes to detect underwater objects. Here's how:


1. A sound wave (ultrasound) is sent into water.
2. The wave reflects off objects (e.g., submarines, fish).
3. The returning echo is detected, and the time delay helps calculate distance.

This principle is vital for navigation, fishing, and oceanography.

Question 14:
Explain the propagation of sound in different mediums (solid, liquid, gas) with examples. How does the speed of sound vary in these mediums? Support your answer with a diagram.
Answer:

Sound propagates as a longitudinal wave through the vibration of particles in a medium. The speed of sound depends on the density and elasticity of the medium.


1. Solids: Sound travels fastest in solids because particles are closely packed, allowing quick energy transfer. Example: Striking a metal rod produces sound heard clearly at the other end.
2. Liquids: Sound travels slower than in solids but faster than in gases due to moderate particle spacing. Example: Whales communicate underwater using sound waves.
3. Gases: Sound travels slowest in gases because particles are far apart, causing delayed energy transfer. Example: Thunder is heard after lightning due to slower sound speed in air.


The speed of sound follows the order: Solid > Liquid > Gas. A diagram showing particle arrangement in each medium can illustrate this concept clearly.

Question 15:
Describe the human ear and explain how it detects sound. Include the role of the eardrum, cochlea, and auditory nerve in the process.
Answer:

The human ear has three main parts: outer ear, middle ear, and inner ear, which work together to detect sound.


1. Outer Ear: The pinna collects sound waves and directs them into the ear canal.
2. Middle Ear: Sound waves hit the eardrum, causing vibrations. These vibrations are amplified by three tiny bones (malleus, incus, stapes) and transmitted to the inner ear.
3. Inner Ear: The cochlea, a fluid-filled spiral structure, converts vibrations into electrical signals via hair cells. The auditory nerve then carries these signals to the brain for interpretation.


This process ensures we perceive sound clearly. A labeled diagram of the ear can help visualize these steps.

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. It propagates as a series of compressions and rarefactions in the medium.


1. Solids: Sound travels fastest in solids because the particles are closely packed, allowing vibrations to transfer quickly.
Example: When you tap a metal rod, the sound is heard clearly at the other end due to efficient particle vibration.


2. Liquids: Sound travels slower in liquids compared to solids but faster than in gases because particles are less tightly packed.
Example: Whales communicate underwater using sound waves, which travel efficiently through water.


3. Gases: Sound travels slowest in gases due to widely spaced particles, reducing the speed of vibration transfer.
Example: When someone speaks, sound waves travel through air (a gas) to reach our ears.


Why sound cannot travel in a vacuum: A vacuum is an empty space with no particles. Since sound requires a medium to propagate, it cannot travel in a vacuum. Example: In outer space, no sound is heard because it is a near-perfect vacuum.


Additional Insight: The speed of sound depends on the medium's elasticity and density. Solids have high elasticity, enabling faster sound travel, while gases have low elasticity, slowing it down.

Question 17:
Explain the working of the human ear with a well-labelled diagram. How does the ear convert sound waves into signals that the brain can interpret? (5 marks)
Answer:

The human ear is a complex organ that detects and processes sound waves, converting them into electrical signals for the brain. It consists of three main parts: the outer ear, middle ear, and inner ear.


1. Outer Ear: The pinna collects sound waves and directs them into the ear canal. These sound waves travel to the eardrum (tympanic membrane), causing it to vibrate.


2. Middle Ear: The vibrations from the eardrum are amplified by three tiny bones called ossicles (malleus, incus, and stapes). These bones transmit the vibrations to the oval window of the cochlea.


3. Inner Ear: The cochlea, a spiral-shaped fluid-filled structure, contains hair cells that convert vibrations into electrical signals. These signals are sent via the auditory nerve to the brain, which interprets them as sound.


Diagram (Labelled): A well-drawn diagram should include:

  • Pinna
  • Ear canal
  • Eardrum
  • Ossicles (malleus, incus, stapes)
  • Cochlea
  • Auditory nerve


Additional Insight: The ear also helps maintain balance through the vestibular system in the inner ear, which detects head movements.

Question 18:
Explain how the human ear works to detect sound, including the roles of the outer ear, middle ear, and inner ear. Also, mention how damage to any part can affect hearing.
Answer:

The human ear is a complex organ that detects and processes sound through three main parts: the outer ear, middle ear, and inner ear.

Outer Ear: The pinna collects sound waves and directs them into the ear canal. These sound waves then strike the eardrum (tympanic membrane), causing it to vibrate.

Middle Ear: The vibrations from the eardrum are amplified by three tiny bones called ossicles (malleus, incus, and stapes). These bones transmit the vibrations to the oval window of the inner ear.

Inner Ear: The cochlea, a spiral-shaped structure filled with fluid, converts these vibrations into electrical signals using tiny hair cells. These signals are sent to the brain via the auditory nerve, where they are interpreted as sound.

Effects of Damage:

  • Damage to the eardrum or ossicles can reduce hearing ability by weakening sound transmission.
  • Damage to the cochlea or hair cells can cause permanent hearing loss, as these cells do not regenerate.
  • Blockages in the ear canal (e.g., earwax) can muffle sounds.

Proper care, such as avoiding loud noises and keeping ears clean, helps maintain healthy hearing.

Question 19:
Explain the concept of echo with a real-life example. Describe the necessary conditions for an echo to be heard clearly and calculate the minimum distance required between the observer and the reflecting surface for an echo to be audible (speed of sound = 344 m/s).
Answer:

An echo is a reflection of sound that arrives at the listener with a delay after the direct sound. It occurs when sound waves bounce off a hard surface and return to the listener's ears. A common real-life example is shouting in a large empty hall and hearing the sound repeat after a short pause.

Conditions for clear echo:

  • The reflecting surface must be large and hard (e.g., mountains, walls).
  • The distance between the observer and the surface must be sufficient for the echo to be distinct from the original sound.
  • The time gap between the original sound and its echo should be at least 0.1 seconds for human ears to perceive them separately.

Calculation of minimum distance:


Speed of sound (v) = 344 m/s
Minimum time gap (t) = 0.1 s
Distance = (Speed × Time) / 2 (since sound travels to the surface and back)
Distance = (344 × 0.1) / 2 = 17.2 meters

Thus, the minimum distance required is 17.2 meters for an echo to be audible clearly.

Question 20:
Describe how the human ear detects sound waves and converts them into electrical signals for the brain to interpret. Include the role of key parts like the eardrum, cochlea, and auditory nerve in this process.
Answer:

The human ear detects sound through a series of steps that convert sound waves into electrical signals:

  • Outer Ear: The pinna collects sound waves and directs them into the ear canal.
  • Eardrum (Tympanic Membrane): Sound waves strike the eardrum, causing it to vibrate. These vibrations are passed to the three tiny bones (ossicles) in the middle ear.
  • Cochlea: The vibrations reach the cochlea, a fluid-filled, spiral-shaped structure in the inner ear. Hair cells inside the cochlea bend due to the fluid movement, generating electrical signals.
  • Auditory Nerve: The auditory nerve carries these electrical signals to the brain, where they are interpreted as sound.

Additional Insight: The cochlea's hair cells are highly sensitive and can be damaged by loud noises, leading to hearing loss. Protecting ears from excessive noise is essential for long-term hearing health.

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 student plucks a stretched rubber band and observes sound. Vibration and frequency are key terms here. Explain how sound is produced and how frequency affects pitch.
Answer:
Case Summary

Plucking a rubber band causes vibrations, producing sound.

Scientific Principle
  • Sound is produced by vibrating objects (NCERT example: tuning fork).
  • Higher frequency increases pitch (real-world: guitar strings).
Solution Approach

We studied that vibrations create sound waves. Our textbook shows frequency determines pitch—faster vibrations mean higher pitch.

Question 2:
In an experiment, two sounds have frequencies 256 Hz and 512 Hz. Identify the higher-pitched sound and explain why humans can hear both.
Answer:
Case Summary

Comparing two sounds with different frequencies.

Scientific Principle
  • 512 Hz is higher-pitched (NCERT example: mosquito buzz vs. drum).
  • Human hearing range: 20 Hz–20 kHz (real-world: dog whistles exceed this).
Solution Approach

We learned pitch depends on frequency. Both sounds are within our audible range, as per NCERT.

Question 3:
A classroom has echoes. Suggest sound-absorbing materials and explain how they reduce reverberation using the reflection concept.
Answer:
Case Summary

Echoes in a classroom due to sound reflection.

Scientific Principle
  • Soft materials (curtains, carpets) absorb sound (NCERT example: auditorium design).
  • Reduces reflection (real-world: recording studios).
Solution Approach

Our textbook shows echoes arise from repeated reflections. Absorbers minimize this, improving clarity.

Question 4:
A tuning fork vibrates at 440 Hz but is barely audible. Link amplitude to loudness and suggest how to increase it.
Answer:
Case Summary

Tuning fork sound is faint despite correct frequency.

Scientific Principle
  • Loudness depends on amplitude (NCERT example: striking a fork harder).
  • Real-world: shouting vs. whispering.
Solution Approach

We studied that amplitude determines loudness. Hitting the fork harder increases vibration amplitude, making it louder.

Question 5:
A student observes that a tuning fork vibrating at 256 Hz produces sound waves in air. Describe how these waves propagate and calculate the wavelength if the speed of sound is 340 m/s.
Answer:
Case Summary
A tuning fork vibrates at 256 Hz, creating sound waves.
Scientific Principle
We studied that sound waves are longitudinal waves. The formula v = f × λ relates speed (v), frequency (f), and wavelength (λ).
Solution Approach

Given v = 340 m/s and f = 256 Hz, we calculate λ as λ = v/f = 340/256 ≈ 1.33 m. Our textbook shows similar calculations for tuning forks.

Question 6:
In an experiment, two students stand 50 m apart and clap. The echo returns after 0.3 seconds. Explain the phenomenon and determine the speed of sound.
Answer:
Case Summary
An echo is heard 0.3 s after clapping 50 m away.
Scientific Principle
We studied that echoes are reflected sound waves. The formula speed = distance/time applies here.
Solution Approach

Total distance = 50 m × 2 = 100 m (to and fro). Speed = 100 m / 0.3 s ≈ 333 m/s. Our textbook shows echoes are used in sonar, similar to this experiment.

Question 7:
A school bell rings, but a student 50 meters away hears it faintly. Identify two reasons for the faint sound and relate it to sound properties.
Answer:
Case Summary
A bell's sound becomes faint over 50 meters.
Scientific Principle
We studied that sound intensity decreases with distance due to energy spreading. Absorption by air also reduces loudness.
Solution Approach
  • Sound waves spread out, reducing energy per area.
  • Air molecules absorb some sound energy. Our textbook shows this with megaphones amplifying sound.
Question 8:
A musician plays a flute and a drum at the same volume. Compare their sound characteristics and explain how they differ.
Answer:
Case Summary
A flute and drum produce sounds at the same volume.
Scientific Principle
We studied that sound has pitch (frequency) and timbre (quality). Flutes produce musical notes, while drums create noise.
Solution Approach
  • Flute: High pitch (clear frequency).
  • Drum: Low pitch (irregular vibrations). Our textbook shows this with sitar and tabla examples.
Question 9:
A student plucks a guitar string and observes the sound produced. Explain how the frequency and amplitude of the sound wave change if the string is tightened more.
Answer:
Case Summary

A guitar string's tension affects sound properties.

Scientific Principle
  • Frequency increases with tension (higher pitch).
  • Amplitude depends on plucking force, not tension.
Solution Approach

Our textbook shows that tightening a string raises its frequency, like in a violin. Real-world, musicians tune instruments this way.

Question 10:
In an experiment, two students stand 50 m apart. One claps, and the other hears the sound after 0.15 s. Calculate the speed of sound and compare it with the textbook value (342 m/s at 20°C).
Answer:
Case Summary

Calculating speed using distance and time.

Scientific Principle
  • Speed = Distance/Time.
  • Temperature affects speed (NCERT Example 12.1).
Solution Approach

Speed = 50/0.15 ≈ 333 m/s. Lower than 342 m/s, possibly due to cooler air. Real-world, sound travels slower in winters.

Question 11:
A hospital uses ultrasound for imaging. How does its frequency compare to audible sound? Why is it preferred over X-rays for certain scans?
Answer:
Case Summary

Ultrasound applications in medicine.

Scientific Principle
  • Ultrasound frequency > 20 kHz (inaudible).
  • Safer than X-rays (NCERT Page 161).
Solution Approach

We studied that ultrasound is high-frequency and non-ionizing. Real-world, it’s used for fetal scans as it’s harmless.

Question 12:
A classroom has echoes when empty but not when filled with students. Explain using reflection and absorption of sound.
Answer:
Case Summary

Echoes depend on room occupancy.

Scientific Principle
  • Hard surfaces reflect sound (echo).
  • Soft materials absorb it (NCERT Activity 12.2).
Solution Approach

Students’ clothes absorb sound, reducing echoes. Real-world, theaters use curtains to minimize echoes.

Question 13:

Rahul conducted an experiment to study the characteristics of sound waves. He used a tuning fork of frequency 512 Hz and observed that the sound produced could be heard clearly at a distance of 50 meters. However, when he repeated the experiment with a tuning fork of 256 Hz, the sound was not audible beyond 25 meters.

Based on this observation:

  • Explain why the higher frequency sound traveled farther.
  • How does the amplitude of the sound wave affect its audibility?
Answer:

Explanation for higher frequency sound traveling farther:
Higher frequency sound waves (512 Hz) have more energy compared to lower frequency waves (256 Hz) when the amplitude is the same. This energy helps the sound wave overcome air resistance and travel longer distances without significant energy loss.

Role of amplitude in audibility:
The amplitude of a sound wave determines its loudness. Greater amplitude means more energy is carried by the wave, making it louder and more audible over longer distances. In Rahul's experiment, if the amplitude of both tuning forks was the same, the higher frequency fork would still be heard farther due to its energy, but increasing the amplitude of the 256 Hz fork could improve its range.

Additional Insight:
Environmental factors like wind, humidity, and obstacles also affect sound propagation, but frequency and amplitude are key intrinsic properties influencing audibility.

Question 14:

In a school auditorium, students noticed that when their teacher spoke normally, the sound was clear at the front but faint at the back. When the teacher used a microphone, the sound was equally clear everywhere.

Analyze this scenario by answering:

  • Why does sound intensity decrease with distance in the absence of a microphone?
  • How does a microphone help in maintaining uniform sound distribution?
Answer:

Decrease in sound intensity with distance:
Sound waves spread out in all directions, and their energy gets distributed over a larger area as they travel. This phenomenon is called inverse square law of sound propagation. As distance increases, the energy per unit area (intensity) decreases, making the sound fainter.

Role of a microphone:
A microphone amplifies the sound by converting it into an electrical signal, which is then boosted and sent to speakers. The speakers redistribute the sound uniformly across the auditorium, ensuring consistent intensity at all distances. This overcomes the natural weakening of sound over distance.

Application:
This principle is used in public address systems, concerts, and theaters to ensure everyone hears clearly, regardless of their position relative to the sound source.

Question 15:

A group of students conducted an experiment to study the relationship between the frequency and pitch of sound. They used a tuning fork of 512 Hz and another of 256 Hz. When struck, the 512 Hz tuning fork produced a higher-pitched sound compared to the 256 Hz fork.

Question: Explain the scientific reason behind this observation and describe how frequency affects the pitch of sound. Also, mention one real-life application where this principle is utilized.

Answer:

Explanation: The pitch of a sound is directly related to its frequency. Higher frequency means more vibrations per second, resulting in a higher-pitched sound. The 512 Hz tuning fork vibrates faster (512 times per second) compared to the 256 Hz fork (256 times per second), hence producing a higher pitch.


Application: This principle is used in musical instruments like guitars or pianos, where shorter or tighter strings produce higher frequencies (and thus higher pitches) when plucked or struck.

Question 16:

During a science fair, Riya demonstrated how sound needs a medium to travel. She placed a ringing alarm clock inside a glass jar and gradually removed the air using a vacuum pump. The sound became fainter and eventually inaudible.

Question: Analyze this experiment and explain why sound became inaudible. Also, compare the speed of sound in air, water, and steel, justifying your answer with scientific reasoning.

Answer:

Analysis: Sound requires a medium (like air, water, or solids) to propagate as it travels via vibrations of particles. When air was removed from the jar, the medium was lost, making the sound inaudible.


Comparison of Speed:

  • Air: ~343 m/s (slowest, due to widely spaced molecules)
  • Water: ~1482 m/s (faster than air, as molecules are closer)
  • Steel: ~5960 m/s (fastest, due to tightly packed molecules allowing quicker energy transfer)

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 vibrating. However, his friend Priya claimed that no sound was produced in a vacuum when she performed a similar experiment. Analyze both observations and explain:

  • Why did the table tennis ball vibrate in Rahul's experiment?
  • How does Priya's observation support the nature of sound waves?
Answer:

Vibration of the table tennis ball: When Rahul struck the tuning fork, it vibrated and produced sound waves. These waves traveled through the air and caused the table tennis ball to vibrate due to the transfer of energy from the sound waves to the ball.


Priya's observation in a vacuum: Sound requires a medium (like air, water, or solids) to travel because it propagates as a longitudinal wave through particle collisions. In a vacuum, there are no particles to vibrate, so no sound is produced. This confirms that sound cannot travel in a vacuum, supporting its wave nature.

Question 18:

In a school auditorium, two students, Ananya and Rohan, stood at opposite ends. Ananya clapped once, and Rohan heard two distinct sounds. The science teacher explained that this was due to the phenomenon of reverberation. Based on this:

  • Define reverberation and explain why Rohan heard two sounds.
  • Suggest two methods to reduce excessive reverberation in the auditorium.
Answer:

Reverberation: It is the persistence of sound due to repeated reflections from surfaces like walls, ceilings, or floors. Rohan heard two sounds because the first was the direct sound from Ananya's clap, and the second was the reflected sound after bouncing off the auditorium's surfaces.


Methods to reduce reverberation:

  • Using sound-absorbing materials like curtains, carpets, or foam panels to minimize reflections.
  • Designing the auditorium with uneven surfaces or angled walls to scatter sound waves instead of reflecting them directly.

Question 19:
A group of students conducted an experiment to study the characteristics of sound waves. They used a tuning fork and a glass of water. When the tuning fork was struck and brought near the water surface, ripples were observed.

Based on this observation, answer the following:

  • What does the formation of ripples indicate about sound waves?
  • How does the frequency of the tuning fork affect the ripples?
Answer:

The formation of ripples indicates that sound waves are mechanical waves and require a medium (like air or water) to travel. When the vibrating tuning fork comes near the water, it transfers its energy to the water molecules, creating ripples.


The frequency of the tuning fork determines the number of ripples formed per second. A higher frequency tuning fork will produce more ripples in the same time compared to a lower frequency one, as frequency is directly related to the rate of vibration.


Additionally, this experiment demonstrates the concept of wave propagation and how energy is transferred through a medium without the actual movement of the medium itself over long distances.

Question 20:
In a classroom, two students were discussing why sound cannot travel in space. One argued that it is because space is empty, while the other said it is due to the absence of a medium.

Explain the scientific reason behind the inability of sound to travel in space, supporting your answer with the properties of sound waves.

Answer:

Sound is a mechanical wave that requires a medium (like air, water, or solids) to propagate. In space, there is no air or any other medium—it is a near-perfect vacuum.


Since sound waves rely on the vibration of particles in the medium to transfer energy, the absence of particles in space means there is nothing to vibrate and carry the sound. This is why sound cannot travel in space.


This property distinguishes sound from electromagnetic waves (like light), which do not require a medium and can travel through the vacuum of space.

Question 21:
Rahul observed that when he clapped his hands near a tall building, he heard the same sound again after a short time.

(i) Name the phenomenon responsible for this observation.
(ii) Explain how this phenomenon occurs.
(iii) If Rahul stands 85 meters away from the building, calculate the time taken for him to hear the reflected sound (Speed of sound in air = 340 m/s).

Answer:

(i) The phenomenon is called echo.

(ii) An echo is produced when sound waves reflect off a hard surface, like a building, and return to the listener. For an echo to be clearly heard, the reflecting surface must be at least 17.2 meters away (as the human ear can distinguish sounds only if they are at least 0.1 seconds apart).

(iii) Calculation:
Total distance travelled by sound = 85 m (to the building) + 85 m (back to Rahul) = 170 m
Speed of sound = 340 m/s
Time taken = Distance / Speed = 170 / 340 = 0.5 seconds.

Question 22:
A school auditorium has a high ceiling and curved walls. Students often complain that they hear a lingering sound even after the teacher stops speaking.

(i) Identify the acoustic phenomenon occurring here.
(ii) Why does this happen in such rooms?
(iii) Suggest two practical ways to reduce this effect in the auditorium.

Answer:

(i) The phenomenon is called reverberation.

(ii) Reverberation occurs when sound waves reflect multiple times off hard surfaces (like high ceilings and curved walls) before fading away. This causes the sound to persist, making it unclear.

(iii) Two ways to reduce reverberation:

  • Using sound-absorbing materials like carpets, curtains, or foam panels on walls and ceilings.
  • Designing the auditorium with irregular surfaces to break up sound reflections.

These methods help absorb sound energy, reducing repeated reflections.

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