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Sound – CBSE NCERT Study Resources

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Study Materials

9th

9th - English (Panorama Part‑1)

Sound

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Overview of the Chapter

The chapter "Sound" from the CBSE Grade 9 English textbook (Panorama Part-1) explores the concept of sound as a form of energy and its significance in communication and the natural world. It delves into the scientific principles behind sound, its production, propagation, and perception, while also touching upon its artistic and cultural dimensions.

Sound: A vibration that propagates as an acoustic wave through a medium such as air or water, perceived by the ear when these waves reach it.

Key Concepts Covered

  • Production of sound and its sources
  • Propagation of sound waves
  • Characteristics of sound: frequency, amplitude, and speed
  • Human perception of sound and the role of the ear
  • Applications of sound in technology and art

Detailed Summary

The chapter begins by explaining how sound is produced through vibrations, using examples like musical instruments and vocal cords. It then describes how these vibrations travel as longitudinal waves, requiring a medium such as air, water, or solids.

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

Further, the chapter discusses the characteristics of sound, including frequency (pitch), amplitude (loudness), and speed, which varies depending on the medium. It explains how the human ear captures sound waves and converts them into electrical signals for the brain to interpret.

The chapter also highlights the importance of sound in daily life, from communication to entertainment, and its applications in technologies like sonar and ultrasound. Additionally, it touches upon noise pollution and measures to mitigate its effects.

Conclusion

Through this chapter, students gain a comprehensive understanding of sound as both a physical phenomenon and a vital part of human experience. The blend of scientific principles and real-world applications makes the topic engaging and relevant.

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:

Sound travels fastest through solids.

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 collects sound waves?
Answer:

The pinna collects sound waves.

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

Humans hear sounds between 20 Hz to 20,000 Hz.

Question 6:
What is an echo?
Answer:

An echo is a reflected sound wave.

Question 7:
What is the speed of sound in air at 20°C?
Answer:

The speed is 343 m/s in air at 20°C.

Question 8:
Name a musical instrument that produces sound by vibration of strings.
Answer:

A guitar produces sound by vibrating strings.

Question 9:
What is ultrasound?
Answer:

Ultrasound is sound with frequency above 20,000 Hz.

Question 10:
What is the pitch of sound?
Answer:

Pitch depends on the frequency of sound.

Question 11:
Which animal uses echolocation to navigate?
Answer:

Bats use echolocation to navigate.

Question 12:
What is noise pollution?
Answer:

Noise pollution is unwanted or harmful sound.

Question 13:
Name the three characteristics of sound.
Answer:

The three characteristics of sound are:
1. Pitch (related to frequency)
2. Loudness (related to amplitude)
3. Quality (related to waveform)

Question 14:
How does sound travel through air?
Answer:

Sound travels through air as longitudinal waves, where particles vibrate back and forth in the same direction as the wave moves, creating compressions and rarefactions.

Question 15:
What is the speed of sound in air at room temperature?
Answer:

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

Question 16:
Why can't sound travel in a vacuum?
Answer:

Sound cannot travel in a vacuum because it requires a medium (like air, water, or solids) for vibrations to propagate. In a vacuum, there are no particles to carry the sound waves.

Question 17:
What is the minimum distance required to hear an echo clearly?
Answer:

The minimum distance required to hear an echo clearly is 17.2 meters (as sound takes about 0.1 seconds to reflect and the human ear can distinguish sounds separated by at least 0.1 seconds).

Question 18:
Define amplitude of a sound wave.
Answer:

Amplitude is the maximum displacement of particles in a medium from their rest position when a sound wave passes. It determines the loudness of the sound—greater amplitude means louder sound.

Question 19:
What is the audible range of sound for humans?
Answer:

The audible range of sound for humans is typically from 20 Hz to 20,000 Hz. Sounds below 20 Hz are called infrasound, and sounds above 20,000 Hz are called ultrasound.

Question 20:
How does a loudspeaker produce sound?
Answer:

A loudspeaker produces sound by converting electrical signals into mechanical vibrations (using a diaphragm), which then create sound waves in the surrounding air.

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 vibrations that travel through a medium (like air, water, or solids) and can be heard when they reach a person's or animal's ear.
The SI unit of sound is the decibel (dB).

Question 2:
What is the role of the eardrum in hearing?
Answer:

The eardrum is a thin membrane in the ear that vibrates when sound waves hit it.
These vibrations are then transmitted to the inner ear, where they are converted into electrical signals sent to the brain for interpretation.

Question 3:
Differentiate between loudness and pitch of sound.
Answer:
  • Loudness depends on the amplitude of the sound wave and determines how strong or faint the sound is.
  • Pitch depends on the frequency of the sound wave and determines whether the sound is high or low.
Question 4:
What is an echo? State one condition necessary for hearing it clearly.
Answer:

An echo is a reflected sound wave that reaches the listener after the original sound.
One condition for hearing it clearly is that the reflecting surface should be at least 17 meters away from the source of sound.

Question 5:
How does the frequency of a sound wave affect its pitch?
Answer:

Higher frequency sound waves produce a higher pitch, while lower frequency waves produce a lower pitch.
For example, a bird's chirp has a higher frequency (and pitch) than a lion's roar.

Question 6:
Name two musical instruments that produce sound by vibrating strings.
Answer:

Two musical instruments that produce sound by vibrating strings are:
1. Guitar
2. Violin

Question 7:
What is the range of audible sound for humans?
Answer:

The range of audible sound for humans is typically from 20 Hz to 20,000 Hz.
Sounds below 20 Hz are called infrasound, and those above 20,000 Hz are called ultrasound.

Question 8:
Explain why thunder is heard after a lightning flash.
Answer:

Thunder is heard after lightning because light travels faster than sound.
Lightning and thunder occur simultaneously, but light reaches us almost instantly, while sound takes longer to travel through the air.

Question 9:
What is noise pollution? Give one example of its harmful effect.
Answer:

Noise pollution refers to excessive or harmful levels of noise in the environment.
One harmful effect is hearing loss due to prolonged exposure to loud sounds.

Question 10:
How does a stethoscope help doctors hear heartbeats clearly?
Answer:

A stethoscope amplifies sound by capturing vibrations from the body and directing them through tubes to the doctor's ears.
This makes faint sounds like heartbeats clearer and easier to analyze.

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 is produced and propagated through a medium.
Answer:

Sound is produced due to vibrations of an object. When an object vibrates, it causes the particles of the surrounding medium (like air, water, or solids) to vibrate as well. These vibrations create compressions (high-pressure regions) and rarefactions (low-pressure regions) that travel through the medium as a sound wave.

For example, when a guitar string is plucked, it vibrates and pushes air molecules, creating sound waves that reach our ears.

Question 2:
Differentiate between loudness and pitch of sound with examples.
Answer:

Loudness refers to how strong or faint a sound is, depending on the amplitude of the wave. A higher amplitude means louder sound (e.g., shouting vs. whispering).

Pitch refers to how high or low a sound is, determined by the frequency of the wave. A higher frequency means a higher pitch (e.g., a child's voice vs. an adult's voice).

Question 3:
Why can't sound travel in a vacuum? Explain with a scientific reason.
Answer:

Sound requires a medium (like air, water, or solids) to travel because it propagates through the vibration of particles. In a vacuum, there are no particles to vibrate, so sound cannot travel. This is why we cannot hear sounds in space, which is a near-perfect vacuum.

Question 4:
Describe how the human ear detects sound waves and processes them.
Answer:

The human ear detects sound in three main steps:
1. Outer Ear: The pinna collects sound waves and directs them into the ear canal.
2. Middle Ear: The eardrum vibrates, and tiny bones (ossicles) amplify these vibrations.
3. Inner Ear: The cochlea converts vibrations into electrical signals sent to the brain via the auditory nerve.

Question 5:
What is an echo? State the minimum distance required for an echo to be heard clearly.
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 sound source and the reflecting surface should be 17.2 meters (at 22°C). This ensures that the reflected sound reaches the ear at least 0.1 seconds after the original sound, allowing the brain to distinguish them separately.

Question 6:
How does the speed of sound vary in different media? Provide a reason.
Answer:

The speed of sound depends on the medium's density and elasticity:
- Solids: Fastest, because particles are closely packed and transmit vibrations quickly (e.g., steel).
- Liquids: Slower than solids but faster than gases (e.g., water).
- Gases: Slowest, due to widely spaced particles (e.g., air).

Question 7:
Explain how sound is produced and travels through a medium.
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 compressions and rarefactions. These disturbances travel through the medium (solid, liquid, or gas) as longitudinal waves.

For example, when a drum is struck, its membrane vibrates, pushing air particles and creating sound waves that reach our ears.

Question 8:
Why can't sound travel in a vacuum? Explain with an example.
Answer:

Sound requires a medium (solid, liquid, or gas) to travel because it propagates through the vibration of particles. In a vacuum, there are no particles to vibrate, so sound cannot travel.

For example, in outer space (a near-vacuum), astronauts cannot hear each other directly because there is no air to carry sound waves. They use radio signals instead.

Question 9:
Describe how the human ear detects sound waves.
Answer:

The human ear detects sound in the following steps:


1. Outer Ear: The pinna collects sound waves and directs them into the ear canal.
2. Middle Ear: The eardrum vibrates, and these vibrations are amplified by three tiny bones (ossicles).
3. Inner Ear: The cochlea converts vibrations into electrical signals via hair cells.
4. Auditory Nerve: These signals are sent to the brain, which interprets them as sound.
Question 10:
What is an echo? State the minimum distance required to hear an echo clearly.
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 sound source and the reflecting surface must be 17.2 meters (at 22°C).

This is because the human ear can distinguish two sounds only if they are at least 0.1 seconds apart. Sound travels ~344 m/s, so the total distance (to and fro) must be 34.4 m, making the minimum distance 17.2 m.

Question 11:
How does the sonar technique work? Mention one application.
Answer:

Sonar (Sound Navigation and Ranging) uses sound waves to detect objects underwater. It works by:


1. Emitting ultrasonic waves into water.
2. Measuring the time taken for the waves to reflect back after hitting an object.
3. Calculating the distance using the formula: Distance = (Speed of Sound × Time) / 2.

One application is in submarines to locate underwater obstacles or enemy vessels.

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 is produced and travels through different mediums using examples from daily life.
Answer:
Introduction

Sound is produced by vibrating objects. Our textbook shows that vibrations create waves, which travel through mediums like air, water, or solids.

Argument 1
  • Example: When a bell rings, vibrations travel as sound waves through air.
  • Example: Underwater, sound travels faster due to water's density.
Argument 2

Solids like metals transmit sound efficiently. We studied how placing an ear on a railway track detects distant trains.

Conclusion

Sound needs a medium and travels differently based on its properties.

Question 2:
Describe the characteristics of sound waves with two real-life examples.
Answer:
Introduction

Sound waves have three key characteristics: frequency, amplitude, and speed. These determine pitch, loudness, and travel.

Argument 1
  • Frequency: A mosquito's buzz has high frequency (pitch).
  • Amplitude: A drum hit hard produces louder sound.
Argument 2

Our textbook shows thunder's slow speed explains delayed hearing after lightning.

Conclusion

Understanding these helps explain everyday sounds.

Question 3:
How does echo occur? Explain with NCERT-based examples.
Answer:
Introduction

An echo is a reflected sound wave. It occurs when sound bounces off hard surfaces.

Argument 1
  • Example: Shouting in mountains causes echoes due to rock reflections.
  • Our textbook mentions auditoriums use soft materials to reduce echoes.
Argument 2

For clear echoes, the obstacle must be at least 17m away.

Conclusion

Echoes demonstrate sound's reflective property.

Question 4:
Compare musical sounds and noise using textbook definitions and examples.
Answer:
Introduction

Musical sounds are pleasant, while noise is unpleasant and irregular.

Argument 1
  • Example: A flute produces musical notes with fixed frequencies.
  • Example: Traffic noise has irregular vibrations.
Argument 2

Our textbook shows musical sounds follow patterns, unlike chaotic noise.

Conclusion

This distinction helps classify sounds around us.

Question 5:
Explain the human ear's role in hearing with a labeled diagram reference.
Answer:
Introduction

The ear converts sound waves into signals for the brain.

Argument 1
  • Outer ear collects waves, eardrum vibrates.
  • Middle ear bones amplify vibrations.
Argument 2

[Diagram: Ear structure] Our textbook shows cochlea converts vibrations to nerve signals.

Conclusion

Each part plays a vital role in hearing.

Question 6:
Explain how sound is produced and propagated using examples from daily life and NCERT concepts.
Answer:
Introduction

Sound is produced by vibrating objects and travels as longitudinal waves. Our textbook shows examples like tuning forks and vocal cords.

Argument 1

Vibrations create compressions and rarefactions in air. For example, a guitar string vibrates to produce sound.

Argument 2

Sound needs a medium (solid, liquid, gas) to propagate. NCERT mentions the bell-jar experiment proving sound cannot travel in a vacuum.

Conclusion

Daily examples like speaking or clapping demonstrate sound production and propagation clearly.

Question 7:
Describe the characteristics of sound waves with reference to amplitude, frequency, and wavelength.
Answer:
Introduction

Sound waves have three key characteristics: amplitude, frequency, and wavelength, which determine their properties.

Argument 1

Amplitude defines loudness. For example, a drum hit hard produces louder sound due to higher amplitude.

Argument 2

Frequency determines pitch. NCERT explains how a mosquito’s buzz has higher frequency than a lion’s roar.

Conclusion

Understanding these characteristics helps us analyze sound in daily life, like differentiating voices or musical notes.

Question 8:
How does reflection of sound lead to echoes? Provide NCERT-based and real-life examples.
Answer:
Introduction

Reflection of sound occurs when waves bounce off surfaces, creating echoes under specific conditions.

Argument 1

For an echo, the reflecting surface must be at least 17m away, as per NCERT. Mountains or large walls reflect sound clearly.

Argument 2

Daily examples include shouting in empty halls or hearing echoes in tunnels due to repeated reflections.

Conclusion

Echoes demonstrate sound’s wave nature and are useful in technologies like sonar.

Question 9:
Compare ultrasonic and infrasonic sounds with applications mentioned in NCERT.
Answer:
Introduction

Ultrasonic (high-frequency) and infrasonic (low-frequency) sounds are beyond human hearing range but have unique applications.

Argument 1

Ultrasonic sounds are used in medical imaging (NCERT example: sonography) and cleaning jewelry.

Argument 2

Infrasonic sounds are produced by earthquakes or elephants for long-distance communication, as per NCERT.

Conclusion

Both types play vital roles in science and nature despite being inaudible to humans.

Question 10:
Explain the human ear’s structure and its role in hearing, referencing NCERT diagrams.
Answer:
Introduction

The human ear has three parts: outer, middle, and inner ear, each crucial for hearing.

Argument 1

The outer ear collects sound waves, while the middle ear amplifies them via the eardrum and ossicles (NCERT Fig. 12.14).

Argument 2

The inner ear’s cochlea converts vibrations into electrical signals for the brain, as explained in our textbook.

Conclusion

This intricate structure enables us to hear sounds from whispers to thunder efficiently.

Question 11:
Explain the process of sound propagation in air with a detailed description of how compressions and rarefactions are formed. How does this relate to the longitudinal wave nature of sound?
Answer:

Sound propagates in air as a longitudinal wave, where particles of the medium vibrate parallel to the direction of wave propagation. When a vibrating object, like a tuning fork, moves forward, it pushes air particles together, creating a region of high pressure called compression. When it moves backward, air particles spread apart, forming a region of low pressure called rarefaction.


This cycle repeats, causing a series of compressions and rarefactions to travel through the air. Since the particles oscillate back and forth in the same direction as the wave, sound is classified as a longitudinal wave. Unlike transverse waves (e.g., light), sound requires a medium (solid, liquid, or gas) to travel because it relies on particle collisions.


For example, when you speak, your vocal cords vibrate, creating compressions and rarefactions in the air that reach the listener's ear as sound. The speed of sound depends on the medium's properties—it travels faster in solids than in liquids or gases due to closer particle spacing.

Question 12:
Describe how the human ear perceives sound, starting from the outer ear to the brain. Include the roles of the eardrum, cochlea, and auditory nerves in this process.
Answer:

The human ear detects sound through a series of steps involving its three main parts: the outer ear, middle ear, and inner ear.


1. Outer Ear: Sound waves enter through the pinna (visible part) and travel through the ear canal to the eardrum (tympanic membrane), causing it to vibrate.


2. Middle Ear: The eardrum's vibrations are amplified by three tiny bones—malleus, incus, and stapes—which transmit the vibrations to the cochlea in the inner ear.


3. Inner Ear: The cochlea, a fluid-filled, spiral-shaped 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.


For instance, when you hear a bell, the sound waves trigger this entire process, allowing your brain to recognize the pitch and loudness. Damage to any part (e.g., ruptured eardrum) can impair hearing, highlighting the ear's delicate structure.

Question 13:
Explain the process of sound production and propagation in humans with a labeled diagram of the human ear.
Answer:

Sound is produced when an object vibrates, creating pressure waves in the surrounding medium (usually air). In humans, sound is produced by the vocal cords in the larynx. When air from the lungs passes through the vocal cords, they vibrate, producing sound waves.

These sound waves travel through the air and enter the outer ear, where they are funneled by the pinna into the ear canal. The sound waves then strike the eardrum, causing it to vibrate. These vibrations are transmitted through the three tiny bones in the middle ear (malleus, incus, and stapes), which amplify the sound. The vibrations then reach the cochlea in the inner ear, where they are converted into electrical signals by hair cells. These signals are sent to the brain via the auditory nerve, allowing us to perceive sound.

Labeled Diagram of the Human Ear:
1. Pinna (Outer Ear)
2. Ear Canal
3. Eardrum (Tympanic Membrane)
4. Malleus, Incus, Stapes (Middle Ear Bones)
5. Cochlea (Inner Ear)
6. Auditory Nerve

Question 14:
Describe how the frequency and amplitude of a sound wave affect its pitch and loudness, respectively, with real-life examples.
Answer:

The frequency of a sound wave determines its pitch. Higher frequency waves produce higher-pitched sounds, while lower frequency waves produce lower-pitched sounds. For example, a whistle has a high frequency, resulting in a high-pitched sound, whereas a drum has a low frequency, producing a deep, low-pitched sound.

The amplitude of a sound wave determines its loudness. Greater amplitude means louder sound, while smaller amplitude means softer sound. For instance, shouting creates sound waves with high amplitude, making it loud, whereas whispering produces low-amplitude waves, resulting in a soft sound.

Real-life examples:
1. Frequency (Pitch): A guitar string vibrates faster when tightened, increasing frequency and pitch.
2. Amplitude (Loudness): Hitting a drum harder increases the amplitude, making the sound louder.

Question 15:
Explain the process of sound production and propagation in humans with a detailed description of the role of the vocal cords and medium. How does sound travel through different mediums? (5 marks)
Answer:

Sound is produced when an object vibrates, creating waves that travel through a medium. In humans, sound is produced by the vocal cords in the larynx (voice box). When air from the lungs passes through the tightened vocal cords, they vibrate, producing sound waves. The pitch of the sound depends on the tension and thickness of the vocal cords.

For sound to propagate, it requires a medium (solid, liquid, or gas) because sound waves are mechanical and need particles to vibrate and transfer energy. Here’s how sound travels in different mediums:

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

Without a medium (like in a vacuum), sound cannot propagate because there are no particles to carry the vibrations.

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.

1. Solids: Sound travels fastest in solids because the particles are closely packed, allowing vibrations to transfer quickly. For example, when you tap a metal rail, the sound reaches your ear faster through the rail than through air.

2. Liquids: In liquids, sound travels slower than in solids but faster than in gases. The molecules are less tightly packed than in solids but still allow efficient energy transfer. Whales communicate underwater using sound waves, which travel long distances.

3. Gases: Sound travels slowest in gases because the molecules are far apart, making energy transfer less efficient. For instance, when someone speaks, sound waves travel through air (a gas) to reach your 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. This is why astronauts use radios (electromagnetic waves) to communicate in space, as sound waves cannot travel there.

Question 17:
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. 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. For 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 because particles are less tightly packed. Whales communicate underwater using sound waves, which travel efficiently through water.

3. Gases: Sound travels slowest in gases due to widely spaced particles. For instance, 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. This is why astronauts use radios (electromagnetic waves) to communicate in space, as sound waves cannot travel there.

Question 18:
Explain the process of sound propagation through a medium, highlighting the role of particles in it. Support your answer with a labeled diagram.
Answer:

Sound is a form of energy that travels as longitudinal waves through a medium like air, water, or solids. The propagation of sound involves the following steps:

  • Vibration: A sound source (e.g., a tuning fork) vibrates, creating compressions and rarefactions in the surrounding medium.
  • Compression: Particles of the medium are pushed closer together, forming high-pressure regions.
  • Rarefaction: Particles move apart, creating low-pressure regions.
  • Wave Motion: These alternating compressions and rarefactions travel through the medium, transferring energy without permanent displacement of particles.

The particles of the medium oscillate back and forth in the same direction as the wave, which is why sound waves are called longitudinal waves. Sound cannot travel in a vacuum because there are no particles to transmit the vibrations.

Diagram: (A labeled diagram showing compressions and rarefactions in a sound wave, with arrows indicating particle motion.)

Understanding this process helps explain why sound travels faster in solids (due to closely packed particles) compared to gases (where particles are far apart).

Question 19:
Explain the concept of sound and its propagation through different mediums. How does the speed of sound vary in solids, liquids, and gases? Provide examples to support your answer.
Answer:

Sound is a form of energy produced by vibrations that travel through a medium as longitudinal waves. It requires a medium (solid, liquid, or gas) to propagate because it relies on the collision of particles to transfer energy.

Propagation of sound:
1. In solids, particles are closely packed, allowing sound to travel fastest due to strong intermolecular forces. Example: Sound travels faster through a metal rod than air.
2. In liquids, particles are less tightly packed than solids but still closer than gases, resulting in moderate speed. Example: Dolphins use sound waves to communicate underwater.
3. In gases, particles are far apart, leading to the slowest speed of sound. Example: Thunder is heard later than lightning due to slower sound propagation in air.

Key factors affecting speed include elasticity, density, and temperature of the medium. Understanding this helps in applications like SONAR and medical ultrasonography.

Question 20:
Describe the human ear and its role in hearing. Explain how sound waves are converted into signals understood by the brain. Include a labeled diagram of the ear for clarity.
Answer:

The human ear is a complex organ divided into three parts: outer ear, middle ear, and inner ear, each playing a vital role in hearing.

Process of hearing:
1. Outer ear: The pinna collects sound waves and directs them through the ear canal to the eardrum (tympanic membrane).
2. Middle ear: Vibrations from the eardrum are amplified by three tiny bones (malleus, incus, stapes) and transmitted to the inner ear.
3. Inner ear: The cochlea converts vibrations into electrical signals via hair cells. These signals are sent to the brain via the auditory nerve.

Diagram (labeled):
[Outer ear: Pinna, Ear canal | Middle ear: Eardrum, Ossicles | Inner ear: Cochlea, Auditory nerve]

This process highlights the ear's role in balancing (vestibular system) and hearing. Damage to any part can lead to hearing loss, emphasizing the need for protection from loud noises.

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:
In our textbook, we studied how sound waves travel. Explain with two examples how medium affects sound propagation.
Answer:
Case Summary

Sound waves need a medium like air, water, or solids to travel. Our textbook shows that without a medium, sound cannot propagate.

Analysis
  • In air, sound travels slower (343 m/s) compared to water (1500 m/s) due to particle density.
  • Solids like steel allow faster sound travel (5000 m/s) because particles are tightly packed.
Conclusion

Thus, the medium's nature directly impacts sound speed and clarity.

Question 2:
We learned about echo and reverberation. Differentiate them using real-life examples.
Answer:
Case Summary

Echo and reverberation both involve sound reflection but differ in time delay.

Analysis
  • Echo is heard distinctly after 0.1s, like shouting in mountains.
  • Reverberation is overlapping reflections (<0.1s), like in empty halls.
Conclusion

Distance and surface texture determine whether we hear an echo or reverberation.

Question 3:
How does amplitude influence sound? Relate it to loudness using NCERT examples.
Answer:
Case Summary

Amplitude is the height of a sound wave, which determines loudness.

Analysis
  • Higher amplitude means louder sound, like a drum hit hard.
  • Lower amplitude creates softer sounds, like a whisper.
Conclusion

Our textbook confirms that amplitude and loudness are directly proportional.

Question 4:
Describe how frequency and pitch are connected. Use examples from daily life.
Answer:
Case Summary

Frequency is the number of vibrations per second, affecting pitch.

Analysis
  • High-frequency sounds (e.g., whistle) have a high pitch.
  • Low-frequency sounds (e.g., bass guitar) have a low pitch.
Conclusion

We observed in class that pitch rises with frequency.

Question 5:
In our textbook, we studied how sound waves travel. Explain how compressions and rarefactions form in a slinky when sound passes through it.
Answer:
Case Summary

We studied that sound travels as longitudinal waves. In a slinky, compressions are regions where coils are close, while rarefactions are where coils are far apart.

Analysis
  • When we push one end, a compression moves forward.
  • Pulling creates a rarefaction.
Conclusion

Our textbook shows this using a slinky example. Sound waves alternate between compressions and rarefactions to travel.

Question 6:
How does the frequency of a sound wave affect its pitch? Give two examples from daily life.
Answer:
Case Summary

Frequency is the number of vibrations per second. Higher frequency means higher pitch.

Analysis
  • A baby's cry has high frequency (high pitch).
  • A drum has low frequency (low pitch).
Conclusion

We learned that pitch depends on frequency. Our textbook uses these examples to explain the concept.

Question 7:
Why can't we hear sound in space? Relate this to the medium required for sound propagation.
Answer:
Case Summary

Sound needs a medium (solid, liquid, gas) to travel. Space is a vacuum.

Analysis
  • No particles in space to vibrate.
  • Example: Bell in a vacuum jar makes no sound.
Conclusion

Our textbook explains that without a medium, sound cannot propagate. This is why space is silent.

Question 8:
Describe how echo is produced. Mention two conditions required for hearing an echo clearly.
Answer:
Case Summary

Echo is the reflection of sound. It occurs when sound bounces off a surface.

Analysis
  • Minimum distance: 17m between source and reflector.
  • Hard surface: Walls reflect sound better.
Conclusion

We studied that echoes need distance and hard surfaces. Our textbook uses hills and buildings as examples.

Question 9:
In our textbook, we studied how sound waves travel. Explain with two examples how medium affects the speed of sound.
Answer:
Case Summary

Sound travels through different mediums like air, water, and solids. Our textbook shows that the speed varies based on the medium's density.

Analysis
  • In air, sound travels at 343 m/s because molecules are far apart.
  • In water, it moves faster (1482 m/s) as molecules are closer.
Conclusion

Solids like steel transmit sound fastest (5960 m/s) due to tightly packed particles.

Question 10:
We learned about echo and reverberation. Differentiate them using two real-life examples.
Answer:
Case Summary

Both echo and reverberation involve sound reflection, but their duration differs.

Analysis
  • Echo: Heard in mountains when sound reflects after 0.1 sec (e.g., shouting in a valley).
  • Reverberation: Multiple reflections in closed spaces (e.g., empty hall).
Conclusion

Echo needs distance, while reverberation occurs in confined areas.

Question 11:
How does amplitude influence sound? Relate it to loudness using textbook examples.
Answer:
Case Summary

Amplitude determines a sound's energy and loudness.

Analysis
  • Higher amplitude means louder sound (e.g., drum hit hard).
  • Lower amplitude creates soft sounds (e.g., whisper).
Conclusion

Our textbook shows amplitude is directly proportional to loudness.

Question 12:
Describe how frequency affects pitch with two examples from daily life.
Answer:
Case Summary

Frequency measures sound waves per second, altering pitch.

Analysis
  • High frequency = high pitch (e.g., bird chirping).
  • Low frequency = low pitch (e.g., thunder).
Conclusion

We studied that pitch depends solely on frequency.

Question 13:

Rahul was playing his guitar when his younger sister Priya noticed that the sound produced by the thicker string was deeper than the thinner one. She wondered why this happens. Based on the chapter Sound, explain the scientific reason behind this observation.

Answer:

The difference in sound produced by the thicker and thinner strings of a guitar is due to the frequency of vibration.

1. Thicker strings have more mass per unit length, which causes them to vibrate slower, producing a lower frequency sound. This results in a deeper or bass tone.

2. Thinner strings vibrate faster due to less mass, producing a higher frequency sound, which is perceived as a sharper or treble tone.

Additionally, the tension and length of the strings also affect the pitch, but in this case, the thickness (mass) is the primary factor. This principle is used in musical instruments to create varied sounds.

Question 14:

During a school trip to a hilly area, students observed that their voices echoed when they shouted near a large cliff. Explain the phenomenon of echo and the conditions required for it to occur, as per the chapter Sound.

Answer:

An echo is a reflection of sound that arrives at the listener with a delay after the direct sound.

Conditions required for an echo:
1. The distance between the sound source and the reflecting surface (like a cliff) must be at least 17 meters. This is because sound travels approximately 340 m/s in air, and the human ear can distinguish between two sounds only if they are at least 0.1 seconds apart.

2. The reflecting surface must be large and hard (like a cliff or wall) to reflect sound effectively without absorbing it.

3. The surrounding area should be relatively quiet to clearly hear the reflected sound.

In hilly areas, these conditions are naturally met, making echoes common. This phenomenon is also used in technologies like sonar and ultrasound imaging.

Question 15:

Read the following case study and answer the question below:

Rahul was experimenting with sound in his physics lab. He observed that when he struck a tuning fork and placed it near a glass of water, ripples formed on the water's surface. He also noticed that the sound was louder when he placed the vibrating tuning fork on a wooden table.

Question: Explain the scientific principles behind Rahul's observations regarding sound waves and their interaction with different mediums. How does this relate to the propagation and amplitude of sound?

Answer:

Answer:

Rahul's observations demonstrate two key concepts of sound:

  • Vibration and Medium: The tuning fork's vibrations create sound waves, which travel through air and cause the water's surface to ripple. This shows that sound needs a medium (air or water) to propagate.
  • Amplitude and Loudness: When the tuning fork was placed on the wooden table, the sound became louder because the table acted as a resonating surface, increasing the amplitude of vibrations. Higher amplitude means greater energy, resulting in louder sound.

This experiment highlights how sound waves transfer energy through different mediums and how the amplitude directly affects loudness.

Question 16:

Read the following scenario and answer the question:

Priya noticed that during a thunderstorm, she saw lightning first and heard thunder a few seconds later. She also observed that the sound of thunder varied—sometimes it was a sharp crack, and other times a long rumble.

Question: Analyze Priya's observations using the concepts of speed of sound and reflection of sound waves. Why does thunder sound different at times?

Answer:

Answer:

Priya's observations can be explained as follows:

  • Speed of Light vs. Sound: Lightning is seen before thunder because light travels faster (~3 × 108 m/s) than sound (~343 m/s in air). The delay helps estimate the storm's distance.
  • Reflection of Sound: Thunder sounds different due to echoes and reflections of sound waves from clouds, hills, or buildings. A sharp crack occurs when the lightning is close, while a rumble results from multiple reflections over longer distances.

This shows how the speed of sound and its interaction with the environment create varying auditory effects.

Question 17:
Rahul was experimenting with sound in his physics lab. He observed that when he struck a tuning fork and placed it near a glass of water, ripples formed on the water's surface. Based on this observation, answer the following:
(i) What property of sound does this experiment demonstrate?
(ii) How does the frequency of the tuning fork affect the ripples?
Answer:

(i) The experiment demonstrates the property of vibrations in sound. When the tuning fork is struck, it vibrates and produces sound waves. These vibrations travel through the air and cause the water molecules to vibrate, creating ripples on the surface. This shows that sound is a form of mechanical wave that requires a medium (air or water) to propagate.


(ii) The frequency of the tuning fork directly affects the ripples. A higher frequency means the tuning fork vibrates faster, creating more ripples in a given time. Conversely, a lower frequency produces fewer ripples. This is because frequency determines the number of vibrations per second, which influences the wave pattern on the water's surface.

Question 18:
In a classroom, the teacher demonstrated how sound travels differently through solids, liquids, and gases using three experiments:
(i) Tapping a desk and listening to the sound.
(ii) Placing a ringing phone inside a water-filled container.
(iii) Blowing air into a bottle to produce sound.
Analyze these experiments and explain how the speed of sound varies in each medium.
Answer:

The experiments show how sound travels at different speeds depending on the medium:

  • Solid (desk): Sound travels fastest in solids because the molecules are tightly packed, allowing vibrations to transfer quickly. When the desk is tapped, the sound is heard almost instantly due to the high elasticity and density of the solid.
  • Liquid (water-filled container): Sound travels slower in liquids compared to solids but faster than in gases. The ringing phone's sound is muffled underwater because water molecules are less tightly packed than solids but still closer than gases.
  • Gas (air in the bottle): Sound travels slowest in gases because the molecules are far apart. Blowing air into the bottle produces sound, but it takes longer to travel through air compared to solids or liquids.

In summary, the speed of sound follows this order: solids > liquids > gases due to differences in molecular arrangement.

Question 19:
Read the following case: In a classroom, Riya observed that when her teacher struck a tuning fork and placed it near a table, the sound was louder than when held in the air. She wondered why this happened.

Based on the case, explain the scientific reason behind Riya's observation and how it relates to the properties of sound.
Answer:

Riya observed that the sound from the tuning fork was louder when placed near the table because of the property of sound reflection and medium density. Sound waves travel faster and more efficiently through solids (like the table) than through air due to the closer arrangement of particles in solids. When the tuning fork was placed on the table, the vibrations were directly transferred to the table, which acted as a resonating surface, amplifying the sound.

Additionally, the table reflected the sound waves, making them more concentrated and louder to Riya's ears. This demonstrates how medium density and reflection affect sound propagation.

Question 20:
Read the case: During a school assembly, students sitting at the back complained they couldn't hear the speaker clearly, while those in front could. The principal decided to install reflectors behind the stage.

Explain how the reflectors would help improve sound clarity for the students at the back, referring to the principles of sound.
Answer:

The reflectors installed behind the stage would help by redirecting and focusing the sound waves toward the students at the back. Sound waves travel in all directions, and without reflectors, some of the waves disperse and lose energy before reaching the back.

The reflectors work on the principle of sound reflection, bouncing the waves forward and preventing them from spreading outward. This ensures that more sound energy reaches the students at the back, improving clarity.

Additionally, reflectors can reduce echo and unwanted noise by directing sound efficiently, making the speaker's voice clearer and louder for everyone.

Question 21:
Read the following case:
In a quiet classroom, Riya clapped her hands once. The sound traveled to the opposite wall, 10 meters away, and echoed back to her. The temperature in the room was 25°C.
Based on this, answer:
(a) Calculate the time taken for the sound to travel to the wall and back to Riya. (Speed of sound at 25°C is 346 m/s)
(b) Why did Riya hear an echo? Explain the condition necessary for hearing an echo clearly.
Answer:

(a) To calculate the time taken for sound to travel to the wall and back:


Total distance = Distance to wall + Distance back = 10 m + 10 m = 20 meters.
Speed of sound at 25°C = 346 m/s.
Time = Distance / Speed = 20 m / 346 m/s ≈ 0.058 seconds.

(b) Riya heard an echo because the sound reflected off the wall and returned to her ears. For a clear echo:

  • The minimum distance between the source and the reflecting surface must be 17.2 meters at 25°C (since the human ear can distinguish sounds only if they are 0.1 seconds apart).
  • The reflecting surface should be large and hard (like a wall) to avoid absorption of sound.

Since the classroom was quiet and the distance (10 m) was less than 17.2 m, Riya likely heard a reverberation (quick succession of echoes) rather than a distinct echo.

Question 22:
Case: A school auditorium has a high ceiling and curved walls. During an assembly, students noticed that even whispers from the stage were audible at the back.
Based on this, answer:
(a) How do the auditorium's design features help in the propagation of sound?
(b) What is the scientific term for this phenomenon, and how does it differ from an echo?
Answer:

(a) The auditorium's design aids sound propagation because:

  • The high ceiling prevents sound waves from escaping upward, directing them toward the audience.
  • The curved walls act as reflectors, focusing sound waves toward the back through reflection and reducing energy loss.
  • Such designs minimize absorption of sound, ensuring clarity.

(b) The phenomenon is called reverberation. Unlike an echo (a distinct repetition of sound after a delay), reverberation is the persistence of sound due to multiple reflections. It occurs when the time gap between reflections is less than 0.1 seconds, blending sounds for better audibility.

Question 23:
Read the following case:
In a classroom, students conducted an experiment to study how sound travels through different mediums. They used a tuning fork to produce sound and observed its behavior in air, water, and a solid (metal rod).
Based on the case, answer:
(a) Why does sound travel fastest in the metal rod compared to air and water?
(b) How does the medium affect the speed of sound?
Answer:

(a) Sound travels fastest in the metal rod because solids have closely packed particles, allowing vibrations to transfer quickly. In air and water, particles are farther apart, slowing down the sound.

(b) The speed of sound depends on the density and elasticity of the medium.

  • In solids, particles are tightly packed, enabling faster energy transfer.
  • In liquids, particles are less dense than solids but still allow sound to travel faster than in gases.
  • In gases (like air), particles are widely spaced, causing slower sound propagation.

Thus, sound travels fastest in solids, followed by liquids, and slowest in gases.

Question 24:
Read the following case:
A group of students observed that when they clapped their hands near a wall, they heard an echo after a short time. They measured the distance to the wall as 17 meters.
Based on the case, answer:
(a) What is an echo, and why does it occur?
(b) Calculate the time taken for the echo to be heard (Speed of sound = 340 m/s).
Answer:

(a) An echo is a reflected sound wave that reaches the listener after the original sound. It occurs when sound bounces off a hard surface (like a wall) and returns to the source. For an echo to be heard clearly, the reflecting surface must be at least 17 meters away (as per NCERT guidelines).

(b) Calculation:
Total distance traveled by sound = 17 m (to wall) + 17 m (back) = 34 meters.
Speed of sound = 340 m/s.
Time = Distance/Speed = 34/340 = 0.1 seconds.
Thus, the echo is heard after 0.1 seconds.

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