SUMMARY: The chapter on Sound in Class 9 Science explores the nature, production, and propagation of sound waves, as well as their characteristics and applications. KEY TOPICS: Nature of sound, production of sound, propagation of sound, speed of sound, reflection of sound, echo, reverberation, characteristics of sound waves, applications of ultrasound, SONAR.
Sound is a form of energy that is produced by vibrating objects. When an object vibrates, it creates pressure waves in the surrounding medium, which we perceive as sound.
Q173 Marks
Explain the concept of frequency in relation to sound.
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Frequency refers to the number of vibrations or cycles that occur in one second. It is measured in hertz (Hz) and determines the pitch of the sound; higher frequencies correspond to higher pitches.
Q183 Marks
What is the difference between pitch and loudness?
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Pitch is determined by the frequency of the sound wave, while loudness is related to the amplitude of the wave. Higher frequency sounds are perceived as higher pitch, and greater amplitude results in louder sounds.
Q193 Marks
Define the term 'wavelength' in the context of sound waves.
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Wavelength is the distance between two consecutive points that are in phase on a sound wave, such as from crest to crest or trough to trough. It is inversely related to frequency; higher frequencies have shorter wavelengths.
Q203 Marks
What is the speed of sound in air at room temperature?
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The speed of sound in air at room temperature (approximately 20°C) is about 343 meters per second. This speed can vary based on temperature and the medium through which sound travels.
Q213 Marks
How does temperature affect the speed of sound?
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The speed of sound increases with an increase in temperature. This is because warmer air has more energy, causing the molecules to vibrate faster and transmit sound waves more quickly.
Q223 Marks
What is an echo and how does it occur?
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An echo is a reflection of sound that arrives at the listener after a delay. It occurs when sound waves bounce off a surface and return to the listener, typically heard when the distance to the reflecting surface is significant enough to allow for the delay.
Q233 Marks
Explain the phenomenon of resonance with an example.
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Resonance occurs when an object vibrates at its natural frequency due to an external force. For example, a tuning fork can cause a glass to vibrate and potentially shatter if the frequency matches the glass's natural frequency.
Q243 Marks
What role does the medium play in the transmission of sound?
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The medium through which sound travels affects its speed and quality. Sound travels fastest in solids, slower in liquids, and slowest in gases due to differences in particle density and elasticity.
Q253 Marks
Describe how sound waves can be classified.
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Sound waves can be classified as longitudinal or transverse. Longitudinal waves have particles that move parallel to the direction of wave propagation, while transverse waves have particles that move perpendicular to the direction of wave propagation.
Long Answer Questions6 questions
Q266 Marks
Explain the nature of sound waves and how they propagate through different mediums. Include examples of solids, liquids, and gases in your explanation.
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Sound waves are mechanical waves that require a medium to travel through. They propagate by causing the particles of the medium to vibrate. In solids, sound travels fastest because the particles are closely packed, allowing efficient transfer of energy. For instance, sound travels through steel at about 5000 m/s. In liquids, sound travels slower than in solids but faster than in gases; for example, in water, sound travels at approximately 1500 m/s. In gases, sound travels slowest due to the larger distance between particles; for example, in air, sound travels at about 343 m/s. This variation in speed is due to the differences in density and elasticity of the mediums.
Q276 Marks
Describe the phenomenon of echo and the conditions necessary for it to occur. How is this principle applied in real life?
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An echo is the reflection of sound that arrives at the listener after a delay. For an echo to occur, there must be a sufficient distance between the source of the sound and a reflecting surface, typically at least 17 meters away, to allow the sound to travel to the surface and back. The sound must also be loud enough to be heard after reflection. This principle is applied in various real-life situations, such as in concert halls where sound engineers design the space to enhance sound quality by managing echoes. Additionally, echolocation is used by animals like bats and dolphins to navigate and hunt by interpreting echoes of their emitted sounds.
Q286 Marks
Discuss the concept of frequency and amplitude in sound waves. How do they affect the characteristics of sound that we perceive?
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Frequency refers to the number of vibrations or cycles of a sound wave that occur in one second, measured in Hertz (Hz). It determines the pitch of the sound; higher frequencies correspond to higher pitches, while lower frequencies correspond to lower pitches. Amplitude, on the other hand, is the maximum displacement of particles from their rest position in a sound wave and is related to the loudness of the sound. Greater amplitude results in louder sounds, while smaller amplitude produces softer sounds. Together, frequency and amplitude define the quality of sound we perceive, influencing how we distinguish between different musical notes and volumes.
Q296 Marks
Explain how sound is produced and transmitted in musical instruments, using the example of a guitar. Include the role of strings and body in sound production.
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Sound in a guitar is produced primarily by the vibration of its strings. When a string is plucked, it vibrates at a specific frequency, producing sound waves. The pitch of the sound depends on the string's thickness, tension, and length; thinner, tighter strings produce higher pitches, while thicker, looser strings produce lower pitches. The body of the guitar amplifies these vibrations. As the strings vibrate, they transfer energy to the air inside the guitar body, creating sound waves that resonate and project outward. The design of the guitar body, including its shape and material, also influences the quality and volume of the sound produced.
Q306 Marks
What is the Doppler effect? Explain how it can be observed in everyday life with examples.
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The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer moving relative to the wave source. When the source of sound moves towards an observer, the sound waves compress, leading to a higher frequency and a higher pitch. Conversely, when the source moves away, the sound waves stretch, resulting in a lower frequency and a lower pitch. This effect can be observed in everyday life, such as when an ambulance passes by; as it approaches, the siren sounds higher in pitch, and as it moves away, the pitch drops. This phenomenon is also used in radar and astronomy to measure the speed of moving objects.
Q316 Marks
Describe the human ear's structure and its role in hearing. How do different parts contribute to the process of sound perception?
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The human ear consists of three main parts: the outer ear, middle ear, and inner ear. The outer ear collects sound waves and funnels them through the ear canal to the eardrum. When sound waves hit the eardrum, it vibrates, transmitting these vibrations to the ossicles in the middle ear, which amplify the sound. The stapes, the last ossicle, connects to the oval window of the cochlea in the inner ear. The cochlea is filled with fluid and lined with hair cells that convert the vibrations into electrical signals. These signals are then sent to the brain via the auditory nerve, allowing us to perceive sound. Each part of the ear plays a crucial role in transforming sound waves into a form that the brain can interpret.
Assertion–Reason Questions8 questions
Q321 Mark
Assertion (A): Sound travels faster in solids than in liquids.
Reason (R): The particles in solids are more closely packed than in liquids.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q331 Mark
Assertion (A): Ultrasound can be used for medical imaging.
Reason (R): Ultrasound waves are inaudible to the human ear.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q341 Mark
Assertion (A): The speed of sound is constant in all mediums.
Reason (R): The speed of sound varies depending on the medium it travels through.
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Correct answer: Option 4 —
A is false, but R is true.
Q351 Mark
Assertion (A): Echo is a reflection of sound.
Reason (R): Echoes can only be heard in open spaces.
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Correct answer: Option 3 —
A is true, but R is false.
Q361 Mark
Assertion (A): Sound cannot travel through a vacuum.
Reason (R): Sound requires a medium to propagate.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q371 Mark
Assertion (A): The frequency of a sound wave determines its pitch.
Reason (R): Higher frequency sound waves produce lower pitches.
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Correct answer: Option 3 —
A is true, but R is false.
Q381 Mark
Assertion (A): Infrasonic sounds are below the audible range for humans.
Reason (R): Infrasonic sounds can be used for communication between animals.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q391 Mark
Assertion (A): A loud sound has a high amplitude.
Reason (R): Amplitude is directly related to the energy of the sound wave.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Statement-Based Questions8 questions
Q401 Mark
Statement 1: Sound can travel through a vacuum.
Statement 2: Sound is a longitudinal wave.
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Correct answer: Option 3 —
Only Statement 2 is true.
Q411 Mark
Statement 1: Ultrasound is sound with frequencies above the audible range.
Statement 2: Infrasonic waves are below the audible range.
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Correct answer: Option 1 —
Both statements are true.
Q421 Mark
Statement 1: The speed of sound is faster in air than in water.
Statement 2: Sound travels faster in solids than in liquids.
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Correct answer: Option 3 —
Only Statement 2 is true.
Q431 Mark
Statement 1: Echo is a reflection of sound.
Statement 2: Sound cannot be reflected.
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Correct answer: Option 2 —
Only Statement 1 is true.
Q441 Mark
Statement 1: The frequency of a sound wave determines its pitch.
Statement 2: The amplitude of a sound wave determines its loudness.
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Correct answer: Option 1 —
Both statements are true.
Q451 Mark
Statement 1: Sound waves require a medium to travel.
Statement 2: Sound can travel faster in gases than in solids.
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Correct answer: Option 4 —
Both statements are false.
Q461 Mark
Statement 1: The human ear can hear sounds up to 20 kHz.
Statement 2: Sound waves can be polarized.
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Correct answer: Option 2 —
Only Statement 1 is true.
Q471 Mark
Statement 1: Noise pollution can affect human health.
Statement 2: All sounds are considered music.
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Correct answer: Option 2 —
Only Statement 1 is true.
