Chapter 11: Sound Long Questions | science Class 9 CBSE

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Chapter 11: Sound Long Questions | science Class 9 CBSE | ByteNotes.in

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Question 1

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Q1. What is sound and how is it produced? Give examples to support your answer.

• Sound is a form of energy that produces a sensation of hearing.
• It is produced by the vibration of objects.
• Vibrating objects disturb surrounding particles in a medium, producing sound.
• Examples: Vocal cords vibrate to produce voice, a tuning fork vibrates to produce sound.
• Rubbing, plucking, blowing, or striking objects causes them to vibrate and produce sound.
• Without vibration, no sound can be produced.
• All sounds originate from vibrating sources.

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Question 2

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Q2. How does sound propagate through a medium? Explain using compressions and rarefactions.

• Sound propagates through a medium as a disturbance called a wave.
• The vibrating object creates compressions (high pressure) and rarefactions (low pressure) in the medium.
• These pressure variations travel through the medium as longitudinal waves.
• Particles of the medium do not move permanently; they oscillate back and forth.
• The disturbance is passed from one particle to another, carrying sound energy.
• The energy, not the matter, moves through the medium.
• Solids, liquids, and gases can act as mediums for sound propagation.

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Question 3

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Q3. Explain why sound waves are called mechanical and longitudinal waves.

• Mechanical waves need a material medium for propagation.
• Sound waves travel by setting particles of the medium in motion.
• Longitudinal waves are those where particles vibrate parallel to the direction of wave propagation.
• Sound travels as successive compressions and rarefactions.
• In slinky experiments, similar forward and backward motion is observed.
• Sound cannot travel in vacuum as there is no medium.
• Hence, sound is classified as a mechanical longitudinal wave.

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Question 4

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Q4. Describe the characteristics of a sound wave with proper definitions.

• Amplitude: Maximum displacement from mean position; determines loudness.
• Frequency: Number of oscillations per second; measured in hertz (Hz); defines pitch.
• Time period: Time taken for one complete oscillation; inverse of frequency.
• Wavelength (λ): Distance between two consecutive compressions or rarefactions.
• Speed (v): Distance traveled by sound wave in unit time.
• Relation: v = λ × frequency.
• These parameters define the nature and behavior of sound waves.

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Question 5

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Q5. Explain how pitch, loudness, and quality of sound are determined by properties of wave.

• Pitch is determined by the frequency of vibration.
• Higher frequency → higher pitch (e.g., whistle); lower frequency → lower pitch (e.g., drum).
• Loudness depends on amplitude of the sound wave.
• Greater amplitude means louder sound; lesser amplitude means softer sound.
• Quality (timbre) allows us to distinguish between sounds with same pitch and loudness.
• Quality depends on waveform and harmonic content.
• Example: Violin and flute can play same note, but sound different due to quality.

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Question 6

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Q6. State and explain the laws of reflection of sound. Mention one activity to verify it.

• Sound obeys same reflection laws as light.
• Law 1: Angle of incidence = Angle of reflection.
• Law 2: Incident ray, reflected ray, and normal lie in the same plane.
• Activity: Use two cardboard tubes and a clock near a wall to verify angles.
• Sound is reflected from a rigid surface like a wall or metal sheet.
• Applications: Megaphones, stethoscopes, auditoriums.
• Reflection allows sound to be directed, amplified, or heard from a distance.

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Question 7

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Q7. What is an echo? What conditions are required to hear an echo clearly?

• Echo is the repetition of sound caused by its reflection from a distant surface.
• The minimum time gap to hear a distinct echo is 0.1 seconds.
• Distance required for echo = (Speed of sound × Time)/2 = 17.2 meters.
• Echoes can be heard in empty halls, near mountains or buildings.
• Echoes result from multiple reflections of sound.
• Used in SONAR and ultrasonic imaging.
• Speed of sound varies with temperature and medium.

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Question 8

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Q8. What is reverberation? How can it be reduced in auditoriums?

• Reverberation is the persistence of sound due to multiple reflections.
• Causes prolonged echo, making sound unclear.
• In auditoriums, it can affect speech and music clarity.
• Reduced by using sound-absorbing materials like curtains, carpets, fiberboards.
• Walls and ceilings can be designed to absorb or scatter sound.
• Seats may be cushioned to absorb sound.
• Good acoustic design ensures minimal reverberation.

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Question 9

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Q9. Differentiate between loudness and intensity of sound with examples.

• Loudness is the perception of sound by the ear; intensity is objective physical quantity.
• Loudness is influenced by ear sensitivity and surroundings.
• Intensity = Energy per unit area per second.
• Two sounds of same intensity may be perceived differently in loudness.
• Measured intensity does not change with distance, but loudness decreases.
• Loudness depends on amplitude; intensity is energy-based.
• Intensity is measurable; loudness is subjective.

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Question 10

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Q10. Describe the range of hearing in humans. Define infrasonic and ultrasonic waves.

• Human hearing range: 20 Hz to 20,000 Hz (20 kHz).
• Sounds below 20 Hz are called infrasound or infrasonic.
• Sounds above 20,000 Hz are called ultrasound or ultrasonic.
• Infrasound is used by animals like elephants and whales.
• Ultrasound is used by bats, dolphins, and medical diagnostics.
• Human sensitivity decreases with age, especially for higher frequencies.
• Devices like hearing aids help people hear better within this range.

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Chapter 11: Sound | Long Questions

Q1. What is sound and how is it produced? Give examples to support your answer.

Q2. How does sound propagate through a medium? Explain using compressions and rarefactions.

Q3. Explain why sound waves are called mechanical and longitudinal waves.

Q4. Describe the characteristics of a sound wave with proper definitions.

Q5. Explain how pitch, loudness, and quality of sound are determined by properties of wave.

Q6. State and explain the laws of reflection of sound. Mention one activity to verify it.

Q7. What is an echo? What conditions are required to hear an echo clearly?

Q8. What is reverberation? How can it be reduced in auditoriums?

Q9. Differentiate between loudness and intensity of sound with examples.

Q10. Describe the range of hearing in humans. Define infrasonic and ultrasonic waves.

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