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NCERT Class 9 Science Important Chapter 10 Sound Waves: Characteristics and Applications
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Sound Waves: Characteristics and Applications
Chapter – 10
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. How is sound produced by objects?
Ans: Sound is produced by the periodic to and fro motion, or vibrations, of an object.
2. How is sound produced in humans?
Ans: In humans, sound is produced by the vibration of vocal cords, which are tightly stretched muscular flaps located inside the voice box or larynx in the throat.
3. Through which states of matter can sound propagate?
Ans: Sound can travel or propagate through solids, liquids, and gases, which act as a medium for the sound waves.
4. Why cannot astronauts talk directly to each other in outer space without special devices?
Ans: Outer space is a near vacuum with no material medium. Since sound needs a medium to propagate, it cannot travel through the vacuum of space.
5. What is a sound wave?
Ans: A sound wave is a disturbance consisting of a series of alternating compressions and rarefactions propagating through a medium, without the actual flow of the medium’s particles.
6. What is a compression in a sound wave?
Ans: A compression is a region in a medium where the particles are pushed together, resulting in a higher density compared to the average density.
7. What is a rarefaction in a sound wave?
Ans: A rarefaction is a region in a medium where the particles move backward and become more spread out, resulting in a lower density compared to the average density.
8. Why is sound considered a longitudinal wave?
Ans: Sound is a longitudinal wave because the particles of the medium vibrate back and forth parallel to the direction of wave propagation.
9. What are mechanical waves?
Ans: Waves that require a material medium for their propagation, such as sound waves and seismic waves, are called mechanical waves.
10. How do particles vibrate in a transverse wave?
Ans: In a transverse wave, the particles of the medium vibrate in a direction perpendicular to the direction of wave propagation.
11. How is the wavelength of a sound wave defined?
Ans: The wavelength is defined as the distance between two consecutive crests (maximum density) or two consecutive troughs (minimum density) in a wave.
12. What is the frequency of a sound wave?
Ans: The frequency of a sound wave is the number of complete density oscillations at a fixed point per unit time, measured in hertz (Hz).
13. How are the time period and frequency of a wave related?
Ans: The time period and frequency are inversely related; the time period is equal to one divided by the frequency (T = 1/\nu).
14. What does the amplitude of a sound wave represent?
Ans: The amplitude is the maximum change in the density of the medium in a compression or rarefaction compared to the average density.
15. What is the mathematical relationship between the speed, wavelength, and frequency of a sound wave?
Ans: The speed of a sound wave is equal to its wavelength multiplied by its frequency (v=\lambda \times \nu).
16. How is the frequency of a sound perceived by humans?
Ans: Humans perceive the frequency of a sound as pitch; sounds with higher frequencies are perceived as having a high pitch, and lower frequencies have a low pitch.
17. How is the amplitude of a sound wave perceived by human ears?
Ans: Humans perceive the amplitude of a sound wave as loudness. Sounds with a larger amplitude carry more energy and are heard louder.
18. What is the audible range of sound frequencies for typical humans?
Ans: The human hearing range, or audible range, is typically from 20 Hz to 20,000 Hz (20 kHz).
19. What is the minimum time gap required between an original sound and its reflection to hear a distinct echo?
Ans: To hear a distinct echo, the time gap between the original sound and the reflected sound reaching the ear must be at least 0.1 seconds.
20. What is the full form of sonar and what is its primary use?
Ans: Sonar stands for Sound Navigation and Ranging. It uses ultrasonic waves to determine the distance, direction, and speed of underwater objects, such as submarines or shipwrecks.
Fill in the Blanks:
1. Sound is produced by the __________ of an object.
Ans: Vibrations.
2. Sound cannot travel through a __________.
Ans: Vacuum.
3. The SI unit of frequency is __________.
Ans: Hertz (Hz).
4. Humans perceive the frequency of sound as __________.
Ans: Pitch.
5. Sonar stands for Sound Navigation and __________.
Ans: Ranging.
True or False:
1. Sound is a longitudinal wave.
Ans: True.
2. Sound can travel through outer space without a medium.
Ans: False.
3. The amplitude of a sound wave determines its loudness.
Ans: True.
4. The audible range of human hearing is 20 Hz to 20,000 Hz.
Ans: True.
5. An echo can be heard if the reflected sound reaches the ear in less than 0.05 seconds.
Ans: False.
Long Questions & Answers:
1. Describe the vacuum bell jar experiment and explain what it proves about the propagation of sound waves.
Ans: The vacuum bell jar experiment is used to demonstrate that sound requires a material medium to propagate and cannot travel through a vacuum. In this experiment, an electric bell is placed inside an airtight glass bell jar connected to a vacuum pump. Initially, when the bell is switched on, its ringing sound is clearly heard through the air inside the jar. However, as the vacuum pump progressively sucks the air out of the jar, the sound of the bell becomes increasingly fainter, even though the bell can still be seen ringing. Once a near vacuum is reached inside the jar, almost no sound can be heard at all, proving that without a medium like air, sound energy cannot be transferred. When air is let back into the jar, the sound gradually becomes loud again, conclusively showing that sound waves are mechanical waves that strictly require a solid, liquid, or gaseous medium for their propagation.
2. Explain how sound propagates through a medium using the concepts of compressions and rarefactions, and describe the movement of the medium’s particles during this process.
Ans: Sound propagates through a medium by creating a series of alternating high-density and low-density regions known as compressions and rarefactions. When a vibrating object, such as an oscillating piston, moves forward, it pushes and displaces the nearby air particles, creating a region of higher density and pressure called a compression, which moves forward as the compressed particles collide with their neighbours. Conversely, when the vibrating object moves backward, the air particles move back towards it, creating a region of lower density and pressure called a rarefaction, which also travels forward following the compression. Throughout this continuous process, the actual particles of the medium do not travel from the source to the listener; instead, they merely oscillate back and forth about their mean resting positions parallel to the direction of the wave’s propagation, which is the fundamental characteristic of a longitudinal mechanical wave.
3. Define the terms wavelength, frequency, amplitude, and speed as they relate to a sound wave, and explain the mathematical relationship between speed, wavelength, and frequency.
Ans: In the context of a sound wave, the wavelength is defined as the distance between two consecutive compressions (crests) or two consecutive rarefactions (troughs). The frequency refers to the number of complete density oscillations that pass a fixed point per unit of time, usually measured in hertz (Hz). The amplitude indicates the maximum change in the density of the medium compared to its average density, which directly relates to the amount of energy the wave carries and its perceived loudness. The speed of sound is the distance a point on the wave, such as a crest, travels in a unit of time, and it depends primarily on the properties of the medium through which it is travelling. Mathematically, these characteristics are linked by the equation stating that the speed of a sound wave is equal to the product of its wavelength and its frequency, meaning that for a given medium where speed is constant, an increase in frequency results in a proportionally shorter wavelength.
4. What is the difference between an echo and reverberation, and what is the minimum distance required to hear a distinct echo in air?
Ans: Both an echo and reverberation are phenomena caused by the reflection of sound waves off hard surfaces, but they differ significantly in how they are perceived by the human ear. An echo is a distinctly heard reflection of a sound that arrives at the listener’s ear at least 0.1 seconds after the original sound was produced, allowing the human brain to distinguish it as a completely separate sound. In contrast, reverberation occurs in large halls or auditoriums when multiple reflections from walls and ceilings arrive at the listener’s ear with a time gap of less than 0.05 seconds, causing the sound to persist, blend together, and sometimes distort the original audio. To hear a distinct echo in air (assuming a sound speed of 340 m/s), the sound must travel a total distance of at least 34 meters in that 0.1-second interval, meaning the minimum distance from the sound source to the reflecting obstacle must be exactly half of that, or 17 meters.
5. What are ultrasonic waves, and what are some of their major applications in nature, medicine, and technology?
Ans: Ultrasonic waves are high-frequency sound waves that exceed 20,000 Hz (20 kHz), which places them completely beyond the upper limit of the human audible range. In nature, nocturnal animals like bats and marine mammals like dolphins utilise these ultrasonic waves for echolocation, emitting short bursts of ultrasound and listening to the returning echoes to navigate safely in the dark and accurately locate their prey or obstacles. In the medical field, ultrasonic waves are heavily utilised for non-invasive diagnostic procedures like ultrasonography to image internal organs without surgery, and for therapeutic purposes such as breaking painful kidney stones into smaller, passable pieces. Technologically, these high-frequency waves are highly valuable in industries for cleaning delicate machine parts, performing ultrasonic welding, detecting hidden defects inside solid metal blocks, and in marine navigation where sonar systems use ultrasound to determine the distance, direction, and speed of underwater objects like submarines or shipwrecks.
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