CBSE Class 9 Science Chapter 10 Sound Waves Characteristics And Applications MCQs Set 01

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Question: When you pluck a rubber band stretched across a cardboard box, why does the sound stop when the vibration stops?
A. Sound is stored in the rubber band material
B. The vibration is necessary to continuously transfer energy to the medium
C. The rubber band becomes too weak to vibrate again
D. Sound waves can only travel for a fixed distance from the source
Show Answer & Explanation

Answer: (B) The vibration is necessary to continuously transfer energy to the medium

Explanation:
Sound is produced when an object vibrates and transfers energy to the surrounding medium. Once the vibration ceases, no new energy is being transferred, so sound production stops. The source must continuously vibrate to maintain sound generation.

Question: In the vacuum bell jar experiment, sound becomes fainter as air is removed, demonstrating what key principle about sound propagation?
A. Sound waves lose energy over long distances
B. A material medium is essential for sound to travel
C. The bell is gradually losing its ability to ring
D. Vacuum itself can carry sound but less efficiently than air
Show Answer & Explanation

Answer: (B) A material medium is essential for sound to travel

Explanation:
As air is progressively removed from the bell jar, the intensity of sound decreases until it becomes inaudible despite the bell still visibly vibrating. This directly shows that sound requires a material medium—without matter to transmit the disturbance, sound cannot propagate.

Question: How do compressions and rarefactions move through air differently from how air particles themselves move?
A. Compressions move forward while particles move backward
B. Particles travel with the wave while compressions stay in one location
C. Compressions propagate forward while particles oscillate about their mean positions
D. Both move in identical ways but at different speeds
Show Answer & Explanation

Answer: (C) Compressions propagate forward while particles oscillate about their mean positions

Explanation:
This is a fundamental distinction in wave mechanics. While compressions and rarefactions (the density disturbances) travel outward from the source, individual air particles only vibrate back and forth around their equilibrium positions. The wave pattern moves forward, but the medium's particles do not travel with it.

Question: If a sound wave has a frequency of 5 Hz, how many complete oscillations occur at a fixed point in 10 seconds?
A. 2 oscillations
B. 5 oscillations
C. 50 oscillations
D. 15 oscillations
Show Answer & Explanation

Answer: (C) 50 oscillations

Explanation:
Frequency is defined as the number of oscillations per unit time. With a frequency of 5 Hz, there are 5 oscillations each second. Over 10 seconds, the total number is 5 × 10 = 50 oscillations.

Question: Why do astronauts in spacesuits communicating during a spacewalk need electronic devices rather than hearing each other directly?
A. Space is too cold for sound to travel
B. Their spacesuits block sound completely
C. Space is essentially a vacuum and sound requires a material medium to propagate
D. Sound travels too slowly to reach across space
Show Answer & Explanation

Answer: (C) Space is essentially a vacuum and sound requires a material medium to propagate

Explanation:
In outer space, there is a near vacuum—almost no air or matter present. Since sound is a mechanical wave requiring a medium to travel, sound cannot propagate in vacuum. Astronauts must use radios or other electronic communication systems built into their suits.

Question: Which characteristic of a sound wave is most directly responsible for how loud or soft we perceive a sound to be?
A. Wavelength of the wave
B. Frequency of the wave
C. Amplitude of the wave
D. Speed of the wave
Show Answer & Explanation

Answer: (C) Amplitude of the wave

Explanation:
Amplitude measures the maximum change in density during compression or rarefaction. Larger amplitude waves carry more energy and are perceived as louder by human listeners, while smaller amplitude waves sound softer. This is the primary link between a physical wave property and our subjective perception of loudness.

Question: Two identical tuning forks are struck, one in air and one inside a sealed container filled with water. Which will be heard as louder by a person listening through the water, assuming both containers are at the same distance?
A. The one in air, because air transmits sound better than water
B. The one in water, because sound travels faster in water and carries more energy
C. Both will sound equally loud because frequency determines loudness
D. The one in air, because water absorbs all sound
Show Answer & Explanation

Answer: (B) The one in water, because sound travels faster in water and carries more energy

Explanation:
• Sound travels approximately 4–5 times faster in water than in air
• Higher speed in denser media correlates with better energy transmission
• The tuning fork in water transfers more energy to a denser medium, resulting in greater intensity at the listener's location
• Intensity is the key factor determining perceived loudness when the sources are identical

Question: From a density vs. distance graph of a sound wave, how would you identify the wavelength?
A. By measuring the height of the highest peak
B. By finding the horizontal distance between two consecutive crests or two consecutive troughs
C. By counting the total number of peaks in the graph
D. By measuring the vertical distance from the average density line to a trough
Show Answer & Explanation

Answer: (B) By finding the horizontal distance between two consecutive crests or two consecutive troughs

Explanation:
Wavelength is defined as the distance between two consecutive crests (regions of maximum density) or two consecutive troughs (regions of minimum density). On a spatial graph, this distance can be measured directly along the horizontal axis.

Question: How would increasing the temperature of air affect the time it takes for sound to travel a fixed distance?
A. The time would increase because warm air is less dense
B. The time would decrease because sound travels faster at higher temperatures
C. The time would remain exactly the same regardless of temperature
D. The effect depends on whether the sound is produced by a musical instrument
Show Answer & Explanation

Answer: (B) The time would decrease because sound travels faster at higher temperatures

Explanation:
The chapter states that speed of sound in air increases with temperature. For example, sound travels at about 331 m/s at 0°C but nearly 344 m/s at 22°C. Since distance is fixed and speed increases with temperature, the time required (calculated as distance ÷ speed) must decrease.

Question: When you hear an echo in a canyon, what is the minimum distance the reflecting surface must be from you for the echo to be clearly distinguishable from the original sound?
A. 5 meters
B. 10 meters
C. 17 meters
D. 34 meters
Show Answer & Explanation

Answer: (C) 17 meters

Explanation:
The brain requires at least 0.1 seconds between two sounds to perceive them as separate. At a speed of 340 m/s, sound travels 34 meters in 0.1 seconds. Since the sound must travel to the surface and back, the minimum distance is half of 34 meters, which equals 17 meters.

Question: A musical note produced by plucking a sitar string contains a fundamental frequency and overtones. What is the primary reason different instruments sound unique even when playing the same note at the same loudness?
A. They use different materials which change the frequency
B. The pattern and intensity of overtones differs among instruments, creating distinct timbre
C. Overtones can only be produced by string instruments, not wind instruments
D. The speed of sound differs depending on the instrument type
Show Answer & Explanation

Answer: (B) The pattern and intensity of overtones differs among instruments, creating distinct timbre

Explanation:
Timbre is the unique quality that makes each instrument distinctive. Even when playing the same fundamental frequency at identical amplitude, instruments differ in their construction, shape, and material—all of which determine which overtones are produced and how intensely. This specific combination of overtones creates the characteristic sound of each instrument.

Question: If a sonar signal takes 2 seconds to return to a ship after being sent into the ocean, and sound travels at 1500 m/s in seawater, how far away is the underwater object?
A. 750 meters
B. 1500 meters
C. 2000 meters
D. 3000 meters
Show Answer & Explanation

Answer: (B) 1500 meters

Explanation:
• Sound travels to the object and back, so total distance = speed × total time
• Total distance traveled = 1500 m/s × 2 s = 3000 m
• This is the round-trip distance
• Distance to object = 3000 m ÷ 2 = 1500 m

Question: During an explosion in a movie, viewers see the flash of light and hear the sound simultaneously. What is physically incorrect about this depiction?
A. Light and sound should both be visible first
B. Sound reaches the viewer much faster than light in reality
C. Light travels at a fixed speed while sound speed varies with medium
D. In real situations, light would be seen before the sound is heard because light travels vastly faster than sound
Show Answer & Explanation

Answer: (D) In real situations, light would be seen before the sound is heard because light travels vastly faster than sound

Explanation:
Light travels at approximately 300,000 km/s while sound travels at roughly 340 m/s in air. This means light reaches the observer almost instantaneously while sound takes considerable time, especially at large distances. In a real explosion, a distant observer would see the flash first and hear the sound much later. Movies showing them simultaneously are scientifically inaccurate.

Question: Why are bats able to locate objects in complete darkness while humans cannot?
A. Bats have larger ears that collect more sound
B. Bats emit and interpret ultrasonic waves which are outside the human hearing range
C. Bats' eyes are extremely sensitive in low light conditions
D. Bats can hear infrasound which carries information about distant objects
Show Answer & Explanation

Answer: (B) Bats emit and interpret ultrasonic waves which are outside the human hearing range

Explanation:
Most bats use echolocation by emitting short bursts of ultrasonic waves (above 20 kHz) which humans cannot hear. These waves reflect off nearby objects, and the bat interprets the returning echoes to determine position and distance. Humans are limited to the audible frequency range of 20 Hz to 20 kHz, making echolocation impractical for us.

Question: In modern concert halls, soft porous materials like curtains and upholstered chairs are intentionally placed on walls and surfaces. What acoustic problem do these materials help solve?
A. They amplify weak sounds so the audience can hear better
B. They reduce unwanted reverberations by absorbing sound energy
C. They speed up the propagation of sound through the hall
D. They prevent echoes from forming at all
Show Answer & Explanation

Answer: (B) They reduce unwanted reverberations by absorbing sound energy

Explanation:
Reverberation occurs when multiple reflections from hard surfaces cause sound to persist after the source stops. Hard, smooth surfaces reflect sound strongly, leading to multiple sound reflections arriving with small time differences (less than 0.05 s). Soft, porous materials absorb sound energy rather than reflecting it, thereby controlling excessive reverberation and ensuring clear, undistorted audio for the audience.

Question: When a tuning fork is struck and held near your ear in different orientations, you still hear the sound. What does this observation tell you about how sound spreads from a vibrating source?
A. Sound travels only in the direction the prongs of the fork point
B. Sound propagates outward in multiple directions from the source
C. Sound requires the fork to be held at a specific angle to reach the ear
D. Sound travels faster when the fork is oriented vertically
Show Answer & Explanation

Answer: (B) Sound propagates outward in multiple directions from the source

Explanation:
In Activity 10.2, holding a vibrating tuning fork near the ear in various orientations still produces audible sound, indicating that the disturbance spreads in all directions from the source rather than along a single path.

Question: According to the chapter, which of the following best explains why the particles in a slinky vibrate back and forth but do not travel along with the disturbance?
A. The disturbance moves faster than the particles can follow
B. Each particle oscillates about its own equilibrium position while the wave pattern itself advances
C. The slinky material is too heavy for particles to move with the wave
D. Particles are held in place by friction with neighboring particles
Show Answer & Explanation

Answer: (B) Each particle oscillates about its own equilibrium position while the wave pattern itself advances

Explanation:
The slinky experiment demonstrates that while the disturbance (regions of closer and spread-out coils) travels along the length, the mark on the slinky only oscillates parallel to the wave direction—it does not move along with the wave pattern.

Question: A source produces sound waves with two different frequencies: 100 Hz and 200 Hz. If both sounds travel through the same medium at the same speed, which sound has the longer wavelength?
A. The 200 Hz sound has a longer wavelength
B. The 100 Hz sound has a longer wavelength
C. Both have the same wavelength since they travel at the same speed
D. Wavelength cannot be determined without knowing the amplitude
Show Answer & Explanation

Answer: (B) The 100 Hz sound has a longer wavelength

Explanation:
Using v = λν, for a fixed speed, a lower frequency corresponds to a longer wavelength. Since 100 Hz is lower than 200 Hz, the 100 Hz sound has the greater wavelength.

Question: In a graph showing density variation against distance for a sound wave, the regions marked as compressions appear above the average density line while rarefactions appear below it. What creates these alternating regions as sound propagates?
A. The source vibrating back and forth, pushing and pulling the medium alternately
B. The reflection of sound from distant surfaces
C. The cooling and heating of the medium
D. The different speeds of sound at different distances
Show Answer & Explanation

Answer: (A) The source vibrating back and forth, pushing and pulling the medium alternately

Explanation:
As illustrated with the piston model in section 10.3, forward motion of the vibrating source compresses the medium (compression), and backward motion creates regions of lower density (rarefaction). These alternate continuously as the source oscillates.

Question: A student measures that 15 density oscillations occur at a fixed point in 3 seconds. What is the time period of this sound wave?
A. 0.2 seconds
B. 5 seconds
C. 15 seconds
D. 45 seconds
Show Answer & Explanation

Answer: (A) 0.2 seconds

Explanation:
Frequency = 15 oscillations ÷ 3 seconds = 5 Hz. Using the relation ν = 1/T (Equation 10.1), the time period T = 1/5 = 0.2 seconds.

Question: Sound travels at 340 m/s in air at 22°C but only 331 m/s at 0°C. If a sound wave has a frequency of 170 Hz at both temperatures, how does its wavelength change?
A. • Wavelength increases at 22°C because sound travels faster • Using v = λν, a higher speed with constant frequency requires longer wavelength • At 22°C: λ = 340/170 = 2 m; At 0°C: λ = 331/170 ≈ 1.95 m
B. The wavelength remains constant because frequency is unchanged
C. The wavelength decreases at 22°C because higher temperature compresses the wave
D. Wavelength cannot be compared without knowing the medium's density
Show Answer & Explanation

Answer: (A) • Wavelength increases at 22°C because sound travels faster • Using v = λν, a higher speed with constant frequency requires longer wavelength • At 22°C: λ = 340/170 = 2 m; At 0°C: λ = 331/170 ≈ 1.95 m

Explanation:
• Wavelength increases at 22°C because sound travels faster
• Using v = λν, a higher speed with constant frequency requires longer wavelength
• At 22°C: λ = 340/170 = 2 m; At 0°C: λ = 331/170 ≈ 1.95 m

Question: In an auditorium, if the time delay between hearing the direct sound from the stage and hearing its reflection from the back wall is 0.04 seconds, will the audience experience this as an echo or reverberation?
A. An echo, because any reflected sound is called an echo
B. Reverberation, because the time delay is less than 0.05 seconds
C. An echo, because the time is less than 0.1 seconds
D. Neither echo nor reverberation, because sound does not reflect in auditoriums
Show Answer & Explanation

Answer: (B) Reverberation, because the time delay is less than 0.05 seconds

Explanation:
According to section 10.7.2, reverberation occurs when sound reflections arrive with a time difference less than 0.05 seconds. Since 0.04 seconds is less than this threshold, the listener perceives the reflections as reverberation rather than as separate sounds (echo).

Question: A dolphin uses echolocation to hunt fish. Its emitted ultrasonic waves have a frequency of 40 kHz. Why are ultrasonic waves more suitable for this purpose than audible frequency sound waves?
A. Ultrasonic waves have higher amplitude and carry more energy
B. Ultrasonic waves have shorter wavelengths, allowing detection of smaller objects and better spatial resolution
C. Ultrasonic waves travel faster through water than audible sounds
D. Audible frequency sounds are absorbed by water while ultrasonic waves are not
Show Answer & Explanation

Answer: (B) Ultrasonic waves have shorter wavelengths, allowing detection of smaller objects and better spatial resolution

Explanation:
Higher frequency sound (ultrasonic) has correspondingly shorter wavelength according to v = λν. Shorter wavelengths allow the dolphin to detect finer details and locate smaller prey with greater precision compared to lower frequency audible sounds.

Question: In Activity 10.6, when a loud sound is produced near a sheet stretched over a container, the grains on the sheet jump. This observation demonstrates which property of sound?
A. Sound is a transverse wave
B. Sound requires a vacuum to propagate
C. Sound carries energy that can be transferred to matter
D. Sound has a fixed frequency regardless of source
Show Answer & Explanation

Answer: (C) Sound carries energy that can be transferred to matter

Explanation:
The jumping of grains shows that the vibrating medium (air) transfers energy to the sheet, causing it to vibrate and move the grains. This directly demonstrates that sound is a form of energy in motion.

Question: Three sound waves have the following characteristics: Wave A has high frequency and large amplitude; Wave B has low frequency and large amplitude; Wave C has high frequency and small amplitude. Arranged from loudest to softest perception, what is the correct order?
A. A, B, C
B. B, A, C
C. A, C, B
D. C, B, A
Show Answer & Explanation

Answer: (B) B, A, C

Explanation:
Loudness is determined by amplitude, not frequency. Both Waves A and B have large amplitude and would sound loud, while Wave C with small amplitude would sound soft. Between A and B, both are equally loud since loudness depends on amplitude. The order from loudest to softest based on amplitude alone is: A and B (equal loudness), then C. Among the given options, B, A, C reflects that large-amplitude waves (B and A) are louder than small-amplitude Wave C.

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