Sound

11. Which among the following is true for propagation of sound waves ?

Which among the following is true for propagation of sound waves ?

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Sound can travel in vacuum and it is a transverse wave in air.
Sound cannot travel in vacuum and it is a longitudinal wave in air.
Sound can travel in vacuum and it is a longitudinal wave in air.
Sound cannot travel in vacuum and it is a transverse wave in air.
This question was previously asked in
UPSC NDA-2 – 2021
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The correct option is B. Sound waves are mechanical waves that require a medium to propagate and cannot travel through a vacuum. In air, sound waves are longitudinal waves, meaning the particles of the medium vibrate parallel to the direction of wave propagation.
Sound transmission involves the vibration of particles in a medium. In air, this vibration creates regions of compression and rarefaction that travel through the air as a wave. Because the particle displacement is along the direction of energy propagation, it is a longitudinal wave. Vacuum lacks the particles needed to transmit these vibrations, hence sound cannot travel in a vacuum.
Examples of longitudinal waves include sound waves in fluids and solids, and seismic P-waves. Examples of transverse waves include light waves, waves on the surface of water, and seismic S-waves (in solids). Transverse waves involve particle vibration perpendicular to the direction of wave propagation.

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12. Reverberation is a phenomenon associated with a

Reverberation is a phenomenon associated with a

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multiple refraction of sound.
multiple reflection of sound.
single refraction of sound.
single reflection of sound.
This question was previously asked in
UPSC NDA-2 – 2021
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The correct option is B. Reverberation is the persistence of sound in an enclosed space after the sound source has stopped, caused by multiple reflections of sound waves from surfaces within the space.
Reverberation is distinct from a single echo, which is a clearly discernible reflection. Reverberation occurs when reflected sound waves arrive at the listener in rapid succession, blending together and prolonging the original sound. It is a phenomenon resulting from numerous reflections.
Refraction of sound is the bending of sound waves as they pass from one medium to another or through a medium with varying properties (like temperature or density gradients). Single reflection is responsible for echoes or contributes to the overall sound experience in a space. Reverberation time is a key parameter in architectural acoustics, describing how long it takes for sound intensity to decay by 60 dB after the source stops.

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13. Which one of the following statements about sound is NOT correct ?

Which one of the following statements about sound is NOT correct ?

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Sound travels at a speed slower than the speed of light
Sound waves are transverse waves
Sound waves are longitudinal waves
Sound travels faster in moist air than in dry air
This question was previously asked in
UPSC NDA-2 – 2020
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Let’s evaluate each statement:
A) Sound travels at a speed slower than the speed of light: This is correct. The speed of light in vacuum is approximately 3 x 10⁸ m/s, while the speed of sound in air is approximately 343 m/s at room temperature.
B) Sound waves are transverse waves: This is NOT correct. Sound waves in a gas or liquid are longitudinal waves, meaning the particle displacement is parallel to the direction of wave propagation (compressions and rarefactions). Sound waves in solids can have both longitudinal and transverse components.
C) Sound waves are longitudinal waves: This is correct for sound in air or liquids.
D) Sound travels faster in moist air than in dry air: This is generally correct. At the same temperature and pressure, moist air is slightly less dense than dry air because water molecules (molecular mass ~18) replace nitrogen (N₂, ~28) and oxygen (O₂, ~32) molecules. While density decreases, the bulk modulus also changes. The effect of lower density dominates, leading to a slightly higher speed of sound in moist air.
The statement that is NOT correct is B.
Sound waves in fluids (like air) are longitudinal waves. Transverse waves have particle displacement perpendicular to the direction of propagation (e.g., light waves, waves on a string).
The speed of sound depends on the properties of the medium (elasticity and density). It is faster in solids than in liquids, and faster in liquids than in gases, primarily due to differences in elasticity (compressibility). Temperature also affects the speed of sound in gases.

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14. Which of the following statements is NOT correct regarding the travel

Which of the following statements is NOT correct regarding the travel of sound waves ?

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Sound waves can travel through water
Sound waves can travel through air
Sound waves can travel through steel
Sound waves can travel through vacuum
This question was previously asked in
UPSC NDA-2 – 2020
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Sound waves are mechanical waves, which means they require a medium (like solid, liquid, or gas) to travel. They cannot travel through a vacuum.
– Sound propagates through a medium by causing vibrations in the particles of that medium.
– In a vacuum, there are virtually no particles, so there is no medium for the sound waves to propagate through.
– Options A, B, and C state that sound waves can travel through water (liquid), air (gas), and steel (solid), respectively. These statements are correct as these are all forms of matter that can act as mediums for sound transmission.
– This is why sound cannot be heard in outer space, which is essentially a vacuum.

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15. Compared to audible sound waves, ultrasound waves have

Compared to audible sound waves, ultrasound waves have

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higher speed.
higher frequency.
longer wavelength.
both higher speed and frequency.
This question was previously asked in
UPSC NDA-2 – 2019
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The correct option is B) higher frequency.
Ultrasound waves are defined as sound waves with frequencies higher than the upper limit of human hearing, which is typically considered to be 20 kilohertz (kHz). Audible sound waves have frequencies between approximately 20 Hz and 20 kHz.
The speed of sound in a given medium is generally constant for all frequencies. The relationship between speed (v), frequency (f), and wavelength (λ) is v = fλ. Since ultrasound has a higher frequency (f) than audible sound in the same medium with constant speed (v), its wavelength (λ) must be shorter. Therefore, compared to audible sound waves, ultrasound waves have higher frequency and shorter wavelength, but the speed remains the same in the same medium.

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16. The loudness of sound depends upon the

The loudness of sound depends upon the

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velocity of sound waves in the medium.
amplitude of the sound waves.
frequency of the sound waves.
frequency and velocity of the sound waves.
This question was previously asked in
UPSC NDA-2 – 2019
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The correct answer is B) amplitude of the sound waves.
Loudness is the perceptual quality of sound that is most closely associated with intensity. Intensity is proportional to the square of the amplitude of the sound wave. A larger amplitude corresponds to a higher intensity and thus a louder sound.
The frequency of sound waves determines the pitch of the sound. The velocity of sound waves in a medium depends on the properties of the medium (like density and elasticity) and temperature, but it does not directly determine the loudness for a given source and medium. While velocity and frequency are related to wavelength ($\text{velocity} = \text{frequency} \times \text{wavelength}$), loudness is fundamentally related to the energy or power carried by the wave, which is proportional to the square of its amplitude.

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17. The frequency of ultrasound waves is

The frequency of ultrasound waves is

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less than 20 Hz
between 20 Hz and 2 kHz
between 2 kHz and 20 kHz
greater than 20 kHz
This question was previously asked in
UPSC NDA-2 – 2018
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The frequency of ultrasound waves is greater than 20 kHz.
– The audible range of sound frequencies for humans is typically considered to be from 20 Hz to 20,000 Hz (20 kHz).
– Sound waves with frequencies below 20 Hz are called infrasound.
– Sound waves with frequencies above 20 kHz are called ultrasound.
Ultrasound has various applications in medicine (imaging, therapy), industry (cleaning, welding), and technology (sonar, pest control). The exact upper limit of human hearing varies with age and individual.

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18. The following six (6) items consist of two statements, Statement I and

The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below.
Statement I :
Sound wave cannot propagate in vacuum.
Statement II :
Sound waves are elastic waves and require a medium to propagate.

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Both the statements are individually true and Statement II is the correct explanation of Statement I
Both the statements are individually true but Statement II is not the correct explanation of Statement I
Statement I is true but Statement II is false
Statement I is false but Statement II is true
This question was previously asked in
UPSC NDA-2 – 2018
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Statement I is true: Sound waves are mechanical waves, meaning they require a material medium (like air, water, or solids) to propagate by vibrating particles. A vacuum is devoid of such particles, so sound cannot travel through it. Statement II is true: Sound waves are elastic waves (a type of mechanical wave) that cause disturbances in the medium’s particles, which are then transmitted through the medium. This reliance on particle interaction directly explains why a medium is required for propagation. Therefore, Statement II is the correct explanation for Statement I.
– Sound is a form of energy that travels as waves.
– Sound waves are longitudinal mechanical waves, causing compressions and rarefactions in the medium.
– The presence of a medium is essential for the vibrations to be transmitted.
Electromagnetic waves (like light, radio waves, X-rays) are not mechanical waves and can travel through a vacuum because they do not require a medium for propagation.

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19. The following six (6) items consist of two statements, Statement I and

The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below.
Statement I :
The pitch of the sound wave depends upon its frequency.
Statement II :
The loudness of the sound wave depends upon its amplitude.

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Both the statements are individually true and Statement II is the correct explanation of Statement I
Both the statements are individually true but Statement II is not the correct explanation of Statement I
Statement I is true but Statement II is false
Statement I is false but Statement II is true
This question was previously asked in
UPSC NDA-2 – 2018
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Statement I is true: The pitch of a sound wave is determined by its frequency; higher frequency corresponds to higher pitch. Statement II is also true: The loudness (intensity) of a sound wave is related to its amplitude; greater amplitude corresponds to greater loudness. However, these are two distinct properties of sound waves that are not causally related to each other in the sense that the mechanism determining loudness does not explain why pitch is determined by frequency.
– Pitch is the perceptual property that allows sounds to be ordered on a frequency-related scale, or more commonly, pitch is the quality that makes it possible to judge sounds as “higher” and “lower”.
– Loudness is the subjective perception of sound pressure. It is related to the intensity of the sound wave, which in turn is proportional to the square of the amplitude.
– Frequency and amplitude are independent physical characteristics of a wave.
While both statements correctly describe fundamental properties of sound waves, the fact that loudness depends on amplitude does not explain why pitch depends on frequency. Therefore, Statement II does not explain Statement I.

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20. Which one of the following statements is not correct ?

Which one of the following statements is not correct ?

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Ultrasonic waves cannot get reflected, refracted or absorbed.
Ultrasonic waves are used to detect the presence of defects like cracks, porosity, etc. in the internal structure of common structure materials.
Ultrasonic waves can be used for making holes in very hard materials like diamond.
Ultrasonic waves cannot travel through vacuum.
This question was previously asked in
UPSC NDA-2 – 2017
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The correct answer is A.
Statement A is incorrect. Like all mechanical waves, ultrasonic waves exhibit properties such as reflection, refraction, absorption, and diffraction when they interact with different media or obstacles.
Statement B is correct; ultrasonic testing (UT) is widely used in non-destructive testing to detect internal flaws in materials. Statement C is correct; high-intensity ultrasonic vibrations are used in ultrasonic machining to drill holes in hard materials. Statement D is correct; ultrasonic waves are mechanical waves and require a material medium (solid, liquid, or gas) to propagate, thus they cannot travel through a vacuum.

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