Sound

Q100. Consider these statements about sound:
I. Sound speed decreases with increasing temperature at constant atmospheric pressure.
II. It increases with humidity.
III. At constant temperature it does not change when atmospheric pressure changes.

Which are correct?

[UPSC NDA II, September 2026]

• (A) I and II only
• (B) II and III only
• (C) I and III only
• (D) I, II and III

Answer: (B) II and III only

Explanation: For an ideal gas, sound speed is proportional to the square root of absolute temperature, so statement I is false. Moisture reduces average molecular mass, increasing speed. At fixed temperature, changing pressure also changes density proportionately, leaving their ratio unchanged.

Q99. The speed of sound in a large ideal-gas container is x. After doubling pressure at constant temperature, it becomes y. Find x/y.

[UPSC NDA I, April 2026]

• (A) 1
• (B) √2
• (C) 2
• (D) 4

Answer: (A) 1

Explanation: The ideal-gas speed of sound is √(γRT/M), determined by composition and absolute temperature. Doubling pressure at the same temperature doubles density as well. Therefore the ratio of elastic modulus to density remains unchanged, and x equals y.

Q98. What is the persistence of sound caused by repeated reflections called?

[RRB NTPC Undergraduate, 18 June 2026]

• (A) Reverberation
• (B) Resonance
• (C) Quality
• (D) Echo

Answer: (A) Reverberation

Explanation: Reverberation is the continued presence of sound after its source has stopped because waves repeatedly reflect from the surrounding surfaces. Unlike a distinct echo, these reflections overlap closely in time and make a sound linger in a hall.

Q97. Why can an echo be heard when a sound is produced beside a large flat wall?

[RRB Group D, 20 August 2026]

• (A) The sound reflects and returns after a delay
• (B) The wall generates an identical new sound
• (C) Sound passes through and returns
• (D) The wall absorbs and slowly releases all sound

Answer: (A) The sound reflects and returns after a delay

Explanation: A sufficiently hard, distant wall reflects part of the outgoing sound wave. The reflected wave returns to the listener after traveling the outward and return distances. A perceptible delay separates this reflected sound from the initial sound.

Q96. A vibrating tuning fork produces which kind of wave in surrounding air?

[RRB Group D, 17 August 2026]

• (A) Circular surface wave
• (B) Longitudinal wave with compressions and rarefactions
• (C) Wave with no particle motion
• (D) Transverse wave with crests and troughs

Answer: (B) Longitudinal wave with compressions and rarefactions

Explanation: The prongs of a vibrating tuning fork alternately squeeze and release neighboring air. This produces regions of high pressure and low pressure moving outward. Air particles oscillate approximately parallel to the propagation direction, characterizing a longitudinal wave.

Q95. Which condition is necessary for a distinct echo?

[RRB Group D, 17 August 2026]

• (A) The listener must move
• (B) At least about 0.1 s between original and reflected sound
• (C) A soft reflecting surface
• (D) An extremely powerful sound source

Answer: (B) At least about 0.1 s between original and reflected sound

Explanation: Human hearing retains an auditory sensation for roughly one-tenth of a second. To distinguish the reflected sound as a separate echo, it should return after this interval. At 340 m/s, the reflecting surface is roughly 17 m away.

Q94. Two sounds have the same frequency but different amplitudes. What characteristic differs?

[RRB Group D, 14 August 2026]

• (A) Direction
• (B) Pitch
• (C) Loudness
• (D) Speed in the same medium

Answer: (C) Loudness

Explanation: Equal frequencies ordinarily give equal pitch, provided other conditions are comparable. Greater amplitude means larger particle displacement and greater sound intensity, so the sound is perceived as louder. In the same medium, wave speed is essentially unaffected by amplitude.

Q93. How does sound normally travel through different material states?

[RRB Group D, 12 August 2026]

• (A) At the same speed in all materials
• (B) Fastest in gases and slowest in solids
• (C) Fastest in liquids and slowest in gases
• (D) Faster in solids than in liquids or gases

Answer: (D) Faster in solids than in liquids or gases

Explanation: Sound speed depends on elastic stiffness as well as density. Under everyday conditions, typical solids transmit sound more quickly than liquids, and liquids more quickly than gases. Strong elastic restoring forces in solids facilitate rapid transmission of disturbances.

Q92. Two sounds have identical amplitudes but different frequencies. Which statement is correct?

[RRB Group D, 12 August 2026]

• (A) Same frequency, different amplitudes
• (B) Same pitch, different loudness
• (C) Same loudness but different pitches
• (D) Different speeds in the same medium

Answer: (C) Same loudness but different pitches

Explanation: Amplitude chiefly determines intensity and perceived loudness, whereas frequency chiefly determines pitch. Therefore, waves with identical amplitudes but different frequencies can sound equally loud while having different pitches. The propagation speed is set mainly by the medium.

Q91. A student hears an echo one second after shouting toward a building 176 m away. What is the speed of sound?

[RRB Group D, 12 August 2026]

• (A) 400 m/s
• (B) 352 m/s
• (C) 330 m/s
• (D) 176 m/s

Answer: (B) 352 m/s

Explanation: For a reflected sound, the wave must travel to the building and back. Its total path is twice 176 m, or 352 m. Dividing that round-trip distance by the one-second delay gives a speed of 352 m/s.

Q90. A musical instrument emits 200 Hz. Which frequency would give a higher pitch?

[RRB Group D, 11 August 2026]

• (A) 150 Hz
• (B) 180 Hz
• (C) 350 Hz
• (D) 120 Hz

Answer: (C) 350 Hz

Explanation: Pitch generally rises as frequency rises. Among the four choices, only 350 Hz exceeds the reference frequency of 200 Hz. The others indicate slower oscillation and hence lower pitch, even if their amplitudes could make them louder.

Q89. What feature is characteristic of longitudinal sound waves?

[RRB Group D, 11 August 2026]

• (A) Only vertical vibrations
• (B) Side-to-side transverse pulses
• (C) Compressions and rarefactions
• (D) Crests and troughs moving transversely

Answer: (C) Compressions and rarefactions

Explanation: A longitudinal sound wave comprises alternating compressed and expanded parts of the medium. These regions have differing pressure and density. Particles oscillate parallel to the direction of wave travel rather than making perpendicular oscillations characteristic of transverse waves.

Q88. How is reverberation in a room commonly measured?

[RRB Group D, 10 August 2026]

• (A) By sound speed
• (B) By counting echoes
• (C) By measuring wave frequency
• (D) By timing sound decay after its source stops

Answer: (D) By timing sound decay after its source stops

Explanation: Reverberation time quantifies how long sound persists as multiple reflections lose energy. Standard acoustic measurement often uses the time required for the sound level to decrease by 60 dB after excitation ceases, helping assess room acoustics.

Q87. A sound wave travels at 250 m/s and has wavelength 0.5 m. Find its frequency.

[RRB Group D, 9 August 2026]

• (A) 500 Hz
• (B) 250 Hz
• (C) 125 Hz
• (D) 1000 Hz

Answer: (A) 500 Hz

Explanation: Use the fundamental wave relationship v = fλ, where speed equals frequency multiplied by wavelength. Rearranging gives f = 250/0.5 = 500 cycles per second. Each cycle passing a point contributes one hertz of frequency.

Q86. How are compressions and rarefactions produced as sound travels in air?

[RRB Group D, 9 August 2026]

• (A) Particles spiral
• (B) Particle separation never changes
• (C) Particles permanently stick together
• (D) Particles alternately crowd together and spread apart

Answer: (D) Particles alternately crowd together and spread apart

Explanation: As a vibrating source pushes nearby air, it creates dense, high-pressure compressions. When it retreats, particles become more separated, creating low-pressure rarefactions. This repeating disturbance travels through air while individual molecules merely oscillate about equilibrium positions.

Q85. Which situation demonstrates that vibration is needed to produce sound?

[RRB Group D, 25 August 2026]

• (A) Holding a still metal rod
• (B) Plucking a stretched guitar string and hearing a note
• (C) Shining a flashlight
• (D) Watching an undisturbed bottle

Answer: (B) Plucking a stretched guitar string and hearing a note

Explanation: Plucking displaces a taut guitar string, which then vibrates repeatedly. Its movement pushes and pulls surrounding air, creating pressure waves heard as a musical tone. The other situations do not show a vibrating source generating audible sound.

Q84. Why does a vibrating tuning fork make ripples when placed against water?

[RRB Group D, 18 August 2026]

• (A) Its sound does not affect water
• (B) It heats the water
• (C) Water ripples spontaneously
• (D) Mechanical vibrations transfer energy to water

Answer: (D) Mechanical vibrations transfer energy to water

Explanation: Vibrating prongs periodically displace the nearby water when they touch its surface. The transferred mechanical energy disturbs the surface and generates ripples. This demonstration visibly confirms that a sounding tuning fork is actually moving back and forth.

Q83. A sound wave has speed 320 m/s and wavelength 0.4 m. What is its frequency?

[RRB Group D, 9 February 2026]

• (A) 800 Hz
• (B) 1280 Hz
• (C) 80 Hz
• (D) 128 Hz

Answer: (A) 800 Hz

Explanation: The wave equation relates speed, wavelength and frequency as v = fλ. Divide 320 metres per second by 0.4 metres per cycle. This gives 800 cycles each second, or a frequency of 800 Hz.

Q82. For a sound wave,
I. Compression is a region of high pressure;
II. Rarefaction is a region of low pressure.

Which is correct?

[SSC MTS 2025 examination, 11 February 2026]

• (A) Only I
• (B) Only II
• (C) Both I and II
• (D) Neither I nor II

Answer: (C) Both I and II

Explanation: Both statements describe the alternating density changes in a longitudinal acoustic wave. Compression occurs where air particles are crowded and local pressure rises. Rarefaction occurs where particles are further apart and local pressure is reduced below equilibrium.

Q81. Why are many concert-hall and cinema-hall ceilings curved?

[SSC MTS 2025 examination, 5 February 2026]

• (A) To direct reflected sound toward all parts of the hall
• (B) To stop sound
• (C) To focus all sound into one direction
• (D) To absorb all sound

Answer: (A) To direct reflected sound toward all parts of the hall

Explanation: Curved reflecting surfaces help redirect sound energy toward different seating areas, improving audibility. Appropriate design distributes sound more evenly, although halls also need absorbing materials to prevent excessive reverberation. Curvature does not simply stop or absorb the waves.

Q80. A microphone captures sound but does not output it, and a CPU processes faster than typical external devices transfer data. Which statements are true?

[RRB NTPC Undergraduate, 16 June 2026]

• (A) Both false
• (B) First true, second false
• (C) Both true
• (D) First false, second true

Answer: (C) Both true

Explanation: A microphone functions as an input transducer, converting acoustic pressure variations into electrical information. A loudspeaker provides output instead. CPUs typically process information much faster than comparatively slow input-output peripherals, so both of the stated propositions are correct.

Q79. SONAR stands for which of the following?

[UPSC NDA II, September 2025]

• (A) Sonographic Natural Ranging
• (B) Simple Operational Navigation Ranging
• (C) Sound Navigation and Ranging
• (D) Simple Operational Natural Ranging

Answer: (C) Sound Navigation and Ranging

Explanation: SONAR is an acronym for Sound Navigation and Ranging. This technology transmits sound pulses, often at ultrasonic frequencies, and analyzes returning echoes. It is widely employed for locating underwater objects, measuring depth and studying the sea floor.

Q78. For sound, which equation correctly relates frequency f, wavelength λ and speed v?

[UPSC NDA I, April 2025]

• (A) f = vλ
• (B) λ = vf
• (C) f = λv
• (D) v = λf

Answer: (D) v = λf

Explanation: A wave advances one wavelength during each cycle. With f cycles occurring each second, the distance traveled per second is f multiplied by λ. Thus wave speed equals frequency times wavelength, written v = fλ.

Q77. A sound wave travels from air into water. How do its speed and wavelength change?

[SSC CGL, 26 September 2025]

• (A) Speed rises and wavelength falls
• (B) Speed falls and wavelength rises
• (C) Both speed and wavelength rise
• (D) Both speed and wavelength fall

Answer: (C) Both speed and wavelength rise

Explanation: Sound usually travels more rapidly in water than in air. The source fixes its frequency, which does not change at the boundary. Since λ = v/f, the higher sound speed in water implies a longer wavelength as well.

Q76. What frequency interval is typically audible to a healthy young human ear?

[SSC CGL, 23 September 2025]

• (A) 0–10,000 Hz
• (B) 20–20,000 Hz
• (C) 2,000–20,000 Hz
• (D) 100–1,000 Hz

Answer: (B) 20–20,000 Hz

Explanation: The conventional audible range extends approximately from 20 Hz to 20 kHz. Frequencies below the lower bound are infrasonic, while those above the upper bound are ultrasonic. Actual limits vary between individuals and generally decline with age.

Q75. What changes when sound passes from air into water?

[SSC CGL, 15 September 2025]

• (A) Its frequency remains constant while wavelength changes
• (B) Frequency rises but wavelength remains fixed
• (C) Both frequency and wavelength fall
• (D) Speed falls while frequency rises

Answer: (A) Its frequency remains constant while wavelength changes

Explanation: Frequency is determined by the vibrating source and remains continuous across a boundary. Changing from air to water alters propagation speed because the elastic properties differ. The wavelength therefore changes to preserve the relationship v = fλ.

Q74. What is required to generate and transmit a sound wave?

[RRB NTPC Graduate, 24 June 2025]

• (A) Air molecules alone
• (B) An electromagnetic carrier alone
• (C) Vibration and an elastic medium
• (D) Only a vacuum

Answer: (C) Vibration and an elastic medium

Explanation: Sound originates when a physical source vibrates. It can be conveyed only through a medium capable of elastic particle motion, such as air, water or steel. A vacuum contains no particles to transfer the disturbance between locations.

Q73. What is the persistence of sound in a hall caused by repeated reflections?

[RRB NTPC Undergraduate, 20 August 2025]

• (A) Reverberation
• (B) Diffraction
• (C) Refraction
• (D) Interference

Answer: (A) Reverberation

Explanation: In a large room, a sound wave rebounds repeatedly from walls, ceilings and other surfaces. Numerous delayed reflections overlap and make the sound continue after its source stops. This lingering effect is called reverberation, distinct from one clear echo.

Q72. Which unit expresses a ratio between two sound intensities?

[RRB NTPC Undergraduate, 28 August 2025]

• (A) Phon
• (B) Pascal
• (C) Hertz
• (D) Decibel

Answer: (D) Decibel

Explanation: The decibel expresses an intensity ratio using a logarithmic scale: 10 log₁₀(I₂/I₁). It is convenient because sound intensities vary over enormous ranges. Hertz measures frequency, while pascal is a pressure unit, not an intensity ratio.

Q71. Which is an example of a longitudinal wave?

[RRB NTPC Undergraduate, 11 August 2025]

• (A) Water-surface ripple
• (B) Light in vacuum
• (C) Sound in air
• (D) Radio transmission

Answer: (C) Sound in air

Explanation: In air, sound moves through alternating compressions and rarefactions. Individual air molecules oscillate along the direction in which the disturbance travels, making it a longitudinal wave. Light and radio waves are electromagnetic and transverse in ordinary free-space propagation.

Q70. In a sound wave, the number of compressions passing a point each second is its what?

[RRB NTPC Graduate CBT-II, 13 October 2025]

• (A) Frequency
• (B) Amplitude
• (C) Wavelength
• (D) Loudness

Answer: (A) Frequency

Explanation: Each complete sound-wave cycle includes a compression and rarefaction. Counting compressions that pass a point per second counts cycles per second, which defines frequency. Hertz is the standard frequency unit; amplitude instead specifies the size of the oscillation.

Q69. What generally happens to the speed of sound in a gas when its temperature increases?

[RRB NTPC Graduate CBT-II, 13 October 2025]

• (A) It increases
• (B) Temperature has no effect
• (C) It decreases
• (D) It stays exactly constant

Answer: (A) It increases

Explanation: For an ideal gas of fixed composition, the sound speed is v = √(γRT/M). Higher absolute temperature raises the thermal motion and elastic response relative to density, making speed increase approximately with the square root of temperature.

Q68. How does a traditional stethoscope carry the sound of a heartbeat to the doctor?

[RRB Group D, 28 November 2025]

• (A) Direct transfer without reflection
• (B) Multiple reflection of sound
• (C) Refraction only
• (D) Conversion to electrical signals

Answer: (B) Multiple reflection of sound

Explanation: A traditional acoustic stethoscope collects vibrations at its chestpiece and channels them through tubing. Repeated reflections at the tube boundaries help guide the sound to the earpieces. It needs no electronic conversion in its basic mechanical design.

Q67. Which property does NOT apply to ordinary sound waves traveling in fluids?

[UPSC NDA I, April 2024]

• (A) They carry energy
• (B) They need a medium
• (C) They are transverse
• (D) They are generally faster in liquids than gases

Answer: (C) They are transverse

Explanation: A fluid cannot support sustained shear stress required by an ordinary transverse bulk sound wave. Its acoustic disturbances propagate longitudinally as alternating pressure and density variations. These waves carry energy and require matter, generally traveling faster in liquids than gases.

Q66. A wave has time period 0.04 second. What is its frequency?

[SSC MTS, 15 October 2024]

• (A) 50 Hz
• (B) 25 Hz
• (C) 40 Hz
• (D) 100 Hz

Answer: (B) 25 Hz

Explanation: Frequency and period are reciprocals: f = 1/T. Substituting a period of 0.04 second gives 1 ÷ 0.04, or 25 cycles per second. Frequency uses hertz, while the time period itself is measured in seconds.

Q65. Which describes sound waves propagating in air?

[UPSC NDA II, September 2023]

• (A) Electromagnetic and transverse
• (B) Mechanical and longitudinal
• (C) Mechanical and transverse
• (D) Electromagnetic and longitudinal

Answer: (B) Mechanical and longitudinal

Explanation: Airborne sound requires matter and is therefore a mechanical wave, not electromagnetic. Air molecules move back and forth parallel to its travel direction, producing moving regions of higher and lower pressure known as compressions and rarefactions.

Q64. For sound in a given ordinary medium, which statement is correct?

[UPSC NDA I, April 2023]

• (A) Sound speed is the same at all frequencies
• (B) Higher frequencies always move faster
• (C) Higher frequencies always move slower
• (D) Longer wavelengths always travel slower

Answer: (A) Sound speed is the same at all frequencies

Explanation: In the standard nondispersive approximation used for ordinary acoustics, wave speed is controlled by the medium rather than the source frequency. Since v = fλ, higher frequency produces proportionally shorter wavelength, maintaining essentially the same speed under identical conditions.

Q63. A firecracker explodes before a cliff. Its echo is heard 5 s later; sound speed is 340 m/s. How far away is the cliff?

[UPSC CDS I, 2023]

• (A) 1700 m
• (B) 170 m
• (C) 85 m
• (D) 850 m

Answer: (D) 850 m

Explanation: The five seconds include the outward journey to the cliff and the reflected return. The total sound path is 340 × 5 = 1700 metres. Half of that is the one-way distance, 850 metres.

Q62. In a mobile phone, which statements are correct?
1. Microphone turns sound into mechanical signal.
2. Microphone turns sound into electrical signal.
3. Speaker turns mechanical signal into sound.
4. Speaker turns electrical signal into sound.

[UPSC CDS I, 2023]

• (A) 1 and 3
• (B) 1 and 4
• (C) 2 and 3
• (D) 2 and 4

Answer: (D) 2 and 4

Explanation: A microphone is an input transducer converting sound pressure fluctuations to electrical signals. A loudspeaker is an output transducer converting an electrical signal into diaphragm movement and then sound. Accordingly statements 2 and 4 correctly identify their principal conversions.

Q61. Sound and light waves in air are respectively of what types?

[UPSC CDS I, 2023]

• (A) Longitudinal and transverse
• (B) Transverse and longitudinal
• (C) Both longitudinal
• (D) Both transverse

Answer: (A) Longitudinal and transverse

Explanation: Sound in air travels as longitudinal pressure oscillations along the propagation direction. Light is an electromagnetic wave with its electric and magnetic fields oscillating transversely to travel. Thus the two waves differ in both physical nature and orientation.

Q60. When does a moving sound source produce a sonic boom?

[UPSC CDS II, 2023]

• (A) When it moves faster than sound
• (B) When it exceeds light speed
• (C) When it is slower than sound
• (D) Only when its speed equals sound speed

Answer: (A) When it moves faster than sound

Explanation: A sonic boom occurs as shock waves form when an object travels supersonically through air. Pressure disturbances accumulate into a strong wave front that reaches observers abruptly. Supersonic means exceeding the local speed of sound, not light.

Q59. In which of these media is sound fastest?

[UPSC CDS II, 2023]

• (A) Glass
• (B) Stainless steel
• (C) Water
• (D) Oxygen

Answer: (B) Stainless steel

Explanation: Stainless steel possesses a high elastic modulus and generally supports sound speeds of several kilometres per second. Water and oxygen support considerably lower speeds, while glass is also typically slower than steel in the comparison intended by this question.

Q58. Which statement about sound is NOT correct?

[SSC CHSL 2022 examination, 20 March 2023]

• (A) Sound travels through vacuum
• (B) Human vocal cords vibrate to produce sound
• (C) Unpleasant sound is termed noise
• (D) Frequency is expressed in hertz

Answer: (A) Sound travels through vacuum

Explanation: Sound is mechanical energy carried by vibration of a material medium. Consequently, vacuum has no particles capable of conveying it. Vocal cords create pressure disturbances, unpleasant disturbances may be called noise, and frequency is correctly measured in hertz.

Q57. In which of these solids is sound speed highest at about 25°C?

[SSC CHSL 2022 examination, 16 March 2023]

• (A) Nickel
• (B) Steel
• (C) Iron
• (D) Aluminium

Answer: (D) Aluminium

Explanation: The textbook values used for these substances put longitudinal sound speed in aluminium near 6420 m/s, above nickel, steel and iron. A material’s resistance to compression or stretching, together with density, determines acoustic speed; density alone cannot rank them.

Q56. What increases when the pitch of a sound increases?

[UPSC NDA II, September 2022]

• (A) Intensity
• (B) Loudness
• (C) Wavelength
• (D) Frequency

Answer: (D) Frequency

Explanation: Pitch follows the frequency of vibrations. More cycles per second make the tone sound higher, irrespective of whether it is quiet or loud. At the same wave speed, increasing frequency actually decreases wavelength, by the relation λ = v/f.

Q55. The amplitude of a sound wave, treated as particle displacement, has what unit?

[UPSC NDA II, September 2022]

• (A) Pressure
• (B) Distance
• (C) Time
• (D) Speed

Answer: (B) Distance

Explanation: Amplitude is the maximum displacement of a vibrating particle from its equilibrium position when discussing a displacement wave. It is therefore measured as a length, such as metres. Sound pressure amplitude is a different quantity measured in pascals.

Q54. Which ear structure converts incoming sound-induced vibrations into electrical nerve signals?

[UPSC NDA II, September 2022]

• (A) Auditory nerve
• (B) Cochlea
• (C) Eardrum
• (D) Eustachian tube

Answer: (B) Cochlea

Explanation: Inside the cochlea, hair cells respond to fluid-driven vibration and convert mechanical deflection into electrical signaling that reaches auditory neurons. The eardrum and middle-ear bones transmit vibrations, while the auditory nerve conveys information toward the brain.

Q53. SONAR measures distance to underwater objects with which waves?

[UPSC NDA II, September 2022]

• (A) Infrasound
• (B) Human-audible sound only
• (C) Ultrasound
• (D) All ranges equally

Answer: (C) Ultrasound

Explanation: Active sonar sends acoustic pulses through water and times their reflections from submerged targets. Ultrasonic pulses are frequently used because their short wavelengths provide good directional resolution. Object distance follows from sound speed multiplied by round-trip time divided by two.

Q52. A sound wave has frequency 1 kHz and wavelength 50 cm. Time to travel 1 km?

[UPSC NDA I, April 2022]

• (A) 5 s
• (B) 4 s
• (C) 3 s
• (D) 2 s

Answer: (D) 2 s

Explanation: Convert 1 kHz to 1000 Hz and 50 cm to 0.5 m. The wave speed is 1000 × 0.5 = 500 m/s. A wave moving at 500 m/s covers 1000 m in 2 seconds.

Q51. Which statement about sound speed is incorrect?

[UPSC NDA II, September 2022]

• (A) Steel conducts sound faster than water
• (B) Sound speed in air decreases when temperature increases
• (C) Sound speed in air increases when temperature increases
• (D) Water conducts sound faster than air

Answer: (B) Sound speed in air decreases when temperature increases

Explanation: For a gas of fixed composition, sound speed increases roughly as the square root of absolute temperature. Therefore its speed does not decrease merely because the air becomes warmer. The remaining statements reflect ordinary sound speeds in steel, water and air.

Q50. Sound travels fastest in which of these?

[WBCS Mains 2021 paper, held May 2022]

• (A) Air
• (B) Water
• (C) Vacuum
• (D) Steel

Answer: (D) Steel

Explanation: Sound cannot travel through vacuum, since no elastic particles exist there. Among air, water and steel, steel has a comparatively large elastic modulus and therefore transmits sound faster under normal conditions than the gas or liquid.

Q49. In ultrasonic machining, abrasive particles move mainly in what direction relative to the workpiece?

[MPPSC Assistant Engineer, Mechanical, 3 July 2022]

• (A) Tangential
• (B) Oscillatory around the whole part
• (C) Circular
• (D) Normal to the workpiece

Answer: (D) Normal to the workpiece

Explanation: Ultrasonic machining uses a tool vibrating at ultrasonic frequencies, driving abrasive particles against the surface in the tool’s approach direction. The normal impacts remove minute fragments of hard, brittle material. Lateral motion is not the intended principal cutting action.

Q48. Which statement about the propagation of sound in air is true?

[UPSC NDA II, September 2021]

• (A) Travels in vacuum and is transverse
• (B) Cannot travel in vacuum and is longitudinal
• (C) Travels in vacuum and is longitudinal
• (D) Cannot travel in vacuum and is transverse

Answer: (B) Cannot travel in vacuum and is longitudinal

Explanation: In air, vibrating particles pass pressure changes to neighboring particles, so a material medium is essential. The molecules oscillate parallel to the direction of travel, establishing that an ordinary sound wave is longitudinal rather than transverse.

Q47. Which of the following cannot serve as a unit of sound-wave frequency?

[UPSC NDA I, April 2021]

• (A) dB
• (B) s⁻¹
• (C) Hz
• (D) min⁻¹

Answer: (A) dB

Explanation: Frequency is a count of repetitions per unit time, so hertz, inverse seconds and inverse minutes can express it. A decibel, however, is a logarithmic level or ratio, commonly associated with sound intensity rather than cycles per time.

Q46. Beats appear when two harmonic waves have frequencies that are?

[UPSC NDA I, April 2021]

• (A) Exactly equal
• (B) Very far apart
• (C) Integer multiples
• (D) Nearly equal

Answer: (D) Nearly equal

Explanation: When two waves of nearly equal frequency overlap, their relative phase changes slowly. At some moments they reinforce each other and at others partially cancel, making the loudness rise and fall. The beat frequency equals the absolute difference between frequencies.

Q45. A sound wave of frequency 4 kHz and wavelength 30 cm travels 2.4 km. How much time does it take?

[UPSC CDS II, 2021]

• (A) 2.0 s
• (B) 0.6 s
• (C) 1.0 s
• (D) 8.0 s

Answer: (A) 2.0 s

Explanation: The wavelength is 0.30 m and frequency is 4000 cycles per second. Consequently v = fλ = 1200 m/s. For a distance of 2400 m, the elapsed time is 2400/1200 = 2 seconds.

Q44. What is approximately the lowest sound frequency audible to humans?

[RRB NTPC CBT-I, 11 March 2021]

• (A) 10 Hz
• (B) 500 Hz
• (C) 50 Hz
• (D) 20 Hz

Answer: (D) 20 Hz

Explanation: Normal young human hearing begins at about 20 Hz and extends upward to roughly 20 kHz. Sounds lower than 20 Hz are classed as infrasound. Hearing limits are approximate and depend on age and individual sensitivity.

Q43. The speed of sound in air at around 20°C is approximately?

[RRB NTPC CBT-I, 27 February 2021]

• (A) 434 m/s
• (B) 343 m/s
• (C) 383 m/s
• (D) 373 m/s

Answer: (B) 343 m/s

Explanation: At roughly 20 degrees Celsius in ordinary air, the usual reference speed is 343 metres per second. Cooling the air reduces the value, while heating it increases the value. Humidity also affects speed slightly by changing air composition.

Q42. Which statement correctly defines wavelength?

[SSC CGL 2020 Tier I, held 23 August 2021]

• (A) Parallel particle oscillations define transverse waves
• (B) Frequency counts oscillations per minute by definition
• (C) Perpendicular oscillations define longitudinal waves
• (D) Shortest distance between two points in the same phase is one wavelength

Answer: (D) Shortest distance between two points in the same phase is one wavelength

Explanation: Wavelength is the smallest spatial separation between consecutive points that are in the same phase, such as adjacent compressions. Parallel medium-particle motion defines longitudinal waves, not transverse waves. Frequency is conventionally expressed as cycles per second.

Q41. Why is thunder heard after its accompanying flash of lightning?

[SSC MTS 2020 paper, held 27 October 2021]

• (A) Sound travels much slower than light
• (B) Sound travels three times faster
• (C) Sound travels twice as fast
• (D) Sound and light travel equally fast

Answer: (A) Sound travels much slower than light

Explanation: Light from lightning reaches an observer almost immediately over ordinary storm distances. Sound travels through air at only about 340 m/s, so the thunder takes noticeably longer. This difference allows a rough estimate of how far away a storm is.

Q40. Which assertion about sound-wave transmission is incorrect?

[UPSC NDA I, April 2020]

• (A) Can travel through water
• (B) Can travel through air
• (C) Can travel through steel
• (D) Can travel through vacuum

Answer: (D) Can travel through vacuum

Explanation: Sound needs interacting material particles to transmit mechanical disturbances. Water, air and steel can all carry such vibrations, though at different speeds. A vacuum has no medium for the pressure variations, making the assertion of sound propagation through vacuum false.

Q39. Which statement about sound is NOT correct?

[UPSC NDA I, April 2020]

• (A) Sound is slower than light
• (B) Sound waves in air are transverse
• (C) Sound waves in air are longitudinal
• (D) Sound travels faster in humid than dry air

Answer: (B) Sound waves in air are transverse

Explanation: Airborne sound propagates through compressions and rarefactions parallel to the wave direction, so it is longitudinal. It is much slower than light; humid air generally raises its speed slightly because water vapour lowers mean molecular mass.

Q38. Which event shows that light travels much faster than sound?

[UPSC CDS II, 2020]

• (A) Star twinkling
• (B) Striking a match
• (C) A thunderstorm
• (D) A mirage

Answer: (C) A thunderstorm

Explanation: During a thunderstorm, lightning and thunder arise from the same discharge. The flash reaches us almost instantly, whereas the much slower acoustic disturbance arrives after a noticeable delay. This difference illustrates the great speed contrast between light and sound.

Q37. Speed of sound is usually greatest in which class of medium?

[WBCS Preliminary Examination 2020]

• (A) Vacuum
• (B) Gases
• (C) Liquids
• (D) Solids

Answer: (D) Solids

Explanation: Solids generally offer strong elastic restoring forces, enabling rapid propagation of mechanical vibrations. Liquids typically conduct sound less quickly, and gases slower still. Vacuum carries no ordinary acoustic sound because it lacks material particles completely.

Q36. Which statements are true?
I. Sound speed decreases from solids to gases.
II. Cooling any medium increases sound speed.
III. Sound speed is lower in aluminium than glass.

[CGPSC Civil Service Prelims 2020]

• (A) I, II and III
• (B) I and II only
• (C) I only
• (D) II and III only

Answer: (C) I only

Explanation: Statement I gives the standard ordering of speeds in common materials. Cooling gases generally reduces sound speed, contradicting II. Aluminium ordinarily has higher longitudinal sound speed than glass, contradicting III. Thus only the first comparison is correct.

Q35. At 20°C, sound speed in water is approximately?

[UPSC NDA I, April 2019]

• (A) 330 m/s
• (B) 800 m/s
• (C) 1500 m/s
• (D) 5000 m/s

Answer: (C) 1500 m/s

Explanation: Sound in liquid water travels at roughly 1480–1500 metres per second around room temperature. This is considerably above its speed in air because water has greater resistance to compression. Metals usually support still faster transmission.

Q34. Compared with ordinary audible sound, ultrasound has what property?

[UPSC NDA II, September 2019]

• (A) Greater speed in the same medium
• (B) Greater frequency
• (C) Longer wavelength in the same medium
• (D) Greater speed and frequency

Answer: (B) Greater frequency

Explanation: Ultrasound consists of acoustic waves with frequencies above about 20,000 Hz. In a specified nondispersive medium, sound speed is largely independent of frequency, so higher ultrasonic frequency means shorter wavelength rather than automatically higher speed.

Q33. A mountain returns a sound echo after 5 s; sound speed is 340 m/s. What is the source-to-mountain distance?

[UPSC CDS II, 2019]

• (A) 0.085 km
• (B) 0.85 km
• (C) 0.17 km
• (D) 1.7 km

Answer: (B) 0.85 km

Explanation: The sound travels to the mountain and returns, making the total path twice the separation. Multiply 340 m/s by five seconds to obtain 1700 metres round trip. Dividing by two gives 850 metres, or 0.85 km.

Q32. Noise pollution is created if sound exceeds which level in the question’s classification?

[WBCS Preliminary Examination 2019]

• (A) 70–75 dB
• (B) 50–60 dB
• (C) 80–99 dB
• (D) 40–65 dB

Answer: (C) 80–99 dB

Explanation: The examination’s stated classification identifies the 80–99 dB range with problematic noise. However, noise exposure risk actually depends on duration, frequency and local regulations; there is no single universal level above which all sound is automatically pollution.

Q31. Statement I: Sound pitch depends on frequency.
Statement II: Sound loudness depends on amplitude.

Choose the correct relation.

[UPSC NDA II, September 2018]

• (A) Both true and II explains I
• (B) Both true but II does not explain I
• (C) I true, II false
• (D) I false, II true

Answer: (B) Both true but II does not explain I

Explanation: Both propositions are correct but describe separate sound characteristics. Oscillation frequency principally determines pitch, whereas amplitude drives intensity and perceived loudness. The amplitude statement does not explain why frequency controls pitch, so both are true without a causal relation.

Q30. Statement I: Sound cannot travel in vacuum.
Statement II: Sound waves are elastic waves that need a medium.

Choose the correct relation.

[UPSC NDA II, September 2018]

• (A) Both true and II explains I
• (B) Both true but II does not explain I
• (C) I true, II false
• (D) I false, II true

Answer: (A) Both true and II explains I

Explanation: Both statements are correct. Elastic sound requires a material through which particles can exert forces on their neighbors. A vacuum contains no such particles, directly explaining why it cannot support propagation of ordinary sound waves.

Q29. Through which of these arrangements can sound NOT travel?

[UPSC CDS I, 2018]

• (A) A copper wire in air
• (B) A silver slab in air
• (C) A glass prism in water
• (D) A hollow wooden pipe placed in vacuum

Answer: (D) A hollow wooden pipe placed in vacuum

Explanation: Copper, silver and glass are solids that support acoustic propagation, while water and air also carry sound. A hollow pipe in vacuum provides no air to transmit the sound along its hollow interior, so this is the intended exception.

Q28. Sound has the greatest velocity in which medium?

[UPPSC Civil Service Prelims 2018]

• (A) Vacuum
• (B) Air
• (C) Water
• (D) Steel

Answer: (D) Steel

Explanation: Sound propagation depends on the stiffness and inertia of material particles. Steel’s large elastic modulus supports fast mechanical disturbances compared with water and air. Vacuum provides no ordinary acoustic propagation at all because it has no transmitting medium.

Q27. Which statement about ultrasound is NOT correct?

[UPSC NDA II, September 2017]

• (A) It cannot be reflected, refracted or absorbed
• (B) It can reveal cracks inside structures
• (C) It can assist machining hard materials
• (D) It cannot travel through vacuum

Answer: (A) It cannot be reflected, refracted or absorbed

Explanation: Ultrasound behaves as mechanical wave energy and can reflect, refract and be absorbed like other sound. Industry uses reflections to detect internal flaws and high-frequency vibrations in machining. Its inability to pass through vacuum remains true.

Q26. Bats detect obstacles by receiving reflected which kind of wave?

[UPSC NDA II, September 2017]

• (A) Infrasound
• (B) Ultrasound
• (C) Radio waves
• (D) Microwaves

Answer: (B) Ultrasound

Explanation: Many bats emit ultrasonic calls and detect returning echoes from insects or obstacles. Differences in return time and other echo characteristics help them judge direction and distance. This biological navigation system is known as echolocation.

Q25. Which material is commonly exploited to produce ultrasonic waves by applying an alternating electrical voltage?

[UPSC CDS II, 2017]

• (A) Ferromagnetic material
• (B) Ferrimagnetic material
• (C) Piezoelectric material
• (D) Pyroelectric material

Answer: (C) Piezoelectric material

Explanation: Piezoelectric crystals deform mechanically when an alternating voltage is applied. At sufficiently high frequency, this deformation generates ultrasonic mechanical vibrations. The reverse piezoelectric effect is therefore fundamental to many ultrasonic transmitters and medical probes.

Q24. Sound waves travel fastest in which of the following?

[MPPSC State Service Prelims 2017]

• (A) Solids
• (B) Liquids
• (C) Gases
• (D) Vacuum

Answer: (A) Solids

Explanation: The usual textbook ranking of acoustic speed is solids above liquids above gases, although specific substances can vary. Solid elastic forces transmit disturbance quickly. Vacuum lacks material particles, so mechanical sound cannot propagate through it at all.

Q23. What is the number of oscillations completed every second called?

[SSC MTS, 4 October 2017]

• (A) Frequency
• (B) Amplitude
• (C) Pitch
• (D) Loudness

Answer: (A) Frequency

Explanation: Frequency measures repetitions per unit time and is commonly stated in hertz, equivalent to cycles per second. Amplitude measures oscillation size, while pitch and loudness are human perceptions related principally to frequency and intensity respectively.

Q22. Which statement concerning sound and hearing is NOT correct?

[UPSC CDS II, 2016]

• (A) Hearing response to intensity is approximately logarithmic
• (B) Ear sensitivity is independent of sound frequency
• (C) Overlapping waves obey superposition
• (D) Longitudinal waves can propagate in solids, liquids and gases

Answer: (B) Ear sensitivity is independent of sound frequency

Explanation: The ear does not respond equally to all frequencies: it is most sensitive to parts of the middle audible range. Loudness perception is approximately logarithmic. Superposition and the possibility of longitudinal waves in various material states are valid.

Q21. Sound speed is 300 m/s. Two people hear a sound 5 s and 6 s after emission. What are their maximum and minimum possible separations?

[CGPSC Aptitude Test 2016]

• (A) 1.8 km and 0.15 km
• (B) 2.2 km and 0.20 km
• (C) 2.8 km and 0.25 km
• (D) 3.3 km and 0.3 km

Answer: (D) 3.3 km and 0.3 km

Explanation: Their distances from the source are 300 × 5 = 1500 m and 300 × 6 = 1800 m. Opposite-side positions produce maximum separation, 3300 m; same-direction positions give minimum separation, 300 m.

Q20. For good acoustics, the walls of a concert hall should primarily do what to excess sound?

[Meghalaya PSC Preliminary Examination 2015]

• (A) Amplify it
• (B) Transmit all of it
• (C) Reflect all of it
• (D) Absorb it

Answer: (D) Absorb it

Explanation: Concert halls require some reflection for distribution but also absorption to control excessive reverberation. Soft furnishings and acoustic panels prevent reflected waves from building up and blurring speech or music. The question emphasizes absorbing excessive sound energy.

Q19. Sound waves cannot travel through which medium?

[UPPSC Civil Service Prelims 2014]

• (A) Solids
• (B) Liquids
• (C) Gases
• (D) Vacuum

Answer: (D) Vacuum

Explanation: Sound is a mechanical wave that transmits energy through interacting particles in a medium. Solids, liquids and gases all permit particle vibration. Vacuum does not contain the matter needed to convey the disturbance, so sound cannot propagate there.

Q18. Why can an astronaut not hear a companion directly on the Moon?

[UPPSC Civil Service Mains 2013]

• (A) Voice frequency is above hearing
• (B) Night temperatures are too low
• (C) No medium exists for sound propagation
• (D) Lunar craters block all waves

Answer: (C) No medium exists for sound propagation

Explanation: The Moon has essentially no atmosphere through which ordinary voice pressure waves can propagate. Direct speech therefore cannot travel between suited astronauts. Their radio equipment converts sound into electromagnetic signals, which are capable of traveling through vacuum.

Q17. Which medical technique uses ultrasonic sound?

[RAS/RTS Preliminary Examination 2012]

• (A) Sonography
• (B) ECG
• (C) EEG
• (D) X-ray imaging

Answer: (A) Sonography

Explanation: Sonography transmits high-frequency sound pulses into tissue and uses returning echoes to construct images. ECG records electrical heart activity, EEG records electrical brain activity, and conventional X-ray imaging relies on electromagnetic radiation rather than ultrasound.

Q16. Ultrasonic waves have frequency in which range?

[UPPSC Civil Service Prelims 2012]

• (A) Above 20,000 Hz
• (B) Below 10,000 Hz
• (C) Exactly 1,000 Hz
• (D) None of these

Answer: (A) Above 20,000 Hz

Explanation: Ultrasound refers to sound whose frequency is above the conventional upper limit of human hearing, roughly 20 kHz. It is useful in imaging, cleaning and nondestructive testing because short wavelengths can offer fine spatial resolution.

Q15. What does the decibel unit commonly quantify in acoustics?

[UPPSC Mains 2011]

• (A) Intensity of light
• (B) Sound intensity level
• (C) Earthquake magnitude
• (D) None of the above

Answer: (B) Sound intensity level

Explanation: Decibel denotes a logarithmic comparison of sound intensity or sound pressure level against a reference. It compresses a very large range of physical intensities into manageable numbers. Earthquake magnitude and optical intensity use different standard descriptions.

Q14. If Va, Vw and Vs are sound speeds in air, water and steel, which relation is correct?

[UP UDA/LDA Special Mains 2010]

• (A) Va < Vw < Vs
• (B) Vs < Vw < Va
• (C) Vw < Vs < Va
• (D) Vs < Va < Vw

Answer: (A) Va < Vw < Vs

Explanation: Under ordinary conditions, air transmits sound at around 340 m/s, water at around 1500 m/s and steel at several thousand metres per second. These typical speeds establish the ascending order air, water, steel.

Q13. Arrange these media in ascending sound speed: 1. Water; 2. Steel; 3. Nitrogen.

[UPPSC Special Mains 2008]

• (A) 3, 2, 1
• (B) 3, 1, 2
• (C) 1, 3, 2
• (D) 2, 1, 3

Answer: (B) 3, 1, 2

Explanation: Nitrogen gas has a much lower acoustic speed than liquid water, and solid steel is generally faster than both. Therefore the ascending order is nitrogen, water, steel, corresponding to sequence 3, 1, 2.

Q12. Assertion: An aircraft at Mach 1 flies faster at 15 km altitude than at Mach 1 near sea level.
Reason: Sound speed depends on the surrounding temperature.

[UPSC CSE Prelims 2007]

• (A) Both true and reason explains assertion
• (B) Both true without explanation
• (C) Assertion true, reason false
• (D) Assertion false, reason true

Answer: (D) Assertion false, reason true

Explanation: Mach 1 means an aircraft travels at the local sound speed. Air at high altitude is commonly colder, producing a lower sound speed, so the assertion is false. The reason is true because gas sound speed depends on absolute temperature.

Q11. Consider:
1. A shorter flute produces lower-frequency sound.
2. Sound propagates through rocks only as longitudinal elastic waves.

Which are correct?

[UPSC CSE Prelims 2007]

• (A) 1 only
• (B) 2 only
• (C) Both 1 and 2
• (D) Neither 1 nor 2

Answer: (D) Neither 1 nor 2

Explanation: A shorter vibrating air column normally produces higher, not lower, frequency. Unlike fluids, rocks can transmit both longitudinal compressional waves and transverse shear waves. Consequently both claims are incorrect, making the fourth option the right choice.

Q10. What is approximately the minimum source-to-reflector distance for a clear echo?

[RAS/RTS Preliminary Examination 2007]

• (A) 10 m
• (B) 17 m
• (C) 24 m
• (D) 30 m

Answer: (B) 17 m

Explanation: A distinct echo requires about 0.1 second separation between original and returning sound. At 340 m/s, the required round trip is roughly 34 metres. Half of that is about 17 metres to the reflecting surface.

Q9. Where is sound speed maximum?

[UPSC CSE Prelims 2006]

• (A) Air at 0°C
• (B) Air at 100°C
• (C) Water
• (D) Wood

Answer: (D) Wood

Explanation: Wood typically transmits longitudinal acoustic vibrations faster than water or air. Heating air increases its sound speed, but not enough to exceed the representative speed in wood. Material stiffness and density jointly determine the sound propagation speed.

Q8. What is meant by Dolby B or Dolby C on older audio recorders?

[UPSC CSE Prelims 2006]

• (A) Frequency modulation
• (B) Amplitude modulation
• (C) Noise-reduction circuit
• (D) Ability to use both AC and DC

Answer: (C) Noise-reduction circuit

Explanation: Dolby B and C were analogue noise-reduction systems used especially with cassette recordings. They process audio during recording and playback to reduce tape hiss while preserving desired sound. They are neither transmission modulation formats nor power-supply specifications.

Q7. How can bats fly and hunt in darkness?

[UPPSC Mains 2005]

• (A) Their pupils are especially large
• (B) Only through unusually good night vision
• (C) All birds can do so
• (D) They emit and detect ultrasound echoes

Answer: (D) They emit and detect ultrasound echoes

Explanation: Bats often orient through echolocation. They emit high-frequency calls and process the echoes returning from targets. By analyzing the time, direction and changes of those returns, they navigate and locate prey even where useful visible light is absent.

Q6. Which of these is measured using decibels?

[MPPSC State Service Prelims 2005]

• (A) Haemoglobin concentration
• (B) Urinary sugar
• (C) Environmental sound level
• (D) Airborne particle count

Answer: (C) Environmental sound level

Explanation: The decibel is a logarithmic unit used to compare acoustic intensity or sound pressure levels to specified references. It is central to environmental noise measurements. Blood chemistry and particle counts are expressed in entirely different physical or chemical units.

Q5. In which medium is sound fastest near 20°C?

[Chhattisgarh PSC Prelims 2004]

• (A) Air
• (B) Granite
• (C) Water
• (D) Iron

Answer: (D) Iron

Explanation: The examination’s comparison identifies iron as the medium with the highest representative sound speed among those listed. Ordinary gases and water are substantially slower; longitudinal waves in metals can travel at several kilometres each second.

Q4. Which statement is NOT correct about sound in air?

[UPSC CSE Prelims 2003]

• (A) Speed rises as temperature rises
• (B) Speed is approximately pressure-independent at fixed temperature
• (C) Speed decreases as humidity rises
• (D) Speed is essentially unaffected by amplitude and frequency

Answer: (C) Speed decreases as humidity rises

Explanation: Increasing humidity replaces some heavier dry-air molecules with lighter water vapour, generally increasing sound speed. At fixed temperature, an ideal gas’s sound speed is independent of pressure, and in ordinary linear acoustics it does not depend on wave amplitude or frequency.

Q3. Which statement about sound waves is correct?

[UPPSC Civil Service Prelims 2002]

• (A) They travel in vacuum
• (B) They travel only in solids
• (C) They travel only in gases
• (D) They travel in both solids and gases

Answer: (D) They travel in both solids and gases

Explanation: Sound requires a material medium but is not confined to one state of matter. It can pass through solids and gases, and also liquids. In a vacuum, however, there are no interacting particles to convey its mechanical vibration.

Q2. Assertion: Reverberation is prominent in large churches and halls.
Reason: Their walls, roof and floor repeatedly reflect sound.

Choose the correct relation.

[UP Lower Subordinate Special Prelims 2002]

• (A) Both true and reason explains assertion
• (B) Both true but reason does not explain assertion
• (C) Assertion true, reason false
• (D) Assertion false, reason true

Answer: (A) Both true and reason explains assertion

Explanation: Large enclosed spaces can sustain overlapping reflections that prolong sounds beyond their production. Walls, ceiling and flooring bounce sound energy through the space, directly causing reverberation. Thus the reason is true and correctly explains the assertion.

Q1. Which statements are correct?
1. The heart vibrates at infrasonic frequencies.
2. Sound travels faster in gases than liquids and solids.
3. Mach number compares speed with sound speed.
4. Ultrasound exceeds 20,000 Hz.

[UPPSC Mains 2002]

• (A) 1, 2 and 3
• (B) 2, 3 and 4
• (C) 1, 3 and 4
• (D) 1, 2 and 4

Answer: (C) 1, 3 and 4

Explanation: Normal heart-rate mechanical oscillations are far below 20 Hz, so the first is accepted as infrasonic in this question. Sound is usually faster in liquids and solids than gases, making statement 2 false. Mach number and ultrasound definitions are correct.