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.
