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.
