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.
