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.
