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.
