Q60. When does a moving sound source produce a sonic boom?
[UPSC CDS II, 2023]
- (A) When it moves faster than sound
- (B) When it exceeds light speed
- (C) When it is slower than sound
- (D) Only when its speed equals sound speed
Answer: (A) When it moves faster than sound
Explanation: A sonic boom occurs as shock waves form when an object travels supersonically through air. Pressure disturbances accumulate into a strong wave front that reaches observers abruptly. Supersonic means exceeding the local speed of sound, not light.
Q59. In which of these media is sound fastest?
[UPSC CDS II, 2023]
- (A) Glass
- (B) Stainless steel
- (C) Water
- (D) Oxygen
Answer: (B) Stainless steel
Explanation: Stainless steel possesses a high elastic modulus and generally supports sound speeds of several kilometres per second. Water and oxygen support considerably lower speeds, while glass is also typically slower than steel in the comparison intended by this question.
Q58. Which statement about sound is NOT correct?
[SSC CHSL 2022 examination, 20 March 2023]
- (A) Sound travels through vacuum
- (B) Human vocal cords vibrate to produce sound
- (C) Unpleasant sound is termed noise
- (D) Frequency is expressed in hertz
Answer: (A) Sound travels through vacuum
Explanation: Sound is mechanical energy carried by vibration of a material medium. Consequently, vacuum has no particles capable of conveying it. Vocal cords create pressure disturbances, unpleasant disturbances may be called noise, and frequency is correctly measured in hertz.
Q57. In which of these solids is sound speed highest at about 25°C?
[SSC CHSL 2022 examination, 16 March 2023]
- (A) Nickel
- (B) Steel
- (C) Iron
- (D) Aluminium
Answer: (D) Aluminium
Explanation: The textbook values used for these substances put longitudinal sound speed in aluminium near 6420 m/s, above nickel, steel and iron. A material’s resistance to compression or stretching, together with density, determines acoustic speed; density alone cannot rank them.
Q56. What increases when the pitch of a sound increases?
[UPSC NDA II, September 2022]
- (A) Intensity
- (B) Loudness
- (C) Wavelength
- (D) Frequency
Answer: (D) Frequency
Explanation: Pitch follows the frequency of vibrations. More cycles per second make the tone sound higher, irrespective of whether it is quiet or loud. At the same wave speed, increasing frequency actually decreases wavelength, by the relation λ = v/f.
Q55. The amplitude of a sound wave, treated as particle displacement, has what unit?
[UPSC NDA II, September 2022]
- (A) Pressure
- (B) Distance
- (C) Time
- (D) Speed
Answer: (B) Distance
Explanation: Amplitude is the maximum displacement of a vibrating particle from its equilibrium position when discussing a displacement wave. It is therefore measured as a length, such as metres. Sound pressure amplitude is a different quantity measured in pascals.
Q54. Which ear structure converts incoming sound-induced vibrations into electrical nerve signals?
[UPSC NDA II, September 2022]
- (A) Auditory nerve
- (B) Cochlea
- (C) Eardrum
- (D) Eustachian tube
Answer: (B) Cochlea
Explanation: Inside the cochlea, hair cells respond to fluid-driven vibration and convert mechanical deflection into electrical signaling that reaches auditory neurons. The eardrum and middle-ear bones transmit vibrations, while the auditory nerve conveys information toward the brain.
Q53. SONAR measures distance to underwater objects with which waves?
[UPSC NDA II, September 2022]
- (A) Infrasound
- (B) Human-audible sound only
- (C) Ultrasound
- (D) All ranges equally
Answer: (C) Ultrasound
Explanation: Active sonar sends acoustic pulses through water and times their reflections from submerged targets. Ultrasonic pulses are frequently used because their short wavelengths provide good directional resolution. Object distance follows from sound speed multiplied by round-trip time divided by two.
Q52. A sound wave has frequency 1 kHz and wavelength 50 cm. Time to travel 1 km?
[UPSC NDA I, April 2022]
- (A) 5 s
- (B) 4 s
- (C) 3 s
- (D) 2 s
Answer: (D) 2 s
Explanation: Convert 1 kHz to 1000 Hz and 50 cm to 0.5 m. The wave speed is 1000 × 0.5 = 500 m/s. A wave moving at 500 m/s covers 1000 m in 2 seconds.
Q51. Which statement about sound speed is incorrect?
[UPSC NDA II, September 2022]
- (A) Steel conducts sound faster than water
- (B) Sound speed in air decreases when temperature increases
- (C) Sound speed in air increases when temperature increases
- (D) Water conducts sound faster than air
Answer: (B) Sound speed in air decreases when temperature increases
Explanation: For a gas of fixed composition, sound speed increases roughly as the square root of absolute temperature. Therefore its speed does not decrease merely because the air becomes warmer. The remaining statements reflect ordinary sound speeds in steel, water and air.
