Q60. If the frequency of a sound wave increases while its speed in air remains constant, what happens to its wavelength?
[RRB Group D — 25 Aug 2026, Shift 1]
(A) Its wavelength remains unchanged
(B) Its wavelength decreases
(C) Its wavelength increases
(D) Its wavelength fluctuates randomly
Answer: (B) Its wavelength decreases
Explanation: The wave equation is v = fλ, where v is speed, f is frequency and λ is wavelength. If the speed remains fixed in the same air, increasing the frequency must reduce the wavelength by the same proportion.
Q59. A sound wave with a frequency of 15 Hz would be best categorized as which type?
[RRB Group D — 21 Aug 2026, Shift 2]
(A) Audible sound
(B) Infrasonic sound
(C) Hypersonic sound
(D) Ultrasonic sound
Answer: (B) Infrasonic sound
Explanation: Infrasound consists of sound waves below approximately 20 Hz, the lower limit of typical human hearing. Since 15 Hz is less than 20 Hz, it is infrasonic. Ultrasound, by contrast, has frequencies above 20,000 Hz.
Q58. A student shouts facing a building 220 meters away. If the speed of sound is 330 m/s, how long after the shout will the echo be heard?
[RRB Group D — 18 Aug 2026, Shift 3]
(A) 1.33 s
(B) 0.67 s
(C) 2.20 s
(D) 0.44 s
Answer: (A) 1.33 s
Explanation: The reflected sound travels from the student to the building and then returns. Its total distance is therefore 2 × 220 = 440 metres. Dividing this by 330 m/s gives 1.33 seconds for the echo.
Q57. Which of the following changes will most likely reduce the clarity of an echo in a large room?
[RRB Group D — 14 Aug 2026, Shift 1]
(A) Painting the walls a light colour
(B) Making the room larger
(C) Installing smooth ceramic tiles
(D) Adding thick curtains to the walls
Answer: (D) Adding thick curtains to the walls
Explanation: Thick curtains are soft, porous sound absorbers. They reduce the amount of acoustic energy reflected from walls, making distinct echoes less noticeable. Hard and smooth surfaces, such as ceramic tiles, instead reflect more sound into the room.
Q56. A student shouts facing a tall building 176 meters away and hears the echo after 1 second. What is the speed of sound in air based on this observation?
[RRB Group D — 12 Aug 2026, Shift 3]
(A) 400 m/s
(B) 352 m/s
(C) 330 m/s
(D) 176 m/s
Answer: (B) 352 m/s
Explanation: The echo covers two journeys: 176 metres toward the building and 176 metres back. Total travel distance is 352 metres. Since the return takes one second, the speed equals 352 ÷ 1 = 352 m/s.
Q55. Which of the following features is unique to longitudinal sound waves?
[RRB Group D — 11 Aug 2026, Shift 3]
(A) They vibrate only up and down in a medium
(B) They travel as side-to-side pulses in a medium
(C) They form regions of compression and rarefaction
(D) They move as crests and troughs through a medium
Answer: (C) They form regions of compression and rarefaction
Explanation: Longitudinal sound waves make medium particles vibrate along the direction of propagation. This creates alternating crowded, high-pressure compressions and spread-out, low-pressure rarefactions. Crests and troughs typically illustrate transverse disturbances, rather than these pressure variations.
Q54. What minimum distance should the reflecting surface be for an echo to be heard?
[RRB NTPC Graduate CBT 2 — 10 Jul 2026, Shift 1]
(A) 5 m
(B) 17.2 m
(C) 34.2 m
(D) 10 m
Answer: (B) 17.2 m
Explanation: A separate echo requires roughly 0.1 second between the direct and returning sounds. Taking sound speed as 344 m/s, the round-trip distance is 34.4 metres; therefore, the reflector must be about 17.2 metres away.
Q53. What do we call the lasting of sound caused by repeated reflections?
[RRB NTPC (UG) CBT-I — 18 Jun 2026, Shift 3]
(A) Reverberation
(B) Resonance
(C) Quality
(D) Echo
Answer: (A) Reverberation
Explanation: Reverberation is the persistence of sound in an enclosure after its source stops. Sound repeatedly reflects from walls, floors and ceilings, making the decay gradual. Unlike a distinct echo, these reflections usually overlap and cannot be separately heard.
Q52. Which of the following is correct regarding sound waves?
I. Compression – Region of high pressure.
II. Rarefaction – Region of low pressure.
[SSC MTS — 11 Feb 2026, Shift 1]
(A) Only I
(B) Only II
(C) Both I and II
(D) Neither I nor II
Answer: (C) Both I and II
Explanation: Both statements correctly describe a longitudinal sound wave. Compression brings neighbouring particles closer, raising local pressure and density. Rarefaction spreads them farther apart, lowering pressure and density. These alternating zones transmit acoustic energy through the medium.
Q51. What technique uses ultrasonic waves to form images of the heart?
[SSC MTS — 9 Feb 2026, Shift 2]
(A) Electrocardiography
(B) Sonography
(C) Echocardiography
(D) Radiography
Answer: (C) Echocardiography
Explanation: Echocardiography uses high-frequency ultrasound pulses directed at the heart. Reflections from the chambers, valves and moving tissues return to a transducer, allowing an image to be created. Electrocardiography instead records the heart’s electrical activity.