Q40. If the temperature of air increases, what happens to the speed of sound and the minimum distance to hear an echo?
[RRB NTPC (UG) CBT-I — 12 Aug 2025, Shift 2]
(A) Speed decreases, minimum distance increases
(B) Speed increases, minimum distance decreases
(C) Speed increases, minimum distance increases
(D) Speed decreases, minimum distance decreases
Answer: (C) Speed increases, minimum distance increases
Explanation: Warmer air transmits sound faster. A distinct echo requires a delay of about 0.1 second, so the minimum reflector distance is approximately d = 0.1v/2. As v rises with temperature, that required distance also rises.
Q39. What happens in the region of rarefaction in a sound wave?
[Maharashtra TET — 10 Nov 2024, Paper II]
(A) Pressure increases
(B) Pressure decreases
(C) Density becomes higher
(D) Pressure and density remain unchanged
Answer: (B) Pressure decreases
Explanation: A rarefaction occurs where vibrating particles move farther apart than normal. The local density and pressure consequently fall below their equilibrium values. It alternates with compression, where particles crowd together and pressure rises as sound travels through a medium.
Q38. What type of waves are sound waves in air?
[MP Vyapam Sub Engineer (Civil) — 23 Sep 2024, Shift 1]
(A) Longitudinal waves
(B) Transverse waves
(C) Electromagnetic waves
(D) Matter waves
Answer: (A) Longitudinal waves
Explanation: Sound in air consists of alternating compressions and rarefactions, and air molecules oscillate parallel to the direction in which sound propagates. Such waves are longitudinal. Unlike electromagnetic waves, they require matter to transmit the mechanical disturbance.
Q37. What is the SI unit of sound pressure?
[SSC JE (Civil) — 5 Jun 2024, Shift 1]
(A) N/m²
(B) Watt
(C) Decibel
(D) Hertz
Answer: (A) N/m²
Explanation: Sound pressure is a small fluctuation of air pressure around the normal atmospheric value. Like any pressure, its SI unit is the pascal, equal to one newton per square metre. Decibels usually express a logarithmic pressure level.
Q36. Among the given options, which one causes an echo?
[SSC MTS — 4 Sep 2023, Shift 1]
(A) Reflection of sound
(B) Refraction of sound
(C) Absorption of sound
(D) Interference of sound
Answer: (A) Reflection of sound
Explanation: An echo is a delayed repetition of a sound after reflection from a surface, such as a distant wall. The wave travels to the surface and back. A noticeable delay distinguishes an echo from the original sound.
Q35. A sound wave of frequency 3 kHz has a wavelength of 25 cm. How much time will it take to cover a distance of 4.5 km?
[MP Patwari — 20 Apr 2023, Shift 2]
(A) 3 seconds
(B) 2 seconds
(C) 5 seconds
(D) 6 seconds
Answer: (D) 6 seconds
Explanation: First convert units: 3 kHz is 3,000 Hz and 25 cm is 0.25 m. The speed is fλ = 750 m/s. Travelling 4,500 m at that speed takes t = 4,500/750 = 6 seconds.
Q34. Which of the following statements is NOT correct regarding sound?
[SSC CHSL — 20 Mar 2023, Shift 4]
(A) Sound can travel through vacuum
(B) The sound of human voice is produced by vibration of vocal cords
(C) Unpleasant sounds are called noise
(D) The frequency of sound is measured in hertz
Answer: (A) Sound can travel through vacuum
Explanation: Sound is a mechanical wave and needs a material medium such as air, water or a solid. It therefore cannot propagate through a vacuum. Vocal-cord vibration generates voice, unwanted sound is noise and hertz measures frequency.
Q33. In which of the following materials is the speed of sound the highest?
[SSC CHSL — 16 Mar 2023, Shift 1]
(A) Nickel
(B) Steel
(C) Iron
(D) Aluminium
Answer: (D) Aluminium
Explanation: Wave speed in a solid depends on its elasticity and density, not density alone. For the values usually used in school reference tables, longitudinal sound travels faster in aluminium than in the listed iron, steel and nickel.
Q32. Which of these statements is/are NOT true?
(i) The audible frequency range for humans is about 20 Hz to 10 kHz.
(ii) Ultrasound is sound with frequency above 10 kHz.
(iii) During earthquakes, ultrasounds are produced before the main shock waves.
[CGPSC State Service Prelims — 2023, General Studies]
(A) (i) and (ii)
(B) (i) and (iii)
(C) (i), (ii) and (iii)
(D) Only (iii)
Answer: (C) (i), (ii) and (iii)
Explanation: Ordinary textbook limits place human hearing at roughly 20 Hz–20 kHz and ultrasound above 20 kHz, so statements (i) and (ii) are incorrect. Earthquakes produce seismic waves, and statement (iii) is also incorrect as phrased.
Q31. Compared with audible sound, ultrasonic sound has:
[BSF Head Constable (Ministerial/RO/RM) — 20 Nov 2022]
(A) A higher speed in the same medium
(B) A higher frequency
(C) Both higher speed and higher frequency
(D) A longer wavelength at the same speed
Answer: (B) A higher frequency
Explanation: Ultrasound lies above the normal audible range, generally exceeding 20,000 Hz. In a fixed medium under the same conditions, its propagation speed is essentially the same as audible sound. Higher frequency means shorter wavelength when speed is unchanged.
