Q100. Consider these statements about sound:
I. Sound speed decreases with increasing temperature at constant atmospheric pressure.
II. It increases with humidity.
III. At constant temperature it does not change when atmospheric pressure changes.
Which are correct?
[UPSC NDA II, September 2026]
- (A) I and II only
- (B) II and III only
- (C) I and III only
- (D) I, II and III
Answer: (B) II and III only
Explanation: For an ideal gas, sound speed is proportional to the square root of absolute temperature, so statement I is false. Moisture reduces average molecular mass, increasing speed. At fixed temperature, changing pressure also changes density proportionately, leaving their ratio unchanged.
Q99. The speed of sound in a large ideal-gas container is x. After doubling pressure at constant temperature, it becomes y. Find x/y.
[UPSC NDA I, April 2026]
- (A) 1
- (B) √2
- (C) 2
- (D) 4
Answer: (A) 1
Explanation: The ideal-gas speed of sound is √(γRT/M), determined by composition and absolute temperature. Doubling pressure at the same temperature doubles density as well. Therefore the ratio of elastic modulus to density remains unchanged, and x equals y.
Q98. What is the persistence of sound caused by repeated reflections called?
[RRB NTPC Undergraduate, 18 June 2026]
- (A) Reverberation
- (B) Resonance
- (C) Quality
- (D) Echo
Answer: (A) Reverberation
Explanation: Reverberation is the continued presence of sound after its source has stopped because waves repeatedly reflect from the surrounding surfaces. Unlike a distinct echo, these reflections overlap closely in time and make a sound linger in a hall.
Q97. Why can an echo be heard when a sound is produced beside a large flat wall?
[RRB Group D, 20 August 2026]
- (A) The sound reflects and returns after a delay
- (B) The wall generates an identical new sound
- (C) Sound passes through and returns
- (D) The wall absorbs and slowly releases all sound
Answer: (A) The sound reflects and returns after a delay
Explanation: A sufficiently hard, distant wall reflects part of the outgoing sound wave. The reflected wave returns to the listener after traveling the outward and return distances. A perceptible delay separates this reflected sound from the initial sound.
Q96. A vibrating tuning fork produces which kind of wave in surrounding air?
[RRB Group D, 17 August 2026]
- (A) Circular surface wave
- (B) Longitudinal wave with compressions and rarefactions
- (C) Wave with no particle motion
- (D) Transverse wave with crests and troughs
Answer: (B) Longitudinal wave with compressions and rarefactions
Explanation: The prongs of a vibrating tuning fork alternately squeeze and release neighboring air. This produces regions of high pressure and low pressure moving outward. Air particles oscillate approximately parallel to the propagation direction, characterizing a longitudinal wave.
Q95. Which condition is necessary for a distinct echo?
[RRB Group D, 17 August 2026]
- (A) The listener must move
- (B) At least about 0.1 s between original and reflected sound
- (C) A soft reflecting surface
- (D) An extremely powerful sound source
Answer: (B) At least about 0.1 s between original and reflected sound
Explanation: Human hearing retains an auditory sensation for roughly one-tenth of a second. To distinguish the reflected sound as a separate echo, it should return after this interval. At 340 m/s, the reflecting surface is roughly 17 m away.
Q94. Two sounds have the same frequency but different amplitudes. What characteristic differs?
[RRB Group D, 14 August 2026]
- (A) Direction
- (B) Pitch
- (C) Loudness
- (D) Speed in the same medium
Answer: (C) Loudness
Explanation: Equal frequencies ordinarily give equal pitch, provided other conditions are comparable. Greater amplitude means larger particle displacement and greater sound intensity, so the sound is perceived as louder. In the same medium, wave speed is essentially unaffected by amplitude.
Q93. How does sound normally travel through different material states?
[RRB Group D, 12 August 2026]
- (A) At the same speed in all materials
- (B) Fastest in gases and slowest in solids
- (C) Fastest in liquids and slowest in gases
- (D) Faster in solids than in liquids or gases
Answer: (D) Faster in solids than in liquids or gases
Explanation: Sound speed depends on elastic stiffness as well as density. Under everyday conditions, typical solids transmit sound more quickly than liquids, and liquids more quickly than gases. Strong elastic restoring forces in solids facilitate rapid transmission of disturbances.
Q92. Two sounds have identical amplitudes but different frequencies. Which statement is correct?
[RRB Group D, 12 August 2026]
- (A) Same frequency, different amplitudes
- (B) Same pitch, different loudness
- (C) Same loudness but different pitches
- (D) Different speeds in the same medium
Answer: (C) Same loudness but different pitches
Explanation: Amplitude chiefly determines intensity and perceived loudness, whereas frequency chiefly determines pitch. Therefore, waves with identical amplitudes but different frequencies can sound equally loud while having different pitches. The propagation speed is set mainly by the medium.
Q91. A student hears an echo one second after shouting toward a building 176 m away. What is the speed of sound?
[RRB Group D, 12 August 2026]
- (A) 400 m/s
- (B) 352 m/s
- (C) 330 m/s
- (D) 176 m/s
Answer: (B) 352 m/s
Explanation: For a reflected sound, the wave must travel to the building and back. Its total path is twice 176 m, or 352 m. Dividing that round-trip distance by the one-second delay gives a speed of 352 m/s.