reflection-of-sound-and-its-applications MCQs for UPSC Prelims

25 practice questions on reflection-of-sound-and-its-applications from the Sound Waves: Characteristics and Applications section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

8 Easy 12 Medium 5 Hard

Sample questions

Q1
medium
Assertion (A): Heavy curtains are hung along the walls of a large hall in order to reduce reverberation. Reason (R): Sound needs a material medium in order to travel from one place to another.
  1. A Both A and R are true, and R is the correct explanation of A
  2. B Both A and R are true, but R is not the correct explanation of A
  3. C A is true, but R is false
  4. D A is false, but R is true
Show answer and explanation

Correct answer: B - Both A and R are true, but R is not the correct explanation of A

(A) is true, because soft heavy fabric soaks up sound energy that a bare wall would send back into the hall. (R) is true as well, since sound cannot cross a vacuum. But needing a medium has nothing to do with why curtains help; absorption at the surface is the reason, so (R) does not explain (A).
Q2
easy
A sound wave arrives at a large smooth wall and is reflected from it. Which statement about that reflection is correct?
  1. A The sound is reflected only when it arrives straight along the normal
  2. B The angle at which the sound leaves the wall is always twice the angle at which it arrived
  3. C The sound leaves the wall at right angles to it, whatever the angle of arrival
  4. D The angle at which the sound leaves the wall equals the angle at which it arrived
Show answer and explanation

Correct answer: D - The angle at which the sound leaves the wall equals the angle at which it arrived

Sound obeys the same laws of reflection as light, so the angle of reflection equals the angle of incidence, and the incident sound, the reflected sound and the normal all lie in one plane. Doubling the angle or fixing it at a right angle has no basis, and sound arriving at a slant is reflected just as surely as sound arriving along the normal.
Q3
medium
The sensation of a sound lasts about $0.1\ \text{s}$ in the human brain, and the speed of sound in air is $344\ \text{m s}^{-1}$. What is the least distance a person must stand from a large wall in order to hear a distinct echo of their own clap?
  1. A $17.2\ \text{m}$
  2. B $34.4\ \text{m}$
  3. C $3.44\ \text{m}$
  4. D $8.6\ \text{m}$
Show answer and explanation

Correct answer: A - $17.2\ \text{m}$

In $0.1\ \text{s}$ the sound covers $344 \times 0.1 = 34.4\ \text{m}$, but that is the journey to the wall and back, so the wall is half of it away, $17.2\ \text{m}$. Quoting $34.4\ \text{m}$ forgets the return trip, $8.6\ \text{m}$ halves the distance twice, and $3.44\ \text{m}$ uses a time of $0.01\ \text{s}$.
Q4
easy
A sound is sent towards a distant wall, is reflected, and returns to the sender after a total time $t$. If the speed of sound is $v$, what is the distance from the sender to the wall?
  1. A $\dfrac{t}{v}$
  2. B $vt$
  3. C $\dfrac{vt}{2}$
  4. D $\dfrac{2v}{t}$
Show answer and explanation

Correct answer: C - $\dfrac{vt}{2}$

In the time $t$ the sound covers the whole journey to the wall and back, a total path of $vt$, so the one-way distance is half of that. Using $vt$ is the standard slip of forgetting the return trip and gives twice the true distance. The other two expressions do not even have the units of a length.
Q5
hard
A faint ticking clock stands across a room. When a large flat board is held behind the clock and tilted so that sound striking it is sent towards a listener, the ticking becomes noticeably clearer. Which idea does this use?
  1. A A board raises the frequency of the ticking, and the ear detects high notes better
  2. B Sound obeys the laws of reflection, so a large surface can direct it towards a chosen point
  3. C A board removes the need for air between the clock and the listener
  4. D Sound travels faster along a wooden board than through air, so it arrives sooner
Show answer and explanation

Correct answer: B - Sound obeys the laws of reflection, so a large surface can direct it towards a chosen point

Sound that would have spread uselessly behind the clock is turned back towards the listener, because the board reflects it with the angle of reflection equal to the angle of incidence. Arriving a fraction of a second sooner would not make a sound clearer, the board cannot change the rate at which the clock ticks, and air is still needed to carry the sound across the room.
Q6
medium
A SONAR pulse sent downwards from a ship returns from a shoal of fish $0.8\ \text{s}$ after it was sent. The speed of sound in sea water is $1500\ \text{m s}^{-1}$. How far below the ship is the shoal?
  1. A $1200\ \text{m}$
  2. B $1875\ \text{m}$
  3. C $300\ \text{m}$
  4. D $600\ \text{m}$
Show answer and explanation

Correct answer: D - $600\ \text{m}$

The pulse travels $1500 \times 0.8 = 1200\ \text{m}$ down to the fish and back, so the shoal lies $\dfrac{1200}{2} = 600\ \text{m}$ below the ship. Taking $1200\ \text{m}$ as the answer ignores the return trip, $300\ \text{m}$ halves the depth once too often, and $1875\ \text{m}$ comes from dividing the speed by the time.
Q7
medium
A newly built auditorium has bare concrete walls and a hard tiled floor, and speech from the stage sounds blurred and muddled. Which change would reduce the problem most?
  1. A Raising the ceiling and making the walls even harder
  2. B Replacing the tiled floor with a sheet of metal
  3. C Hanging heavy curtains and fitting carpets and cushioned seats
  4. D Polishing the concrete walls until they are perfectly smooth
Show answer and explanation

Correct answer: C - Hanging heavy curtains and fitting carpets and cushioned seats

Blurred speech is reverberation, so the cure is to absorb sound rather than bounce it back, and soft porous materials such as curtains, carpets and cushioned seats do exactly that. Polishing the walls, laying metal and hardening the surfaces all make the reflections stronger, and a higher ceiling gives the sound even longer to rattle around.
Q8
easy
What does SONAR send out in order to find objects under water?
  1. A Infrasonic waves made by ocean currents
  2. B Pulses of ultrasonic sound, and it then detects their echoes
  3. C Radio waves, which pass easily through deep water
  4. D Beams of light, and it then detects their reflections from the sea bed
Show answer and explanation

Correct answer: B - Pulses of ultrasonic sound, and it then detects their echoes

A SONAR transmitter sends short ultrasonic pulses into the water and a receiver picks up the echo returning from the sea bed or from an object. Light is absorbed within a short depth of water, ocean currents are not a controllable source of pulses, and radio waves are absorbed quickly by sea water.

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