Sound Waves: Characteristics and Applications MCQs for UPSC Prelims
125 practice questions covering Sound Waves: Characteristics and Applications, organised into 1 topics.
125 questions include a written explanation.
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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.
ABoth A and R are true, and R is the correct explanation of A
BBoth A and R are true, but R is not the correct explanation of A
CA is true, but R is false
DA 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
hard
A student sees a sound wave drawn as a smooth wavy curve and concludes that air particles must move up and down across the direction in which the sound travels. What is wrong with this conclusion?
AThe particles actually move in small circles rather than in straight lines
BThe curve itself is wrong, because a sound wave can never be drawn as a smooth curve
CThe curve only shows how density varies along the wave; the particles still vibrate along the direction of travel
DNothing is wrong, because air particles really do move up and down as sound passes
Show answer and explanation
Correct answer: C - The curve only shows how density varies along the wave; the particles still vibrate along the direction of travel
The wavy curve is a graph of density or pressure against distance, not a picture of the path the particles take. Along the real wave the particles shift back and forth in the direction of travel, forming compressions and rarefactions. Drawing such a graph is perfectly correct, and air particles in a sound wave do not follow circular paths.
Q3
easy
Two sounds reaching a listener have the same pitch and the same loudness, but they are still easy to tell apart. What must be different about them?
ATheir frequency
BTheir quality
CTheir wavelength
DTheir amplitude
Show answer and explanation
Correct answer: B - Their quality
Equal pitch means equal frequency, and in the same air an equal frequency means an equal wavelength as well. Equal loudness means much the same amplitude. Quality is the only characteristic left that can differ, and it is exactly what lets a listener separate two sources.
Q4
easy
A sound wave arrives at a large smooth wall and is reflected from it. Which statement about that reflection is correct?
AThe sound is reflected only when it arrives straight along the normal
BThe angle at which the sound leaves the wall is always twice the angle at which it arrived
CThe sound leaves the wall at right angles to it, whatever the angle of arrival
DThe 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.
Q5
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?
A$17.2\ \text{m}$
B$34.4\ \text{m}$
C$3.44\ \text{m}$
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 $0.01\ \text{s}$.
Q6
medium
A vibrating object completes $900$ full vibrations in $3\ \text{s}$. What is its frequency?
A$300\ \text{Hz}$
B$900\ \text{Hz}$
C$2700\ \text{Hz}$
D$150\ \text{Hz}$
Show answer and explanation
Correct answer: A - $300\ \text{Hz}$
Frequency is vibrations divided by time, so $900 \div 3 = 300\ \text{Hz}$. Quoting $900\ \text{Hz}$ forgets to divide by the time, $2700\ \text{Hz}$ multiplies by the time instead of dividing, and $150\ \text{Hz}$ comes from dividing by $6$ rather than by $3$.
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