Physics · Science General

Acoustics and Sound Waves

2,160 Questions

Acoustics and sound waves deal with mechanical vibrations traveling through media like air and water. Key concepts include wave reflection, beats, echoes, the Mach number, and infrasound. These physics fundamentals are regularly tested in general science sections of multiple competitive exams.

Sound wave propagationEchoes and reflectionWave interferenceMach numberInfrasound frequency

Acoustics and Sound Waves Questions

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

A source of sound A emitting waves of frequency 1800 Hz is falling towards ground with a terminal speed v. The observer B on the ground directly beneath the source receives wave of frequency 2150hz. The source A receives waves, reflected from frequency nearly: (Speed of sound = 343 m/s)

  1. 2150 Hz

  2. 2500Hz

  3. 1800Hz

  4. 2400Hz
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Frequency received by source A is
$f=1800\left( \dfrac { 343+V }{ 343-V }  \right) $
$for\quad V;$
$2150=1800\left( \dfrac { 343 }{ 343-V }  \right) $
$343-V=\dfrac { 1800\times 343 }{ 2150 } $
$V=56\quad m/s$
$\therefore \quad \quad f=1800\left( \dfrac { 399 }{ 287 }  \right) \simeq 2500Hz$
Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

The time of reverberation of a room A is one second. What will be the time (in seconds) of reverberation of a room, having all the dimensions double of those of room A:

  1. 2

  2. 4
  3. $\frac{1} {2}$

  4. 1
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Time of reverberation $\propto$ $\frac{V} {A}$ (sabine's formula)

Where V = volume of room and A = area of room
Area of new room becomes 4 times of A and Volume becomes 8 times of V
Time of reverberation will be 2 seconds

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

When all of the audience come out of the audotirium the reverberation time became$\frac{4}{3}$ of that when packed with the audience. If all $400$ persons come out, the ratio of the absorption of the auditorium to that a person is 

  1. $1600$
  2. $1200$
  3. $3000$
  4. $1000$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

T = k * V / A. T_empty = k * V / A_auditorium. T_full = k * V / (A_auditorium + 400 * A_person). Given T_empty = 4/3 * T_full. So (A_auditorium + 400 * A_person) / A_auditorium = 4/3. 1 + 400 * A_p / A_a = 1.333. 400 * A_p / A_a = 0.333. A_a / A_p = 400 / 0.333 = 1200.

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

The reverberation time of a hall of volume $200m^{-3}$ is $1.7s$. The reverberation time if $20$ persons having absorptions $0.4$ entered the hall, nearly is 

  1. $1.5s$
  2. $1.4s$
  3. $1.3s$`
  4. $1.2s$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

T1 = 0.161 * V / A1 = 1.7. A1 = 0.161 * 200 / 1.7 = 18.94. New absorption A2 = A1 + 20 * 0.4 = 18.94 + 8 = 26.94. T2 = 0.161 * 200 / 26.94 = 1.195s, which is approx 1.2s.

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

Fill in the blank.

___________ is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space.

  1. Reverbaration

  2. Echo

  3. Response

  4. Heat
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A reverberation, or reverb, is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space which could include furniture and people, and air. 

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

What will happen if the reverberation time in a big hall is too long?

  1. Sound will persist for some time in the hall

  2. The reflection may become an echo.

  3. Multiple reflection may occur

  4. All the above will happen
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The multiple reflection of sound from the walls of a room or hall causing sound to persist for some time is called reverberation. If the hall is too long , the reflection will become an echo.

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

A man standing between two cliffs hears the first echo of a sound after 2 sec and the second echo 3 sec after the initial sound. If the speed of sound be $330   {m}/{sec}$ the distance between the two cliffs should be

  1. 1650 m

  2. 990 m

  3. 825 m

  4. 656 m
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let $d _{1}$ and $d _{2}$ be the distances of first and second cliff from man .

Echo after 2s will be heard when sound travels a distance of $2d _{1}$ , because an echo comes back to initial point after reflection .
    therefore  by ,   distance=speed$\times$time ,
         or                 $2d _{1}=330\times2=660$ ,
         or                 $d _{1}=660/2=330m$ ,
similarly Echo after 3s will be heard when sound travels a distance of $2d _{2}$ ,  
    therefore   
         or                 $2d _{2}=330\times3=990$ ,
         or                 $d _{2}=990/2=495m$ ,
as the man is in between the cliffs , therefore distance between cliffs willbe ,
             $d=d _{1}+d _{2}=330+495=825m$

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

(A).  The reverberation time is dependent on the shape of the enclosure,  position of the sources and observer.
(B) The unit of absorption coefficient in S.I. system in metric is sabine

  1. both (A) and (B) are true

  2. both (A) and (B) are false

  3. (A) is true but (B) is false

  4. (A) is false but (B) is true
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
The reverberation time is independent on the shape of the enclosure, position of the sources and observer.
Absorption coefficient is unitless.
Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

A hall has volume 4x6x10 m$^3$. If the total sound absorption of the hall is 27.2metric sabine and 40 visitors are in the hall and each is equivalent to 0.5metric sabine sound absorption then the reverberation time is

  1. 0.8644s

  2. 0.05s

  3. 0.72s

  4. 1.8s
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$volume =240m^{3}$
$Area =27.2m^{2}+40\times 0.5m^{2}$
=47.2
$R.T.=0.161\times \frac{V}{A}$
=0.8186s

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

The reverberation time of a room is t seconds. Another room of double the dimensions with the walls of the same absorption coefficient will have a reverberation time :

  1. $t^{2}$
  2. $2 t$
  3. $t/2$
  4. t$^{3}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given: $\dfrac{V _{1}}{V _{2}}=\dfrac{1}{8} \
\ \dfrac{A _{1}}{A _{2}}=\dfrac{1}{4}$
$R.T. _{1}=t $
$R.T=0.161\dfrac{v}{A}$
$\dfrac{R.T. _{1}}{R.T. _{2}}=\dfrac{0.161 \times \dfrac{v _1}{A _1}}{0.161 \times \dfrac{v _2}{A _2}}=\dfrac{v _{1}A _{2}}{v _{2}A _{1}}=\dfrac{1}{8}\times \dfrac{4}{1} =\dfrac{1}{2}$
$R.T _{2}=2R.T _{1}$
          $=2\times t$
          $=2t$ 

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

The reverberation times in a cinema theatre are 3s, 2s when it is empty, filled with audience respectively.  The reverberation time when the theatre is half filled  with audience is

  1. 2.3 s

  2. 2.4 s

  3. 2.5 s

  4. 2.6 s
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

let Area be A, when empty
let Area of audience be $A _{2}$ when full
so Area of cinema theatre when full $=A _{1}+A _{2}$
Area when half full $=A _{1}+\frac{A _{2}}{2}$
$R.T. \alpha \frac{1}{A}  ; \frac{R.T _{1}}{R.T _{2}}=\frac{A _{2}+A _{1}}{A _{1}}$
$3A _{1}=2A _{1}+2A _{2}  ; A _{1}=2A _{2} ; A _{2}=^{A _{1}}/ _{2}$
$R.T _{3}\alpha \frac{1}{A _{1}\frac{A _{2}}{2}} =\frac{4}{5 A _{1}}$
$ this \frac{4}{5} times     3 sec =2.4 sec$