Physics · Science General

Acoustics and Sound Waves

2,006 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 conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A driver blows the horn when he is at a distance of $1020m$ from a stiff. He hears the echo after $11/2sec$. At what rate is he approaching the cliff. (velocity of sound in air $=340m/s$)

  1. $72m/s$
  2. $31m/s$
  3. $31mph$
  4. $60kmph$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Distance = 1020 m. Total sound distance = 2 × 1020 = 2040 m. If echo is heard after 1.5 s (assuming 11/2 means 1.5), time available for driver approach = (2040/340) - 1.5 = 6 - 1.5 = 4.5 s. Speed = 1020 / 4.5 = 31 m/s (approximately).

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

What should be the minimum distance (in m) between source and reflector in water so that echo is heard distinctly ? (The speed of sound in water $= 1400\; ms^{-1}$)

  1. 60

  2. 140

  3. 90

  4. 70
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off. 
As the speed of sound in water is 1400 m/s in the question, the distance traveled by sound in 0.1 second is 35 m, which is calculated from the formula 
$Distance\; traveled=velocity\; of\; sound \times time\; taken$. That is, 140 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 70 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

The speed of sound in air and sea-water are given to be $340\ m/s$ and $1440\ m/s$ respectively. A ship sends a strong signal straight down and detects its echo after $1.5$second. The depth of the sea at that point is 

  1. $2.16\ kms$
  2. $1.08\ kms$
  3. $0.51\ kms$
  4. $0.255\ kms$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\displaystyle V _{air}=340m/s, V _{water}=1440m/s$
Echo is the sound that we hear after reflection of sound wave from the reflecting surface.
i.e., total distance travel by sound wave $=2d$
velocity$\displaystyle=\frac{distance}{time}$
$\displaystyle 1440=\frac{2d}{1.5}\Rightarrow d=\frac{1440\times 1.5}{2}=1080.0m=1.08kms$
Hence, option $B$ is the correct answer.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

What should be the minimum distance between the source of sound and the obstacle to hear an echo?

  1. 17 m

  2. 34 m

  3. 8.5 m

  4. 51 m
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The sensation of any sound persists in our ear for about 0.1 seconds.  If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off. As the speed of sound is 340 m/s, the distance travelled by sound in 0.1 second is 34 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A man beats drum at a certain distance from the mountain. He slowly increases the rate of beating and finds that the echo is not heard distinctly, when the drum beating is at the rate of $40$ per minute. He moves by $80 m$ towards the mountain and finds that the echo is again not heard distinctly when the rate of beating of the drum is $1$ per second. What is the original distance of the man from the mountain?

  1. $120 m$
  2. $240 m$
  3. $270 m$
  4. $340 m$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Echo is not heard distinctly, when next beat overlaps with echo simultaneously.
Time per beat $=$ time taken by reflected beat to reach listener.
d being the distance and drum beating at rate of $40$ per minute.
$\dfrac{2d}{v}=\dfrac{60}{40}=\dfrac{3}{2}$
and $\dfrac{2d-80}{v}=1$
Solving we get $d=240m$

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

An echo will be heard if the minimum distance between the source of sound and the obstacle is

  1. 1 m

  2. 10 m

  3. 15 m

  4. 17 m
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

An echo is heard when a sound takes minimum of $0.1 \ s$ to reach the observer after getting reflected from the obstacle.
Let the distance between the distance between the source (or observer) and the obstacle be $x$.
Thus distance traveled by sound to hear echo is $2x$.
Speed of sound  $v = 340 \ m/s$
$\therefore$  $2x = vt$
Or  $2x = 340\times 0.1$
$\implies  \ x = 17 \ m$

Multiple choice physics acoustics conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

The echo will be heard if the original sound reflected by an obstacle reaches our ears after

  1. 10 s

  2. 5 s

  3. 1 s

  4. 0.1 s
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

When sound waves travel in a medium, it gets reflected from the obstacle and we hear the same sound produced even after some time. This reflected sound is known as echo.
Human ear can hear an echo if the reflected sound reaches the ear after at least $0.1 \ s$. If the reflected sound reaches the human ear in less than $0.1\ s$, then we cannot hear the echo.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A girl blew a whistle while standing in front of a cliff. She heard an echo after $4s$. Find the distance of the cliff from the boy if velocity of sound in air is $332m{s}^{-1}$

  1. $332m$
  2. $664m$
  3. $166m$
  4. $1328m$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Time taken by the sound to go from the boy to the cliff, $t=\dfrac{4}{2}s=2s$

Speed of sound in air, $v=332m{s}^{-1}$ 

So, distance of the cliff from the boy, $s=v\times{t}=332\times2m=664m$.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A boy shouts while standing in front of a hill. He hears an echo after $6s$. If the speed of sound  in air is $340 m/s$, what is the distance of the hill from the position of the body?

  1. $519 m$
  2. $1020 m$
  3. $2040 m$

  4. None
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let distance of hill from the position of the body be $d$.

Echo is heard after $6s$

Speed of sound, $v=340m/s$

So, $d=v \times {\dfrac{t}{2}}$
or, $d=340 \times {\dfrac{6}{2}}=1020m$

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

An echo returns in $3 s$. What is the distance of the reflecting surface from the source?

  1. $1020 m$
  2. $22.66 m$
  3. $510 m$
  4. $2.28m$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Time taken by sound to travel from the source to the deflecting surface $t=\dfrac{3}{2}=1.5s$

Speed of sound $v=340m/s$

So, distance between the surface and the source $=v \times t=340 \times 1.5=510m$

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

An echo returned in $3 s$. What is the distance of the reflecting surface from the source, given that the speed of sound is ${ 342ms }^{ -1 }$?

  1. $520 m$
  2. $515 m$
  3. $530 m$
  4. $513 m$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

If $d$ be the distance between reflecting surface and the source. 

For the echo, the sound travels distance $2d$ in $3s$. 
Thus, $2d=vt$ or $d=vt/2=\dfrac{342\times 3}{2}=513 m$ 

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

Sound produced by a thunderstorm is heard 10 s after the lightning is seen. The approximate distance of the thunder cloud :
(Given speed of sound $= 340 m s^{1}$)

  1. 3.4 Km

  2. 340 m

  3. 3.4 m

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Lightning and thunder are produced simultaneously, but the thunder is heard a few seconds after the lightning is seen. This is because the speed of light in air is more than the speed of sound in air.
The speed of light is given as $3\times { 10 }^{ 8 }$ m/s. The speed of sound is 340 m/s. Thus, the entire time of 10 seconds (as mentioned in the question) between seeing the light and hearing the thunderstorm is taken by the sound to travel to the observer. Hence, the distance traveled by the thunder is given as follows.
$s=vt$; where,
s is the distance traveled, v is the velocity of sound and t is the time taken.
That is,  $330m/s\times 10s = 3400 m = 3.4 km.$
Thus, the distance traveled by the thunder is $3.4 km.$

Multiple choice physics acoustics conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

The ceiling and wall behind the stage of good conference halls or concert halls are made:

  1. Straight

  2. Curved

  3. Circular

  4. Rectangular

Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In concert or conference halls, music or other sounds that are produced on the stage must be carried through the air to people in the crowd. Some of these sound waves go directly to the people in the crowd, without bouncing off on anything first. However, other sound waves from the stage first go to areas like walls and ceilings, and the sound either bounces off or is absorbed. When sound waves bounce off a surface, the new direction they travel is related to the angle they strike the surface. 
Thus, acoustical engineers and architects sometimes need to place panels on the ceilings and walls to reflect sound in a specific direction, that is, back to the audience. Using panels that are curved have a similar effect. Hence, ceiling and wall behind the stage of good conference halls or concert halls made curved so that sound after reflection reaches all the corners of the hall and the whole audience.

Multiple choice physics study of sound conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

What should be the minimum distance between the listener and the reflector to hear an echo of sound propagating with a speed $v\   m s^{-1}$?

  1. $v/10$ m
  2. $10 v$ m
  3. $v/20$ m
  4. $20v$ m
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
if there is a sound reflector at a distance d from the source, then the time interval between original sound and its echo at the site of the source is given as $T=\dfrac { 2d }{ v } $
 $d=\dfrac { v\times t }{ 2 } = \dfrac { v\times 1 }{ 2\times 10 } $ as Time = 0.1 s.
Hence, $d=\dfrac { v }{ 20 }$.

Multiple choice physics acoustics conditions for hearing echo echo reflection of sound and echo ultrasound and its applications

A man fires a gun and hears its echo after 5 s. The man then moves 310 m towards the hill and fires his gun again. This time he hears  the echo after 3 s. Calculate the speed of sound. 

  1. $330\,m\, s^{-1}$
  2. $350\,m\, s^{-1}$
  3. $210\,m\, s^{-1}$
  4. $310\,m\, s^{-1}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The speed of the sound is calculated as follows.
Let d be the distance between the man and the hill in the beginning.
$v=\dfrac { 2\times d }{ t } $ = $\dfrac { 2\times d }{ 5 } \rightarrow eqn\quad 1$
He moves 310 m towards the hill. Therefore the distance will be (d - 310) m. Therefore,
$v=\dfrac { 2\left( d-310 \right)  }{ 3 } \rightarrow eqn\quad 2$
Since velocity of sound is same, equating (1) and (2), we get
$\dfrac { 2d }{ 5 } =\dfrac { 2\left( d-310 \right)  }{ 3 } $
3d = 5d - 1550
2d = 1550
d = 775 m
Hence, the velocity of sound v=$\dfrac { 2\times 775 }{ 5 } $ (substituting in equation 1)
$v=310 m/s$