Tag: echo

Questions Related to echo

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 

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

  1. $2.16\ kms$

  2. $1.08\ kms$

  3. $0.51\ kms$

  4. $0.255\ kms$

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Correct Option: B
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.

State whether the given statement is True or False :

A girl is sitting in the middle of a park of dimension $\displaystyle 12m\times 12m.$ On the left side of it there is a building adjoining the park and on right side of the park, there is a road adjoining the park. A sound is produced on the road by a cracker. The girl will be able to hear the echo of this sound.

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

  1. True

  2. False

Show answer
Correct Option: B
Explanation:

Like all waves, sound waves can be reflected. Sound waves suffer reflection from the large obstacles. As a result of reflection of sound wave from a large obstacle, the sound is heard which is named as an echo. Ordinarily echo is not heard as the reflected sound gets merged with the original sound. Certain conditions have to be satisfied to hear an echo distinctly (as a separate sound).
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 is 340 m/s, the distance traveled 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. 
In this case, the girl is seated in the middle of the park. So, the sound travels through a distance of 6 m up to the girl and then another 6 m up to the building. The total distance is just 12 m which is less than the minimum distance required for the sound to echo. Hence, the echo cannot be heard in this case. 

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

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

  1. 17 m

  2. 34 m

  3. 8.5 m

  4. 51 m

Show answer
Correct Option: A
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.

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?

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

  1. $120 m$

  2. $240 m$

  3. $270 m$

  4. $340 m$

Show answer
Correct Option: B
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 $d=240m$

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

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

  1. 1 m

  2. 10 m

  3. 15 m

  4. 17 m

Show answer
Correct Option: D
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  $2x = 340\times 0.1$
$\implies  \ x = 17 \ m$

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

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

  1. 10 s

  2. 5 s

  3. 1 s

  4. 0.1 s

Show answer
Correct Option: D
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.

A man is sitting in Gandhi Park of dimension 6m $\times$ 6m. On the left side of it, there is a building adjoining the park, and on the right side of the park, there is a road adjoining by park. A sound is produced on the road by a diwali bomb :

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

  1. the man will hear the echo of this sound

  2. no sound will be heard by the man

  3. no echo will be heard

  4. none

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Correct Option: C
Explanation:

The minimum distance between source of sound and obstacle , to hear an echo is 17m , as the dimensions of the park are $l=6m  , b=6m$ , therefore  distance between road and building is 6m , therefore echo of the bomb sound will not be produced as distance between source of sound (bomb) and building is less than 17m i.e. echo will not be heard .

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 $332m{s}^{-1}$

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

  1. $332m$

  2. $664m$

  3. $166m$

  4. $1328m$

Show answer
Correct Option: B
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, $s=v\times{t}=332\times2m=664m$.

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?

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

  1. $519 m$

  2. $1020 m$

  3. $2040 m$

  4. None

Show answer
Correct Option: B
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, $d=340 \times {\dfrac{6}{2}}=1020m$

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

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

  1. $1020 m$

  2. $22.66 m$

  3. $510 m$

  4. $2.28m$

Show answer
Correct Option: C
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 $=v \times t=340 \times 1.5=510m$