Tag: echo

Questions Related to echo

The minimum distance between the sound and the reflecting surface, in order to hear an echo, must be

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

  1. $0.65\space m$

  2. $1.65\space m$

  3. $16.5\space m$

  4. $165\space m$

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Correct Option: C
Explanation:

Let the distance between the source of sound and the obstacle be $x$ metres .

Thus the distance travelled by sound to reach back to the source   $d = 2  x$
As the minimum time required to hear an echo is equal to $ 0.1  sec$
As velocity of sound in air    $ v = 330   m/s$
Thus  $d = v  t$
$2  x = 330  \times 0.1             \implies x = 16.5   m$

Velocity of sound in the atmosphere of a planet is 600m$s^{-1}$. The minimum distance between the source of sound and the obstacle to hear echo should be

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

  1. 60 m

  2. 25 m

  3. 30 m

  4. 17 m

Show answer
Correct Option: C
Explanation:

Let the distance between the source of sound and the obstacle be $x$ metres .

Thus the distance travelled by sound to reach back to the source   $d = 2  x$
As the minimum time required to hear an echo is equal to $ 0.1  sec$
Thus  $d = v  t$
$2  x = 600  \times 0.1             \implies x = 30   m$

A source of sound emitting a tone of frequency 200 Hz moves towards an observer with a velocity v equal to the velocity of sound if the observer also moves away from the source with the same velocity v, the apparent frequency heard by the observer is

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

  1. $50 Hz$

  2. $100 Hz$

  3. $150 Hz$

  4. $200 Hz$

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Correct Option: D

A table is revolving on its axis at 5 revolutions per second. A sound source of frequency 1000 Hz is fixed on the table at 70 cm from the axis. The minimum frequency heard by a listener standing at a distance from the table will be (speed of sound = 352 m/s)

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

  1. 1000 Hz

  2. 1066 Hz

  3. 941 Hz

  4. 352 Hz

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Correct Option: C
Explanation:

$\begin{array}{l} For\, \, source\, \, vS=r\omega =0.70\times 2\pi \times 5=22\, m/s \ the\, \, source\, \, is\, \, receding\, \, the\, \, man.\, it\, \, is\, \, given\, \, by\, \, { \eta _{ \min   } }=n\frac { v }{ { v+{ v _{ s } } } } Hz \ Minimum\, \, frequency\, \, is\, \, heard\, \, when=1000\times \frac { { 352 } }{ { 352+22 } } =941 \end{array}$

A person clapped his hands hear cliff and heard a echo after 5s. The minimum distance of the cliff from the person if the speed of sound is take as 346 m/s 

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

  1. 17.11 m

  2. 117.2 m

  3. 173 m

  4. 865 m

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Correct Option: D

A mas fires a bullet standing between two cliffs. First echo is heard after $3$ seconds and second echo is heard after $5$ seconds. If the velocity of sound is $336m/s$, then the distance between the cliffs is

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

  1. $5\times 336 m$

  2. $4\times 336 m$

  3. $3\times 336 m$

  4. $2\times 336 m$

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Correct Option: B

A rail engine approaches a hill at a speed of $54\ kmph$ whistling a sound of frequency $160\ Hz$. The frequency of echo heard by the engine driver is (velocity of sound in air $335\ ms^{-1})$.

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

  1. $167.5\ Hz$

  2. $152.5\ Hz$

  3. $175\ Hz$

  4. $145\ Hz$

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Correct Option: C

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 $=340m/s$)

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

  1. $72m/s$

  2. $31m/s$

  3. $31mph$

  4. $60kmph$

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Correct Option: B

State whether given statement is True or False
A man standing 25 m away from a wall produces a sound and receives the reflected sound.The man will be able to hear a distinct echo.

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

  1. True

  2. False

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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 the question, the wall is 25 m away from the man and hence, the man will be able to hear the echo distinctly.

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 $= 1400\; ms^{-1}$)

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

  1. 60

  2. 140

  3. 90

  4. 70

Show answer
Correct Option: D
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.