Tag: ultrasound and its applications

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Questions Related to ultrasound and its applications

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

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

  1. $0.65\space m$
  2. $1.65\space m$
  3. $16.5\space m$
  4. $165\space m$
Reveal answer Fill a bubble to check yourself
C Correct answer
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  $d = v  t$
$2  x = 330  \times 0.1             \implies x = 16.5   m$

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

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

  1. 60 m

  2. 25 m

  3. 30 m

  4. 17 m
Reveal answer Fill a bubble to check yourself
C Correct answer
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  $d = v  t$
$2  x = 600  \times 0.1             \implies x = 30   m$

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

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

  1. $50 Hz$
  2. $100 Hz$
  3. $150 Hz$
  4. $200 Hz$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

According to the Doppler effect, if both the source and observer move at the same velocity in the same direction, the relative velocity between them is zero. Therefore, the frequency heard remains the same as the source frequency.

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

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)

  1. 1000 Hz

  2. 1066 Hz

  3. 941 Hz

  4. 352 Hz
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

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

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

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 

  1. 17.11 m

  2. 117.2 m

  3. 173 m

  4. 865 m
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The total distance traveled by the sound is speed times time: 346 m/s * 5 s = 1730 m. Since the sound travels to the cliff and back, the distance to the cliff is half of the total distance: 1730 / 2 = 865 m.

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

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

  1. $5\times 336 m$
  2. $4\times 336 m$
  3. $3\times 336 m$
  4. $2\times 336 m$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The distance to the first cliff is d1 = (v * t1) / 2 and the second is d2 = (v * t2) / 2. The total distance between the cliffs is d1 + d2 = (v/2) * (t1 + t2) = (336/2) * (3 + 5) = 168 * 8 = 1344 m. This equals 4 * 336 m.

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

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 $335\ ms^{-1})$.

  1. $167.5\ Hz$
  2. $152.5\ Hz$
  3. $175\ Hz$
  4. $145\ Hz$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The engine is moving toward the hill at 15 m/s (54 km/h). The frequency of the echo is f' = f * (v + vs) / (v - vs), where v is speed of sound and vs is speed of engine. f' = 160 * (335 + 15) / (335 - 15) = 160 * 350 / 320 = 175 Hz.

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

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.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
B Correct answer
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.