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14. waves and sound — Physics STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
An observer while going on scooter hears sound of two sirens of same frequencies from two opposite directions. If he travels along the direction of one of the siren, then he
  • A
    Listens resonance
  • ✓
    Listens beats
  • C
    Will not listen sound due to destructive interference
  • D
    Will listen intensive sound due to constructive interference
✓

Answer

Correct option: B.
Listens beats
b
Let the two waves produced due to the two sirens have frequency and velocity as $v_1, v_1$ and $v_2, v_2$

Since, both are sound waves, we have $v_1=v_2=v$

For one wave the traveler is moving towards the source,

$\therefore v^{\prime}=v\left(\frac{v-v_0}{v}\right)$

For the other the traveler is moving away from the source

$\therefore v^{\prime}=v\left(\frac{v+v_0}{v}\right)$

Seeing this, we can conclude that the waves are in phase with each other and are fluctuating repeatedly.

Since, they are sound waves they are periodic in nature. Hence, we got two waves that are periodic and are repeatedly fluctuating. This is the condition of beats. So, beats will be produced and the traveler will listen to beats.

Option $(B)$ is the correct answer.

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