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Sound — Physics STD 11 Science — Question

Maharashtra BoardEnglish MediumSTD 11 SciencePhysicsSound3 Marks
Question
State the expression for apparent frequency when source of sound and listener are
i) moving towards each other
ii) moving away from each other
✓

Answer

i. Let,
$n =$ actual frequency of the source.
$n_0 =$ apparent frequency of the source,
$v =$ velocity of sound in air.
$v_s =$ velocity of the source.
$v_l =$ velocity of the listener.ii. Apparent frequency heard by the listener is given by,
$n = n _0\left(\frac{v \pm v_L}{v \pm v_s}\right)$
Where upper signs $(+ ve$ in numerator and $-ve$ in denominator$)$ indicate that source and observer move towards each other. Lower signs $(-ve$ in numerator and $+ve$ in denominator$)$ indicate that source and listener move away from each other.
iii. If source and listener are moving towards each other, then apparent frequency is given by,
$n = n _0\left(\frac{v+v_L}{v-v_s}\right)$ i.e., apparent frequency increases.
iv. If source and listener are moving away from each other, then apparent frequency is given by,
$n = n _0\left(\frac{v-v_L}{v+v_s}\right)$ i.e., apparent frequency decreases.

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