Question

A source of sound produces waves of wavelength 60 cm when it is stationary. If the speed of sound in air is 320 m/s and source moves with speed 20 m/s, the wavelength of sound in the forward direction will be approximately:

• A. 56.2 cm
• B. 60.8 cm
• C. 64.4 cm
• D. 68.6 cm
  

Answer 1

The correct option is A 56.2 cm

Apparent frequency is given by:
$n^{\prime }=n\left(\frac{v}{v-v_s}\right)$

For a medium, frequency is inversely proportional to wavelength.
${\lambda }^{\prime }=\left(\frac{v-v_s}{v}\right)\lambda$
${\lambda }^{\prime }=\left(\frac{320-20}{320}\right)60$
${\lambda }^{\prime }=56.25cm$