Question

A source is emitting sound of a single frequency. A wall is advancing towards the source at a speed of 3cm/s. If the velocity of sound in air is 330m/s the percentage change of wavelength of the note as perceived by an observer near the source is

• A. 20%
• B. 2%
• C. 0.02%
• D. 12.2%

Answer 1

The correct option is C 0.02%

The distance between source and observer is decreasing with a speed of $3\times 2=6cm/s$

$f^{{}^{\prime }}=f\left(\frac{330-0}{330-2\times \left(0.03\right)}\right)$

$f^{{}^{\prime }}=f\left(1.0002\right)$

which is $0.02$ increasing in $f$

If $f$ is increasing by $0.02$ %, wavelength will be decreased by $0.02$ %.