Question

Source of sound and the observer are mutually at rest. If the speed of sound is changed, then the frequency of sound heard by the observer will appear to be

• A. increased
• B. decreased
• C. unchanged
• D. decreasing exponentially

Answer 1

Answer: (C)

unchanged

Answer 2

The apparent change in the frequency of the source due to a relative motion between the source and observer is known as Doppler's effect.
The perceived frequency is given by $f'=f\left(\frac{v-v_{o}}{v-v_{s}}\right)$ where $f$ is original frequency, $v$ the speed of sound, $v_{o}$ of observer, $v_{s}$ the speed of source.
In the given case there is no relative motion between source and observer,
since both are at rest, hence frequency of sound heard by the observer will remain unchanged.