Q.An observer moves toward a stationary source of sound of frequency $400\ \text{Hz}$ at a speed of $15\ \text{m s}^{-1}$. Taking the speed of sound in air as $340\ \text{m s}^{-1}$, find the frequency heard by the observer.

Given: $f_0=400\ \text{Hz}$, $v_o=15\ \text{m s}^{-1}$ (observer moving toward stationary source), $v=340\ \text{m s}^{-1}$, $v_s=0$.

Since the observer moves toward a stationary source:

$f' = f_0\,\frac{v+v_o}{v} = 400\times\frac{340+15}{340} = 400\times\frac{355}{340}$ …

Identify only the observer is moving, toward the source, so apply $f'=f_ …

• Using the source-moving formula ($v_s$ in the denominator) by mistake for what is actually an observer-moving problem. …