Q.A car's horn sounds a note of frequency $500\ \text{Hz}$ as the car moves toward a stationary pedestrian at $20\ \text{m s}^{-1}$. Taking the speed of sound as $340\ \text{m s}^{-1}$, find the frequency of the horn as heard by the pedestrian.

Given: $f_0=500\ \text{Hz}$, $v_s=20\ \text{m s}^{-1}$ (source moving toward stationary observer), $v=340\ \text{m s}^{-1}$, $v_o=0$.

Since the source moves toward a stationary observer:

$f' = f_0\,\frac{v}{v-v_s} = 500\times\frac{340}{340-20} = 500\times\frac{340}{320}$ …

Identify that only the source is moving, toward the observer, so use $f'=f_0 …

• Using $v+v_s$ in the denominator instead of $v-v_s$ (that form is for a receding source, which would lower, not raise, the pitch). …