Question

The horn of a car emits sound with a dominant frequency of 2400 Hz. What will be the apparent dominant frequency heard by a person standing on the road in front of the car if the car is approaching at 18.0 km h⁻¹? Speed of sound in air = 340 m s⁻¹.

Answer 1

Given:
Frequency of sound emitted by horn $f_0$ = 2400 Hz
Speed of sound in air v = 340 ms⁻¹
Velocity of car $v_s$ = 18 kmh⁻¹ = $18\times \frac{5}{18}\mathrm{m}/\mathrm{s}$ = 5 m/s

Apparent frequency of sound $\left(f\right)$ is given by:

$f=\left(\frac{v}{v-v_s}\right)\times f_0$

On substituting the values, we get:

$$\begin{gathered} f=\left(\frac{340}{340-5}\right)\times 2400 \\ =2436\mathrm{Hz} \end{gathered}$$