Question

A truck moving at a ratr of 50 m/s sounds its horn with a frequency of 800 Hz. Find the frequency heard by the stationary observer, as the truck approaches him. The ambient temperature is about 40°C.

Answer 1

Find the speed of sound at 40°C:

$$c=331\sqrt{1+\frac{40}{273}}=354\text{ m/s}.$$

The frequency heard by the stationary observer, as the truck approaches:

$$f_r=\frac{f_s}{1-v/c}=932\text{ Hz}.$$