A bumblebee buzzes past you at 3 m/s. The frequency of the hum made by its wings is 152 Hz. Assume the speed of sound to be 342 m/s. What is the observed frequency of the hum as the bumblebee approaches you?

As the bumblebee approaches me, the frequency of sound becomes higher than the 152 Hz because of the velocity with which the bee approaches at 3 m/s.

The speed of the bee causes me to detect an increase of frequency higher than 152 Hz.

This phenomenon is called Doppler Effect.