IB Physics SL C 5 1 Doppler Effect Topic Practice

This question is about the Doppler effect.

Georgia carries out an experiment to measure the speed of mosquitoes. She sets up a microphone to record the sounds of passing mosquitoes.

One mosquito is moving in a straight line with constant speed and passes very close to the microphone as seen in the diagram. The mosquito produces a sound of constant frequency. The speed of sound in air is $340 \mathrm{~m} \mathrm{~s}^{-1}$.

The maximum frequency recorded is 751 Hz and the minimum frequency recorded is 749 Hz . Explain this observation.