Final answer:
The increase in bass frequency response when the source is close to the microphone is called the Doppler effect.
Step-by-step explanation:
The increase in bass frequency response when the source is close to the microphone is called the Doppler effect. It occurs because the sound waves are compressed and have a shorter wavelength when the source is moving towards the microphone, resulting in a higher frequency. This effect is commonly observed when a moving car passes by and the sound of its engine seems to change pitch as it approaches and then moves away.