Final answer:
The proximity effect causes an increase in bass response when a directional microphone is moved closer to a sound source. It is a specific phenomenon related to microphone design and placement, not to be confused with the Doppler effect or the occurrence of a sonic boom.
Step-by-step explanation:
The phenomenon that causes an increase in bass response when a directional microphone is brought closer to a sound source is known as the proximity effect. This effect typically occurs with directional microphones such as cardioid, supercardioid, and hypercardioid microphones. When these microphones are moved closer to the sound source, the bass frequencies are boosted, which can make the sound appear warmer or more resonant. This effect does not occur with omnidirectional microphones, as they respond equally to sound from all directions.
The proximity effect is quite distinct from concepts such as acoustic amplification, bass enhancement, frequency modulation, the Doppler effect, and sonic booms. Specifically, it is not related to the Doppler effect, which describes a change in frequency or pitch perceived by an observer when the sound source and observer are in relative motion. A sonic boom, another concept frequently discussed alongside Doppler, occurs when an object moves faster than the speed of sound, creating a loud shockwave. These concepts are centered on the relative motion and speed of sound sources, rather than the microphone placement's influence on sound frequency response.