Final answer:
The increase in bass response when a directional microphone is moved closer to the sound source is referred to as the proximity effect. This effect emphasizes lower frequencies due to reduced sound wave diffusion.
Step-by-step explanation:
The phenomenon you're describing is known as the proximity effect. This occurs when a directional microphone is moved closer to a sound source, resulting in an increase in bass response. Directional microphones are designed to be most sensitive to sounds coming from specific directions, and less so to sounds from other directions. As the microphone gets closer to the source, the sound waves have less room to diffuse and spread out, which emphasizes lower frequencies. Essentially, the direct sound becomes more pronounced relative to the reflected and ambient noise in the environment.
This principle is related to how frequency is perceived depending on the movement of the source or the observer. For example, when the source of a sound moves closer to an observer, the observed frequency increases—a phenomenon known as the Doppler effect. Similarly, in a room where a stereo system has been set up, the way that sound interacts with the environment—reflecting off walls, being absorbed by furniture, etc.—can cause variations in volume and tone, explaining why some areas might have duller or excessively loud sounds.