Microphones become more directional at higher frequencies, so off-axis reverberant pickup tilts dark.

Evidence

Most practical microphones do not have an even response at all angles as a function of frequency. In general, they become more directional as the frequency increases. As most microphones are designed to give a specified on-axis frequency response in order to capture the direct sound accurately, this results in a response to the reverberant sound that falls with frequency. … However, in recordings in which the reverberant field dominates, for example classical music recording, the effect is significant. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2430

Categorization

• DEFINE: Explains why mic directivity is frequency-dependent and what that does to the ratio of direct to reverberant sound.
• XREF: Connects to the chapter’s direct-to-reverberant sound balance framework the reader is tracking for listening.