Horn size sets the lowest frequency a coverage angle can hold.

Evidence

As an example, if one wanted a constant directivity horn that had a cover­ age of 90°  40° from 500 Hz then the mouth would have to be at least 0.62 m for the 90° direction and 1.28 m for the 40° direction. This is not small! You can have a smaller horn but you must recognize that it will not have this coverage angle at 500 Hz. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2495

Categorization

• DEFINE: Defines Fθmin implicitly: the frequency floor below which a horn’s nominal coverage pattern no longer holds.
• XREF: Connects to the earlier chapter material on directivity and coverage angle for loudspeakers.