The loudspeaker manufacturing industry uses a term that is misleading to describe how a speaker behaves.
That term is “COVERAGE ANGLE”.
Anyone who knows how a loudspeaker works can tell you that there is no such thing as a consistent coverage angle for a speaker. The relevant question is “at what frequency?” Otherwise there is no reference point and no useful data to act upon.
Nathan Lively & Daniel Lundberg have provided us with the following:
sounddesignlive.com – How To Find Speaker Coverage Calculator