A loudspeaker’s physical size sets a hard floor on the frequencies it can direct.

Evidence

However, irrespective of the technology used, there is a fundamental lower limit on the frequencies at which the speakers are directive that is determined by their size. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2499

Categorization

• QUESTION: How exactly does radiating area relate to wavelength to set this limit?