Subwoofers radiate omnidirectionally because their wavelengths far exceed diaphragm size

Evidence

So, for a 12-inch (0.3 m) subwoofer operating below 80 Hz, the wavelengths are longer than 14.1 ft (4.3 m) and the sound will radiate omnidirectionally. Closed box subwoofers are “pressure” sources. It therefore doesn’t matter which way a subwoofer diaphragm faces: the radiated sound will not change. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1008

Categorization

• DEFINE: Pressure-source behavior: when wavelength is large relative to the source dimension, radiation is omnidirectional regardless of which way the diaphragm faces.