Room acoustics
Room modes
Standing waves at frequencies set by a room's dimensions that create strong peaks and nulls in bass response at different seats.
A room mode occurs when a sound wave fits a whole number of half-wavelengths between two parallel surfaces, reinforcing itself into a resonance that is loud in some positions and nearly absent in others.
Modes are why bass differs so much from seat to seat and why a subwoofer can sound boomy at one frequency and thin at another. Below a few hundred hertz the room, not the speaker, dominates.
The fundamental for a dimension is f = c / 2d, so a 5 m length with c at 343 m/s gives a mode near 34 Hz, with harmonics at 69, 103, and so on. Dimensions that are simple multiples of each other stack modes; ratios near 1 : 1.4 : 1.9 spread them out.