Room modes
Standing waves between a room's walls that make some bass notes boom and others vanish.
When half a wavelength fits a whole number of times between two parallel surfaces, sound bounces back and forth in phase with itself. The lowest axial mode sits at the speed of sound divided by twice the room length — five metres gives roughly 34 Hz, plus every integer multiple. Add tangential modes between four surfaces and oblique ones between all six, weaker but more numerous.
In a small room the modes are sparse below a few hundred hertz, and there they shape the bass more than the speakers do. In the middle of the room a note can disappear while in the corner it is ten decibels too loud. The fixes are tediously physical: move the listening position, put bass traps in the corners. Room correction can tame the peaks but can never fill the nulls.