Between parallel walls (front and back wall, side walls, ceiling and floor) so called standing waves arise. Their characteristics are dependent on the distance between and the nature of the walls. You can even calculate at what frequencies you will likely encounter standing waves if you know the dimensions and nature, there are calculation tools on the internet that conduct those calculations.
Standing waves become noticeable as "boomy" bass, because the bass at the affected frequency is much louder than the other bass notes. In addition that bass energy decays relatively slow, so the bass is booming quite a long time and therefore seems slow.
And if that wasn't enough, next to bass exaggeration also cancellations exist. At these frequencies the bass is downright absorbed, and therefore is too low in volume. As a result we get a bass frequency response that has peaks and dips (see picture) instead of a linear frequency response:
A subwoofer with a sealed or ported (bass-reflex) enclosure disperses the bass omnidirectional. As a result all three major room resonances between front- and back-wall, between the side-walls and between ceiling and floor are stimulated. The chassis placement (front, back, upfire, downfire) in this case is irrelevant, they all radiate omnidirectional.
Our MusikBass subwoofer using the RiPol-principle by Axel Ridtahler radiates mainly to twards the listening position and towards the back wall with 180 degrees inverted phase. As a result the stimulation of room modes is diminished. For an explanation of the advantages of that principle please also read our section "10 advantages".