That is a ported sub. That is known as a Helmholtz resonator, and is an acoustic transformer. At the operating frequency of the deep bass, the pressure inside the box is very high, and that limits cone displacement. So the air movement is coming out of the port, which you will see if you put a candle in front of the port.
The whole problem with sealed designs, is that only cone movement displaces air. A loudspeaker cone is a very poor coupler to the air in the room in the lowest frequencies. Pipes and horns are even more efficient.
Take a look at this recent sub design, I did for a member. Take a look at cone displacement with frequency, and the vent air velocity especially.
Also take a look at the impedance curve. The tuning frequency is right at the mid point between the two peaks of impedance, around 22 HZ. Note how in the region of the tuning frequency cone displacement falls, but vent air velocity increases. As you fall below peak port air velocity, cone excursion increases, but sound level drops away at 24 db per octave.