Allow me to correct and highlight a frequent misconception. The Varitunes (commercial SMT name for the boxes with the opening) are not bass traps but helmholtz resonators.
What is the difference? I will try a very simplistic explanation (probably sending the lightning down from acoustic experts
)
Bass traps do exactly what they are called; trap (some) low frequency soundwaves and absorbs them, they are not returned to the room. In doing so, they reduce the energy of the frequency they absorb, thus making the remainder of the spectrum to be less clouded by this reduced frequency. The effect is that it may be just right, very hard to achieve in low frequency, or just not enough or worse, often leading to the surrounding frequencies to be experienced themselves to have become too dominant. The effect is that the following round of absorption treatment has to be started...
Helmholtz resonators, like the Varitunes, do not absorb bass. They are effectively tuned so that only the desired frequency is 'allowed in', not necessarily the frequency that is too dominant (for example as a result of a standing wave between ceiling and floor) but based on a real-time spectrum analysis. Anyway, the energy of the frequency is effectively for a very brief moment (following its frequency) kind of swallowed inside but of course it cannot stay there as the volume of the resonator is limited, what comes in has to come out. So that frequency wave is 'spit out' again but just a fraction of a second later, causing a phase shift of that frequency causing the energy still to be there but brought in line with the other frequencies that are not too dominant.
Beware, many brands selling helmholtz resonators stuff them with absorbant material. In doing this, they make a hybrid solution that is neither as effective as a bass trap nor as a true resonator. Not so with SMT whose Varitunes are completely empty, to act as a pure resonator making their effect maximized as a resonator but causing them to be picky in tuning though this should only be done once after installation.
Hope this helps!