Bass reflex
Bass reflex is a system of loudspeaker housing that enhances the reproduction of specific sound frequencies, in most cases the lower (bass) frequencies.
In contrary to normal loudspeaker systems, that are built into a closed airtight box, a bass reflex system has an opening, in front of a resonance area.
This area consists of a plastic pipe in many cases although square casing compartments are also used. The size of the area determines the resonant frequency the system will work at.
Most bass reflex systems are designed to have a frequency resonating that is just below the spectrum that can be adequately reproduced by the loudspeaker. This way, the total reproduced Frequency spectrum is larger than originally intended by the loudspeaker manufacturer.
Some systems consist only of the resonance area and are intended only for this. They have no output directly from the speaker but only from the resonance area. They are meant as support for a larger speaker system like in theaters.
The advantage of this system is that you can reproduce more frequencies with a loudspeaker than originally intended. The disadvantage is that the resonated frequency is very specific and therefore the reproduced sound is, in many cases, not very accurate.