What does bass-reflex mean?
To extend the response in the bass of an enclosure, is added within the body a blowhole opening outwardly. A tuned cavity is thus created, based on the principle of the Helmholtz resonator: at a certain frequency, called "tuning frequency", determined by the length and the diameter of the blowhole as well as by the volume of the enclosure , the system comes into resonance, allowing to "acoustically amplify" the sound (we can test by speaking near the neck of a bottle, which is also a Helmholtz resonator ...). By judiciously placing the tuning frequency, you can (to a certain extent!) Compensate for the level drop in the low end of the spectrum. The choice of the diameter of the blowhole is made according to the air flow, in practice according to the diameter and the excursion of the loudspeaker.