The bass-reflex enclosure differs from the closed chamber by the presence of one or more vents. The bass-reflex uses the principle of the Helmholtz resonator consists of an internal volume and a vent that opens to the outside, in its simplest form.
The principle of the bass-reflex is to recover the sound waves inside the load by the depression in the rear of the boomer (equivalent to the forward pressure) to return them in phase with sound waves issued frontally.
The bass-reflex name comes from the English reflect, the idea being to reflect the sound that is inside the box like a mirror that reflects the original image. It adjusts the agreement of the vent (to avoid important phase shifts) over a frequency region where the displacement of the membrane is kept, almost zero, the sound radiation is then relayed through the vent so as to best reduce the bass distortion in the vicinity of the resonance frequency.
The volume of the enclosure and the agreement of the vent (depth, diameter) are determined by the characteristics of the speaker. The work of Thiele & Small have identified a number of technical parameters used in calculating the loads of all types ... All speakers are indeed not suitable for bass-reflex. To clear roughly, we say that a speaker with a QTS between 0.25 and 0.45 works in Bass-reflex, but it is possible to use speakers with a QTS smaller than 0.25 if the VAS is high.
Art & Acoustique Appliquée
