Bass reflex
[1][2] This port enables the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical sealed- or closed-box loudspeaker or an infinite baffle mounting.
The port generally consists of one or more tubes or pipes mounted in the front (baffle) or rear face of the enclosure.
Unlike closed-box loudspeakers, which are nearly airtight, a bass reflex system has an opening called a port or vent cut into the cabinet, generally consisting of a pipe or duct (typically circular or rectangular cross section).
The design is popular among consumers and manufacturers (speakers cabinets can be smaller and lighter, for more or less equivalent performance) but the increase in bass output requires close matching of driver, the enclosure, and port.
Acoustically, they behave largely the same around their tuning frequency as a port, as they also act as a Helmholtz resonator excited by the rear side of the bass driver's diaphragm.
They also sidestep the midrange pipe resonances that can be an issue on ported enclosures in full range systems.
They are also considerably more expensive than a port tube, as they are effectively a speaker driver minus its voice coil and motor magnet.
The effect of the various speaker parameters, enclosure sizes and port (and duct) dimensions on the performance of bass reflex systems was not well understood until the early 1960s.
Benson[11][12] and Richard H. Small[4][13][14][15] presented the theoretical foundations for the synthesis of bass reflex loudspeaker systems to meet specified low-frequency performance criteria were developed into a series of "alignments" (sets of the relevant speaker parameters) that allowed designers to produce useful, predictable responses.
Keele[16] extended the design options by presenting a new set of 6th-order vented-box loudspeaker system alignments.
Due to the latter, a flat steady-state bass response does not occur at the same time as the rest of the sonic output at higher frequencies in the operating region.
For this reason, high-powered systems using a bass reflex design are often protected by a high-pass filter that removes signals below the vent tuning frequency.
Although some may consider that this is due to the port resonance imposing its characteristics to the note being played, it is simply the result of a non-maximally flat frequency response function.
Ports often are placed on the front baffle, and may thus allow transmission of unwanted midrange frequencies reflected from within the box into the listening environment.
[19] Enclosures with a rear-facing port mask these effects to some extent, but they cannot be placed directly against a wall without causing audible problems.
[23] Asymmetrical loading of the driver cone during high level usage can be reduced by placing a baffle at the inside end of the port tube.
