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A Bass reflex system (also known as a ported, vented box or reflex port) is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.

Contents 1 Explanation 2 History 3 Advantages 4 Limitations 5 See also 6 References

In contrast to closed box loudspeakers, which are substantially air tight, a bass reflex system has an opening called a port or vent which consists of an opening, generally backed with a pipe or duct of circular or rectangular cross section. The air mass in this opening resonates with the "springyness" of the air inside the enclosure in exactly the same fashion as the air in a bottle resonates when a current of air is directed across the opening. The frequency at which the box/port system resonates, known as the Helmholtz resonance, is determined by the cross sectional area and length of the duct and the volume of air inside the enclosure.

When all features are compared, for home use the advantages tend to outweigh the disadvantages because they allow more bass extension out of a smaller box. This is probably why the design is so popular among consumers and manufacturers. Nevertheless the augmentation in bass output is invariably achieved at the expense of temporal integrity of the signal and the buildup of more resonances, and as such it can be undesirable in settings where the utmost accuracy of reproduction is desired, e.g. in monitoring facilities, recording studios etc.

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 1970s. At that time, pioneering analyses by A.N. Thiele ^[1] and Richard H. Small ^[2] related these factors to a series of "alignments" (sets of the relevant speaker parameters) that produced useful, predictable responses. These made it possible for speaker manufacturers to design speakers to match various sizes of enclosures and enclosures to match given speakers with great predictability. All of this is constrained by the laws of physics, which is discussed in detail in Thiele and Small's work. It is not possible to have a small speaker in a small enclosure producing extended bass response at high efficiencies (ie, requiring only a low-powered amplifier). It's possible to have two of these parameters, but not all. The sound pressure produced is dependent on the efficiency of the speaker, the mechanical or thermal power handling of the driver, the power input and the size of the driver.

This resonant system augments the bass response of the driver, and if designed properly, can extend the frequency response of the driver/enclosure combination to below the range the driver could reproduce in a sealed box. The enclosure resonance has a secondary benefit in that it limits cone movement in a band of frequencies centered around the tuning frequency, reducing distortion in that frequency range.

The tradeoff for this augmentation is that at frequencies below 'tuning', the port unloads the cone and allows it to move much as if the speaker were not in an enclosure at all. This means the speaker can be driven past safe limits at frequencies below the tuning frequency with much less power than in an equivalently sized sealed enclosure. For this reason, high powered systems using a bass reflex design are often protected by a filter that removes signals below a certain frequency. One such filter is the rumble filter often built in to receivers or amplifiers designed to be used with LP records because of the undesired LF rumble from the mechanical parts of the turntable. Because of the complex frequency dependent loading, ported enclosures generally result in poorer transient response at low freqencies than in well-designed sealed box systems. The audible effects of this in a properly designed system are debatable. A poorly designed bass reflex system, generally one that is tuned too high, can ring at the tuning frequency and create a 'booming' one-note quality to the bass frequencies.

Ported systems are more complex than sealed box systems, and are more expensive, all other things being equal.

loudspeaker enclosure

1. ^ Thiele, A. N., "Loudspeakers in Vented Boxes: Parts I and II," J. Audio Engineering Soc., Vol 19, No. 5, May 1971, pp 382-392 (Reprinted from a 1961 publication in Proc. IRE Australia).
2. ^ Small, Richard H., "Vented-Box Loudspeaker Systems, Part I: Small-Signal Analysis", J. Audio Engineering Soc., Vol 21, No. 5, June 1973, pp 363-444.