Bass reflex
From Wikipedia, the free encyclopedia
Bass reflex (also known as a ported, vented box or reflex port) is a type of loudspeaker enclosure that utilizes 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.
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[edit] Explanation
In contrast to closed box loudspeakers, which are substantially air tight, a bass reflex system has an opening called port or vent which consists of 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 in a smaller box. This is probably why the design is so popular among consumers (and manufacturers.)
[edit] History
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, of course, is constrained by the laws of physics. One cannot have a very small speaker in a very small enclosure producing a high volume, extended bass response with a low power amplifier.
[edit] Advantages
This resonant system augments the bass response and if designed properly can extend the frequency response below the range the driver could reproduce in a sealed box. The resonance has a secondary benefit in that it limits cone movement in a band of frequencies centered around the tuning frequency, allowing lower distortion reproduction in that frequency range.
[edit] Limitations
The tradeoff for this band of augmentation is that at frequencies below tuning the port unloads the cone and allows it to move much as if the speaker were not housed in an enclosure. This means that the speaker can be driven past safe limits at frequencies below the tuning frequency with much less power than in an equivalently sized sealed system. For this reason, high powered systems using the 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 sometimes produced by such systems. The ported box system is more complex than the sealed box system, and as such it generally has poorer transient response. 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.
[edit] See also
[edit] References
- ^ 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).
- ^ 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.
