Bass reflex
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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.
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[edit] Explanation
In contrast to closed box loudspeakers, which are substantially airtight, 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 "springiness" 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 dependent on the effective length and cross sectional area of the duct, the internal volume of the enclosure, and the speed of sound in air.
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. The design is popular among consumers and manufacturers but the increase in bass output is invariably achieved at the expense of temporal integrity of the signal and the buildup of unwanted resonances. Reflex designs can be undesirable in settings where the utmost accuracy of reproduction is desired, e.g. in monitoring facilities, recording studios etc.
[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 1960s. 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 (i.e., 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.
[edit] Advantages
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.
[edit] Limitations
The trade-off 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 frequencies than in well-designed sealed box systems. Whether or not the effects of this in a properly designed system are audible 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 factors being equal. Ported enclosures usually require more airspace than a sealed enclosure; this makes it hard to fit a proper sized ported enclosure into a small space.
[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.
