Bass reflex

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.

"Reflex port" is the distinctive feature of a very popular variety of loudspeaker enclosures (Bass reflex) which enhances the reproduction of the lowest frequencies generated by the woofer. The port generally consists of one or more tubes mounted in the front (baffle) or the rear side of the loudspeaker. This allows extending the (deep end of the) frequency response of the system, which means that the loudspeaker can reproduce the sound of musical instruments that generate low frequency (bass) in a better way than that of an equally sized sealed enclosure.

Though advantageous in regards of deep bass, the bass reflex cabinet is known to have poor transient response compared to the sealed enclosure cabinet at frequencies near the lower limit of the frequency response. In some cases the frequency range in which the transient response is subjectively adequate can be widened by lowering the lower frequency limit.

Achieving a balanced bass reproduction from a sealed box is simpler than properly aligning the components of a bass reflex system, and home constructors often select a sealed box for their first project.

Explanation

Unlike closed box loudspeakers, which are substantially airtight, a bass reflex system has an opening called a "port" or "vent", consisting of an opening that is 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, depends upon the effective length and cross sectional area of the duct, the internal volume of the enclosure, and the speed of sound in air.

When speakers are designed for home use, manufacturers tend to consider the advantages of porting to outweigh the disadvantages. The design is popular among consumers and manufacturers but the increase in small cabinet bass output is invariably achieved at the expense of temporal integrity of the signal and the build-up 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.

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 [ 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).] and Richard H. Small [ 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. ] 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 attributes, but not all. The sound pressure produced depends upon the efficiency of the speaker, the mechanical or thermal power handling of the driver, the power input and the size of the driver.

Advantages

Such a 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 would 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.

Limitations

By their own nature, resonant systems cannot start and stop instantly. Ported speakers stagger "two" resonances, one from the driver and boxed air and another from the boxed air and port, in order to achieve their bass output, whereas sealed boxes only exhibit one resonance. This causes increased time delay (increased group delay imposed by the twin resonances), both in the commencement of bass output and in its cessation. Therefore a flat steady-state bass response does not occur at the same time as the rest of the sonic output; rather it starts later (lags) and accumulates over time as a longish resonant "tail". Because of this complex, frequency-dependent loading, ported enclosures generally result in poorer transient response at low frequencies than in well-designed sealed box systems.

Another 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, or from the strong subsonic excitation caused by warped vinyl discs. Unfortunately, electrical filtering adds further frequency-dependent group delay. Even if such filtering can be adjusted not to remove musical content, it may interfere with sonic information pertaining to the size and ambiance of the recording venue, information which often exists in the low bass spectrum.

Whether or not the effects of these in a properly designed system are audible, remains a matter of debate. Many people prefer (or are used to) this kind of overindulgent bass, especially if they live in buildings that absorb LF energy (such as typical dry-wall construction housing). A poorly designed bass reflex system, generally one that is tuned too high or too loose, can ring at the tuning frequency and create a 'booming' one-note quality to the bass frequencies. This is due to the port resonance imposing its characteristics to the note being played, and is grossly exacerbated if the port resonance coincides with one of the resonant modes of the room, a not too rare occurrence. In general, the lower in frequency a port is tuned, the less objectionable these problems are likely to be.

Ports often are placed in the front baffle, and may thus transmit unwanted midrange coloration generated inside the box. If poorly designed, a port may generate "wind noise" or "chuffing", due to turbulence around the port openings at high air speeds. Enclosures with a rear facing port mask these effects to some extent, but they are not designed to be placed directly against a wall. They require some free space around the port so they can perform as intended. Poor room placement can significantly reduce the performance of this type of loudspeaker. Some manufacturers incorporate a floor-facing port within the speaker stand or base, offering predictable and repeatable port performance within the design constraints.

See also

*Loudspeaker enclosure

References