Reflex Cabinet Designs
Published in PM November 2008
Sound Advice : Tech Notes
Last month I described the simplest kind of loudspeaker cabinet, the sealed or infinite-baffle type. This month we'll take a look at the reflex cabinet, which is the most common cabinet design by far and used by probably 95 percent of all hi-fi, studio monitor and stage PA speaker systems.
As we have already seen, the sealed type of cabinet, if designed carefully, allows for a well-controlled and smooth frequency response. The bass tends to start falling away from a relatively high frequency, but it does so at a gentle 6dB per octave, which means there is still some useful audible bass energy being put into the room even at pretty low frequencies. More importantly, there should be no phase shifts or high-Q resonances to worry about, and the time domain behaviour is normally very well controlled indeed — which helps to make a very revealing and accurate hi-fi speaker or studio monitor. However, if you want the ability to deliver high volumes and copious amounts of bass energy using a sealed-box design, it will have to be big — usually way too big for domestic acceptability, and impractically big for live sound applications. (Although the latest generations of very long-throw bass drivers offer useful improvements in this situation.)
The bass reflex cabinet design's fundamental strength is that it makes use of the cabinet's inherent resonance to help to increase bass efficiency, which means that far more powerful (read 'loud') systems can be created with a more extended bass response for a given cabinet size. This is usually considered an important design goal, whether we are talking about home hi-fi, professional studio monitors or large PA systems. Everyone wants lots of powerful bass reproduction but from the smallest cabinets, for either aesthetic or practical reasons.
The key aspect that differentiates a bass reflex cabinet from a sealed cabinet is, of course, that it has one or more openings known as vents or ports (and hence the often-used alternative term: a ported loudspeaker). At the simplest level, these ports essentially allow some of the low-frequency sound reproduced by the rear of the bass driver within the cabinet to be heard by the listener via the port, and this sound contribution is tailored during the design by adjusting the area of the port opening(s) and internal length of the port(s), as well as the physical spacing between the rear of the bass driver and the port entry point(s). Although there are a lot of variables to play with here, the bass reflex concept is probably the best understood and most researched of all loudspeaker cabinet types, and the fundamental work in this area by messers Thiele and Small is now often referred to in cabinet and speaker design parameters. There are several well-regarded software packages to help optimise the various parameters too, but given the number of variables and the complexity of their interaction there are inherently a lot of compromises and trade-offs to be made in the design of a reflex cabinet. So it's probably not too surprising that the myriad different speaker manufacturers all choose slightly different ways of achieving the same notional end goal.
If you think about the basic construction of a ported speaker cabinet, the drive unit attempts to pressurise the air inside the box when the cone moves inwards, and that compressed air then tries to escape through the port vent. That potentially results in a significant airflow through the port, and if you place your hand in front of a port opening you can often feel blasts of air with each bass note or kick drum strike. If that airflow is allowed to become turbulent it can create audible 'chuffing' noises, so different manufacturers employ various mechanisms and port designs to prevent that. Options include flared port openings, tubes within the port to maintain a laminar flow, rounded slots instead of tubes, or rear-firing vents instead of frontal openings. Some designs even use a 'passive radiator', which is effectively a secondary, non-powered drive unit, the cone of which covers the port opening and moves in response to the changing internal cabinet pressure.
Whichever way it is designed, a ported cabinet is inherently a resonant system with similarities to someone blowing across a drinks bottle. The size, shape and position of the ports, the overall cabinet size and shape, and the bass driver's characteristics must all be carefully adjusted and optimised to control not only the resonant frequency but also the 'Q' of the resonance — its 'peakiness', if you like — the combination serving to bolster the naturally falling low-frequency response of the bass driver and thus provide more bass for a given cabinet size.
If the system is 'over-damped' (meaning the resonance is very tightly controlled) then the bass response tends to fall off in a similar way to the sealed-cabinet design, but when damped optimally, a ported loudspeaker is both very efficient and can have a surprisingly well-extended and smooth bass response — even for relatively modest-sized cabinets. And that's why they are so popular with both manufacturers and users alike.
However, if a reflex cabinet system is under-damped, the resonance will not be very well controlled at all, resulting in a significant peak in the low-frequency amplitude response before it falls away very steeply. This is sometimes done quite deliberately because it tends to give the impression of an extremely powerful bass output — but one that will exhibit a narrow, dominant frequency range. This gives rise to a kind of thuddy, one-note bass line and monotonic but powerful bass drum kicks, but in some PA situations that can give the impression of a system that is far more powerful than it really is. However, whereas the bass response of a sealed cabinet falls off relatively slowly below the system's resonant frequency, that of a reflex cabinet falls off much more quickly, at 12 dB per octave (or even higher rates sometimes), so there is often very little deep bass energy from a modest-sized reflex cabinet. Coupled with the inherent phase shifts involved in the reflex approach, bass reflex designs produce a different kind of bass sound to a sealed cabinet — it's a small but quite noticeable effect when your ears get used to hearing the difference, and one that influences not only musical perception but also the way a track is mixed.
The under-damped resonance also gives rise to a 'ringing' effect, where bass energy is effectively stored inside the cabinet and then released over time, smearing transients and often making it hard to differentiate between kick drums and bass guitars! This can often be seen in the 'waterfall' plots that are frequently included in technical loudspeaker reviews or specifications. It looks like a mountain ridge, protruding out in time at a low frequency from the near-vertical cliff face decay of the rest of the frequency range. Simplistically, it takes sound waves a short time to propagate from the rear of the bass driver out through the port, and when it gets there it needs to be in the same polarity as the frontal sound wave so that they add constructively, rather than try to cancel one another out! All of which means that it has to have enough time to go through a 180-degree phase shift over the desired low-frequency range — hence the delay and energy storage effects (which are more severe with high-Q tunings). This delay and energy storage smears the transients in time and, in the worst case, it can be very hard to separate bass instruments and kick drums from one another as all the bass sounds seems to be at the same pitch and have the same character and amplitude envelope. Obviously, careful design and optimisation can minimise or even avoid these effects, but to get it absolutely right involves relatively complex designs and elaborate components, and that makes a really good reflex speaker a fairly expensive one.
A relatively recent development of the reflex design is the cardioid cabinet, which is already quite popular in some of the large PA systems and is beginning to be adopted in some professional monitor speakers too (such as some of the larger designs from Musikelectronic Geithain). Often at large outdoor concerts it is necessary to control the direction of sound radiation (to minimise sound reaching nearby homes, for example), and whereas conventional mid and HF cabinets are inherently fairly directional and have controlled dispersion, low bass tends to be omnidirectional and goes everywhere! One technique to try to control bass radiation is to set up secondary bass bins along the axis that the sound is not required, and to adjust the timing (phasing) of their output so that the bass from the main and secondary systems cancels each other out (in that specific direction).
However, a more convenient and compact alternative is to manipulate the low-frequency sound to provide much the same kind of directional cancellation — either by using additional rear-facing drivers in the cabinet, or by using clever port technology in specialised reflex cabinets. In these ported designs, the vents are placed at the rear of the cabinet and the phasing of signals reproduced from them is arranged very carefully, so that the emerging wavefronts support the frontal sound from the main drivers (in the usual way), but cancel any bass sound travelling towards the rear, thereby creating a cardioid polar response at very low frequencies. The rear-facing bass drivers achieve the same thing, but with active sound generation instead of borrowing sound from the inside of the bass drivers. In either case, this technique is similar in concept to the way a phase-shift cardioid microphone capsule works (but in reverse, of course). This directionality is not only useful at outdoor concerts, but also in halls and control rooms, where it significantly reduces LF spill on stage (enabling a much cleaner sound and greater headroom) and also reduces some of the problems of standing waves in enclosed spaces. 0
Published in PM November 2008
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