Operating rather like a subwoofer in reverse, this crafty device controls low frequencies but takes up far less space than conventional bass traps.
Without doubt the most difficult acoustic problem to resolve for the vast majority of project studios is the challenge of low-frequency standing waves or ‘room modes’. These create a rather ‘lumpy’ low-frequency response in most rooms, where some bass notes boom and ring on for a considerable time, while other notes are very weak. These fundamental problems affect recording spaces as well as listening environments. Every room is different, as the affected frequencies relate to the room’s width, length and height dimensions, and the ratios between them.
The underlying physics concern the way that low-frequency sound waves reflect from the boundary surfaces within the room. When the reflected part of the sound wave interacts with the direct signal from the source, their relative phases make them either combine ‘constructively’ into a large, louder sound wave, or ‘destructively’ to partially cancel each other out. And these interactions are different in different parts of the room because their relative phases change with distance.
This kind of sound-wave interaction occurs at all frequencies, but at mid and high frequencies the wavelengths involved are so short that the interaction is largely chaotic and random, and is generally inaudible at best, or forms a mild tonal coloration at worst. However, at low frequencies where the sound wavelengths and room dimensions start to coincide, these additions and cancellations can become very pronounced, producing substantial peaks and even deeper nulls.
An obvious solution to this problem is simply to prevent sound reflections from the wall boundaries, but while that is easy to achieve at mid and high frequencies with simple foam or mineral wool absorber panels, it gets progressively more difficult and impractical as the frequency is reduced. The traditional solution is to install dedicated ‘bass traps’ so that as low-frequency sound radiates through the room from the source it reaches a bass trap, at which point all of its energy gets absorbed and there’s nothing left to bounce back into the room. No reflected LF sound waves means no additive peaks or destructive cancellations with the source sound wave, and so the room’s LF response inherently becomes flatter and more neutral, which is what we want and need for accurate monitoring (or recording, for that matter).
Frustratingly, though, while this concept is simple enough, the practical implementation is not. The simplest forms of sound absorber — like foam and mineral wool — work by converting the kinetic energy of air particles in motion into heat. In effect, the moving air particles lose all their energy by having to fight their way through the porous material of the absorber. An absorber is therefore going to be most efficient when placed where the air is moving the most, and the position along a sound wave where the air has greatest velocity is at a quarter of its wavelength.
For the bottom ‘E’ on a bass guitar, the fundamental frequency is 41Hz and the wavelength is 8.4m. Therefore, for the greatest efficiency, an ideal bass absorber needed to prevent boundary reflections from a 41Hz fundamental needs to extend over 2.1 metres (over 6.5 feet) away from the wall surface! Obviously, as the frequency increases the wavelength gets shorter, and by the ‘E’ two octaves above at 164Hz the wavelength is only 2.1 metres, and the optimum absorber just over 0.5m thick.
Sadly, for most people, taking one metre out of the floor area’s dimensions by lining the walls and ceiling with half a metre of foam or mineral wool isn’t a very practical option, let alone managing to accommodate more than two metres everywhere (although many professional studio control rooms do, or at least get pretty close to this kind of arrangement!).
Of course, there are various alternative and more sophisticated bass trap strategies, such as damped membranes and Helmholtz resonators. These more compact solutions can reduce the physical space requirements considerably, but many of them are specialised ‘tuned’ designs which deal with only a very small frequency range, compared with the broad-spectrum absorption achieved by several metres of porous material. This means they are typically designed and constructed in response to each individual room’s specific issues, making them relatively expensive and far less attractive as a DIY solution.
Intriguingly, modern technology offers another solution in the form of the ‘active bass absorber’, often known more descriptively as an ‘anti-wall device’. In essence, an active bass absorber is a form of subwoofer with a built-in microphone and some closed-loop signal processing. There are several variations on the general theme, but the basic idea is that the microphone senses the arrival of a low-frequency sound wave and the electronics calculate the level and phase of that same sound wave after it’s reflected from the adjacent room boundary a few hundred microseconds later. This is not as easy as it sounds because of the speed-of-sound delays associated with the inherent physical spacing between the microphone, the boundary and the subwoofer.
In theory, though, the active drive unit then generates a sound wave with an inverted polarity but the same level as the naturally reflected sound wave, and it is phased (or timed) precisely such that the two cancel each other out more or less completely. In some practical situations it may actually be more desirable not to kill the reflections completely, in the interests of preserving the duration of any LF ‘reverberation’ in the room needed to balance correctly with that of higher frequencies. (Note that, although ‘reverberation’ is not strictly the appropriate term where the sound wavelength is comparable to the room dimensions, it conveys the intended notion more readily in the context of this review.)
Another way of thinking of this kind of technology is that, in effect, the system works to maintain a zone around it of constant ambient atmospheric pressure, essentially preventing the pressure build-up that naturally occurs near a room boundary when sound waves arrive. The practical effect of the active bass absorber, then, is that the original source sound wave simply disappears and never comes back — almost as if there is no wall there at all — hence the ‘anti-wall’ moniker.
With such a simple concept it will come as no surprise to learn that active bass absorbers aren’t new — I remember reading about early designs in the pro-audio trade magazines maybe as much as 25 years ago — but they are not easy things to design or build, not least because of the requirements for high power handling and the criticality of calculating the compensating sound-wave generation. It’s easy to design a system in which the arriving and generated sound waves cancel completely at the sensing microphone, but that’s not what’s actually needed. If the subwoofer’s low-frequency output doesn’t accurately cancel the sound waves reflecting from the adjacent boundary it will end up radiating additional unwanted LF energy back into the room, which won’t be very helpful at all!
Of course, over the last 25 years the technology of subwoofer drive units and power amplifiers has developed substantially, as have sophisticated signal-processing techniques, so there has never been a better time to design and construct an effective active bass absorber system.
Swiss monitor loudspeaker manufacturers PSI have done exactly that with their new AVAA C20 active bass trap. The product’s odd acronym stands for ‘Active Velocity Acoustic Absorber’, and it is designed to serve as an active replacement for a conventional broadband bass-trap panel. The manufacturers claim it behaves acoustically much like “a hole in the wall” of somewhere between five and 20 times its own size — meaning somewhere between one and four square metres, depending on the acoustic properties of the environment — which is pretty impressive for a box which is only 509mm high and 424mm wide, and which weighs 13kg! The AVAA is roughly a tenth of the size of a conventional broadband bass trap of comparable performance, which is extraordinary.
Since the tri-corner area of a room is where the greatest number of room modes coalesce, the AVAA has been designed with a trapezoidal profile to fit neatly into that space. The two rearward side panels are set at 90 degrees to each other to sit snugly against the adjacent walls, while the inside corner is chamfered off to leave a 50mm deep ‘chimney’ for convective cooling of the amplifier chassis (and to access the power switch). The company have released no specifications for the drive unit, power amplifier or the signal processing, other than to say that it is an all-analogue system.
At the front, the two visible surfaces are constructed of a tough perforated-metal mesh which bows gently outwards. Visible behind this mesh is a white ‘micro-perforated sheet’, which presents a known acoustic resistance into the chamber behind, in which the active drive unit resides. This faces into the rear of the enclosed, and very heavily damped, reflection-free cabinet. Mounted just in front of the micro-perforated sheet is the unit’s pressure-sensing microphone.
The operating principle — which is protected by several patents — is that the acoustic resistance created by the micro-perforated sheet works in combination with the internal drive unit to reduce the acoustic pressure behind the membrane almost to zero. In that way there can be no reflected sound energy. As the incident sound waves pass through the acoustic resistance, the sound pressure is reduced and some sound energy is lost straight away, with most of the rest being absorbed by the controlled movement of the drive unit’s diaphragm. The drive unit’s analogue control signal is derived from the microphone, which senses the acoustic pressure just in front of the resistance membrane. The system’s aim is to create a region of substantially reduced acoustic pressure around the AVAA, and it operates over a frequency range of 15Hz up to 150Hz.
An LED near the centre of the front panel glows green when the unit is working normally, and turns red when switched (remotely) to standby or if the protection systems are activated (for excessive heat or peak-level limiting). The rear panel carries an on/off switch (near the top, and so easily reachable when placed in a corner), along with the usual IEC power inlet, fuse holder and voltage selector. There are also couple of remote-control terminals near the bottom of the unit, along with a small rotary sensitivity control. Normally, this knob is turned fully clockwise so that the active bass absorption operates at maximum efficiency. However, in very small rooms (less than 10m square floor area) or intentionally reverberant spaces (RT60 for frequencies below 200Hz of more than two seconds) it may be necessary to reduce the unit’s efficiency, either to prevent acoustic instability or to permit a greater level of LF ‘reverberation’, and in such cases the sensitivity control can be turned down.
In most rooms, installation is simply a matter of deciding where to place the AVAA devices. PSI originally recommended a minimum of four units for most rooms between 20 and 80 square metres, and suggested that difficult or larger rooms would require six to eight units. Experience has scaled that advice back a bit, and if the room is already reasonably well treated, then two AVAAs may well be sufficient. The ideal AVAA locations can usually be found by ear just by moving around listening for the ‘hot spots’ where bass modes gather strongly. Normally, the AVAA units would be placed in corners, but they may also prove beneficial if placed along side walls in some cases; it really depends upon where the dominant room modes occur within the room. Once positioned, the AVAA units can be switched on and the acoustic improvements assessed. There is no calibration or setup process involved at all, and because the unit is dealing with very long wavelengths, the positioning is not actually that critical either — six to 12 inches one way or the other really won’t upset things.
I was supplied with four AVAA units for this review, each fitted with a bespoke remote-controlled relay unit, which made system performance evaluation much easier as I was able to switch the units on and off individually or all together from a convenient wireless remote controller.
The AVAAs were installed in the room with my PMC IB1 monster three-way monitors, and I am aware of modest peaks in that room around 30 and 90 Hz, with a suck-out around 60Hz. The distributor’s initial recommendations were to put a unit in each corner on the left-hand side of the room, with a third on the right-hand side wall just in front of the monitors, and a fourth on the rear wall just right of centre — these positions being suggested by a simple walk around listening for hot spots. The reduction in the duration of room resonances was instantly noticeable, and there was also an improvement in LF linearity, although some peaks and troughs remained audible as I moved around in the room.
Over the following week I experimented considerably with different placement options — something which is extremely easy, as the units’ only electrical connection is a mains power cable. I found the AVAAs noticeably more effective when placed in corners, as you might expect, but especially so in the front wall corners (ie. behind the monitors), for a reason I can’t explain! In this particular room, a door is located near the front-left wall corner, and opening that door could set the nearest corner unit into self-oscillation, generating a low-mid pitch which could be ‘played’ by moving the door back and forth in a very Theremin-like way! Hours of fun for all the family, perhaps, but I don’t suppose it did the AVAA drive unit much good!
In the end, and as much for reasons of pragmatism as optimised acoustics, I ended up with one pair of units stacked on top of each other in the rear-left corner, and the other two units in both front corners. This arrangement proved remarkably effective in reducing the room resonances quite substantially as well as levelling out both the LF peaks and troughs to a useful degree and across a wide listening area. However, I learned subsequently from PSI that each AVAA unit controls the acoustic impedance over a region extending about 1-1.5 metres around it, and so there is no benefit from placing two units within about two metres of each other as their spheres of influence overlap. So placing one unit on top of the other, as I did (largely because there was nowhere else for it to go!), made little difference over the bottom unit on its own — and that does tie in with my experience as switching the top unit off made little audible difference. I suspect that mounting that fourth unit on the wall up in the ceiling corner might have been more effective, but that wasn’t practical during the review period.
Overall, I found PSI’s AVAA active bass absorbers to be highly effective and easy to use, and there’s no doubt that they are considerably more compact and easier to install than any equivalent passive acoustic bass-trap panels. However, it is important to note that — just as with mechanical bass traps — multiple units are required to achieve any significant improvement. One unit on its own isn’t very effective, and I only noticed a significant improvement with two units powered on. By design, the AVAAs also do nothing for frequencies above about 150Hz, so traditional acoustic treatment is still required to control the reverberation and nearfield reflections at higher frequencies.
Inevitably, the most significant drawback to the use of the AVAA active bass absorber system is the cost. Each individual AVAA unit will, in additional to making a virtual acoustic hole in the wall, knock a $£2000-sized hole in the bank balance, too! However, while that might sound a lot, it really isn’t in comparison to a high-quality, monitoring-grade powered subwoofer — which is the closest equivalent technology. But, whereas a high-quality 5.1 monitoring system rarely needs more than one (or occasionally two) powered subwoofer(s), a typical AVAA installation requires three or four units, and maybe more in challenging cases to properly ‘level out’ a room with a troublesome LF response!
For a budget of $£4000 to $£8000 (to cover two to four AVAAs) you might hope to achieve as good, if not better, room correction by hiring a good acoustician and going down the traditional route of bespoke tuned acoustic absorbers. But how much floor and wall space would you lose with that approach, and how disruptive would the installation process be? Taking a longer-term view, it’s also a lot easier to pick up a few AVAA units and move them to your next studio space than it is to rip out custom-built and specially tuned bass traps which may not actually work in the new space anyway.
Clearly, these AVAA units don’t present a universal solution for every studio or listening environment, purely on cost grounds. But they do offer a unique combination of significant conveniences in being remarkably compact, very easy to install and set up, and demonstrably effective in practice. For some, those benefits will be sufficient to justify the high cost. What is certain is that this is undoubtedly a very interesting technology, and PSI’s implementation of it is impressively effective and very easy to use. I’d definitely recommend an audition of the system for rooms or situations where the considerable practical benefits of active bass absorption outweigh the financial forfeit.
The only other commercially available active bass trap I’m aware of at the current time is the Bag End E-Trap, although that has a different operating principle and has to be manually tuned to a specific room mode frequency.