Product Review

PSI Audio
AVAA C214 Active Bass Trap

Sound On Sound Magazine
January 2024

by Phil Ward

Standard low‑frequency acoustic treatment needs a lot of space, but PSI’s active traps fit anywhere.

If you’re a regular reader of Sound On Sound you will be familiar with advice that goes something along the lines of: “Before you get too hung up on the sound of your monitors, you need to manage the influence that your room has on what you hear.” The significance of that advice can’t really be overstated, and even if you work in a room without a single item of acoustic treatment, having just a basic understanding of how the room might be imprinting itself on your work is better than not giving it a thought. If you want your work to translate successfully to other playback systems in other spaces, the influence of the room in which you mix is a pretty major factor.

When I mention ‘acoustic treatment’ you probably imagine I’m thinking of bass traps or absorption or diffusion panels, and yes, those kinds of products are part of the equation. But big items of soft furniture, bookshelves, curtains and carpets can also be effective tools in the battle to manage room acoustics. What all of these items have in common is that they are passive: they sit in the room and do nothing but absorb or diffuse excess acoustic energy, and as such can’t be targeted at specific troublesome frequencies.

As long as your room displays a reasonably short reverberation time at mid and high frequencies (say, less than 500ms), and isn’t prone to major flutter echoes, those troublesome frequencies tend to reside below around 150Hz, where audio energy with a wavelength that equals some multiple of the room dimensions bounces between the primary room boundaries to generate resonant room modes. We’ve all experienced that phenomenon whereby the same mix sounds bass‑light at one position in the room and bass‑heavy somewhere else. This is because the two listening positions correspond to locations where room modes respectively suppress and boost frequencies that happen to coincide with elements of the mix. It’s a cruel trick of architecture that room sizes and their resonant modes often correspond with the bass frequencies of music. If we all lived and worked in rooms three times the size, I perhaps wouldn’t be writing this review.

Cancellation Culture

Low‑frequency room modes are often dealt with using passive bass traps, which dissipate low‑frequency energy either through plain absorption or a damped low‑frequency panel resonance. Such passive bass traps can be highly effective, but they need to be pretty large, and so typically consume significant studio real estate. However, in 2016, Swiss monitor company PSI Audio launched an alternative approach to bass trapping in the shape of the AVAA C20 active bass trap, which SOS Technical Editor Hugh Robjohns reviewed in that year’s August issue. The AVAA C20 looks a little like an active subwoofer, but it conceptually operates in reverse, reducing bass energy in the room rather than increasing it. It achieves this by employing a microphone to sense instantaneous acoustic pressure in the room, the analogue of which is then used to control the motion of an array of amplifier‑driven subwoofer‑style drivers in order to equalise the acoustic pressure. When the pressure rises with the arrival of a low‑frequency peak, the drivers move backwards to reduce it, and if the pressure drops on the arrival of a low‑frequency trough, the drivers move forwards to increase it. The principle is related to that employed in noise‑cancelling headphones, just on a grander scale. Normalising the acoustic pressure means that low‑frequency energy arriving at the bass trap, rather than being reflected back into the room, is effectively evaporated. The result is that the low‑frequency resonant room modes are suppressed. Hugh’s review of the AVAA C20 concluded that it worked very wel, and PSI have now introduced their second active bass trap: the AVAA C214. AVAA, by the way, stands for Active Velocity Acoustic Absorber.

The new AVAA C214 employs a somewhat different form factor from the C20, being cylindrical rather than trapezoid, and has a neat optional mounting bracket available that enables it to be hung from walls rather than just sit on the floor — although the latter is a perfectly acceptable mode of installation. The C214 incorporates very little in the way of onboard user facilities: just a mains power socket, a couple of configuration buttons and a status LED on the rear panel. It’s clearly intended as a ‘set and forget’ device.

At 21.5cm in diameter by 63.5cm high, the C214 is very much smaller than any effective passive bass trap and is also smaller than its predecessor. The most significant difference between the old C20 and the new C214 is that the sensing and drive electronics within the newer product are entirely digital. As well as bringing significant technical benefits in terms of faster reaction time, that has provided PSI with the opportunity to develop a control and configuration app.

App & Away

The AVAA control app lets you set the maximum gain for each AVAA unit, and assign them to different groups.

The AVAA control app lets you set the maximum gain for each AVAA unit, and assign them to different groups.The control app enables the gain level of the C214 to adjusted over WiFi, and for multiple C214s to be grouped. Changing the gain level increases or decreases the C214’s sensitivity and consequently its effectiveness, but increasing the gain too far risks introducing instability (the C214 is a mic connected in close proximity to a speaker, so the potential for instability is inherent). Being too ambitious with C214 gain can trigger a feedback‑like audible signature, though the phenomenon isn’t quite as worrying as it would be in other audio contexts, firstly because it’s a simple matter to wind the gain back a notch, and secondly because establishing the instability point is part of the setup procedure.

As with conventional passive bass traps, more is generally better, and multiple C214s are very likely to be more effective than a single unit. I was loaned four C214s to experiment with in my 5 x 4 x 2.2m studio room, and following the advice in the installation manual, I began by positioning one in each corner — corners being where the pressure changes due to low‑frequency standing wave modes are likely to be greatest.

The next step was to connect them to mains power, switch them on and put them into WiFi connection mode by pressing and holding one of the rear‑panel configuration buttons. Using the app it was then straightforward to get each unit connected to the studio WiFi network. I chose to group the four units into a front pair and a rear pair. The front pair were those located in the corners either side of my Neumann KH150 nearfield monitors, and the rear pair were those located in the corners at the other end of the room. Experimenting with the C214 gain setting convinced me that +3dB was a good maximum to guarantee stability; +4.5dB was borderline.

Measuring Up

Before discovering the subjective effect of the C214s in my room, I fired up FuzzMeasure to analyse their influence on the measured in‑room frequency response. Diagram 1 shows the 1/12‑octave smoothed, wide‑band frequency response at the primary listening position with both KH150s playing. Note that the KH150s were previously optimised for the room using Neumann’s MA 1 monitor alignment package (which, by default, adds a few dB of low‑shelf gain below 100Hz). Measuring with the MA 1 alignment engaged meant that the C214s potentially had less to work with in terms of low‑frequency modes, but it also raised the question of whether active bass trapping is still worthwhile once monitoring is optimised through room alignment

Diagram 1: Frequency response (12th‑octave smoothed) of a pair of Neumann KH150s at the listening position, with AVAA engaged (green trace) and disabled (red).

And to answer both that question and that of the general effectiveness of the C214, the red curve of Diagram 1 shows the frequency response with the C214s switched off, and it reveals evidence of modes at around 36Hz, 67Hz and 87Hz. The green curve shows the result of switching on all four C214s, and the difference is pretty clear: the low‑frequency room modes are suppressed significantly and the low‑frequency response is generally more linear.

Diagram 2: Unsmoothed frequency response between 20‑200 Hz, again with AVAA engaged and disabled (green and red, respectively).

Diagram 2 shows the same data but zoomed in on the 20‑200 Hz band and now unsmoothed. Again, it’s clear that engaging the C214s works well to suppress the room modes. But what, I hear you ask, if your budget doesn’t stretch to four C214s? Diagrams 3 and 4 respectively show what happens when first the rear pair, and then the front pair, of C214s are switched off. Neither scenario is as good as the full‑fat arrangement of four C214s, but two at the front is significantly more effective than two at the rear. To put some numbers on that, and looking at the 67Hz mode for example, four C214s reduced the mode by around 6dB, two C214s at the front of the room yielded a 3dB reduction and two C214s at the rear of the room resulted in a 2dB reduction. Remember, though, that these measurements reflect only the effect of the C214s at the primary listening position. The very nature of room modes means that a different measurement position may well result in different numbers.

Diagram 3: 20‑200 Hz response, comparing all four AVAA units (green) with only the front pair engaged (purple).

I also tried and measured a variety of other room arrangements of various numbers of C214s but found nothing as effective as putting one in each corner. For example, I had high hopes for two C214s at the front and two halfway down the room, figuring that bringing the second pair closer to the listening position might be more beneficial than locating them in the corners. It was pretty good, but still not as good as the rear corner location.

Diagram 4: Four AVAA units engaged (green) compared with only the rear pair engaged (orange).

The last curve, Diagram 5, shows, just for completeness, the effect of varying the C214 gain. The orange curve shows 0dB gain, the green curve shows +3dB and the purple curve shows +4.5dB. As expected, suppression of room modes increases with C214 gain.

Diagram 5: Comparing all four AVAA units when set to 0dB (orange), +3dB (green) and +4.5dB (purple).

"The AVAA C214 is a technologically advanced, easy to use and brilliantly effective product... it’s a triumph for PSI."

User Experience

So measurement reveals the objective effectiveness of the C214s in suppressing low‑frequency room modes, but what about the subjective result? It’s very impressive. Bearing in mind, firstly, that despite my studio showing low‑frequency modes I’ve never found them to be particularly troublesome, and secondly that my monitors are already optimised for the space, the effect of the C214s was to very noticeably clean up and control the low‑frequency character of the room. And cleaning up the low end has subjective benefits further up the music band: less low‑frequency hash from room modes means voice‑band midrange detail in particular is less likely to be masked. It probably goes without saying that, having established that four C214s working was the best option objectively, it also proved the best subjectively, but the improvement brought by just two C214s was very noticeable and worthwhile — even more so with the KH150 room alignment switched off.

Making low‑frequency mix decisions is often one of the most difficult aspects of music production, and quite often that’s down to the effect of room modes. The AVAA C214 is a technologically advanced, easy to use and brilliantly effective product that provides a neat solution to a genuine need, and in those terms it’s a triumph for PSI. But it is expensive, and there’s the rub, because even just two C214s command around the equivalent price of a pair of high‑end nearfield monitors. Or to put it perhaps more starkly, a single C214 costs more than the pair of Neumann KH150 monitors I currently use in my studio. However, there are two ways of looking at this. One view might be that the C214 constitutes an unaffordable luxury, but an alternative view is that, bearing in mind just how much money is spaffed on studio gear generally, investing in something that has the potential to fix one of the most fundamental issues could be absolutely justified. Even so, sadly, for most of us AVAA C214s will remain an aspiration. In commercial studios, however, where work is plentiful, day rates are not insulting and finances are viable, a brace of C214s may well absolutely be worth the investment.

Alternatives

PSI’s original active bass trap, the AVAA C20, is still available and is significantly less expensive than the C214, so it may well provide an alternative option. A second alternative would be conventional passive bass traps, but while the cost will likely be an order of magnitude less than the C214, you’ll need a lot of free space for installation and the result may not be as effective.

Pros

• Highly effective
  
• Neatly designed
  
• Easy to install and use
  

Cons

• Expensive
  

Summary

PSI’s AVAA C214 active bass trap is well designed and effective in use, and could potentially solve the room‑mode problems that bedevil countless studios. It is, however, frustratingly expensive.

Information

$3950 each.

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