Fresh From the Bench: PSI Audio AVAA C214 Digital Active Bass Absorber

PSI Audio launched the AVAA C20 in 2017, the first commercially available analog active bass trap for small rooms. The easy-to-set-up active acoustics device absorbs undesired room modes in studios, being as efficient as passive absorbers many times its size. With AVAA C214, PSI has introduced a digital controlled active bass trap that’s better suited for listening and small studios rooms. But how effective is it?
 
When we read technical literature about room acoustics it usually starts with something along the lines of: Your room and loudspeakers have the biggest influence on the sound quality... Their influence is way bigger than the quality of players, DACs and amplifiers... Before spending money on esoteric DACs, $100,000 US record players, and amplifier monoblocks with power cables thick as baby arms... you should pay attention to your room and invest in some acoustic treatment instead.

That is, of course, easier said than done. Room acoustics is a vast field of expertise and can be complicated. Hiring the services of a good acoustician is not easy and very likely the project will cost much more than imagined. And doing it yourself requires time and skills. Not every treatment works as intended, and some panels come with side effects that might easily take you down a rabbit hole. Worse, ending up with a great sounding acoustic cave with no daylight might not be what you envisaged in the first place. That is why many audio enthusiasts tend to shy away from doing serious efforts to treat their rooms, particularly when it comes to dealing with the lower frequencies and room modes.

Enter PSI Audio, the famous Swiss studio monitor manufacturer, which provides an effective solution for the more serious of these challenges, ironing out many of the problems associated with conventional room treatment.

What Is It and How Does It Work?
The PSI Audio AVAA C214 is a digital controlled active absorber (Photo 1). It is active, since it uses a microphone, signal processing, an amplification stage and speakers. The processing and amplification circuitry is analog, although control is digital since it’s done through an app (more on this later). The cryptic AVAA product name can be decoded as “active velocity acoustic absorber” and 214 for 2× 14cm, describing the two 140mm (5.5“) drivers (Photo 2). Although these are small diameters, the frequency range that the AVAA acts upon is specified as being from 15Hz to 150Hz.
  An AVAA captures sound through its built-in microphone and lets the drivers act as absorbers to the specific incoming sound. To achieve that, the AVAA is constantly re-tuning itself. The membrane’s excursion is feeding back into the amplifier, allowing for precise control of the whole system and fetching non-linearities. Since the AVAA “absorbs velocity,” it must be placed near reflective boundaries or in corners, where the velocity is at its peak. PSI states that the placement is critical for success.

In my tests, I could confirm that it is important to place an AVAA near corners or walls, with a little less effect reached near walls. But placement is not as complex as it is with speakers or passive acoustic panels for mid and high frequencies. The benefit of this active approach is the higher efficiency per volume achieved in comparison to passive approaches. PSI claims that one AVAA replaces absorbent material 35 times its volume. The tube itself measures 63.5cm/25” in length and 21.5cm/8.5” in diameter (Photo 3).
  To illustrate how the AVAA works, PSI describes the acoustic effect as a black hole or open window. One C214 creates a virtual “bass-trap” five to 20 times its own size, large enough to capture modal frequencies. I have never measured the acoustic effect of an open window, but now I feel really motivated to do so.

To get the best results, it is recommended to use at least two AVAAs for symmetry in one room. In rooms between 20m2 (215ft2) and 80m2 (861ft2) two AVAAs are sufficient, while bigger rooms require more. Figure 1 shows the concept drawing from PSI Audio’s patent. For consultation and details on these matters, PSI is to be contacted directly.
  One of the advantages of these digital bass traps is that we are effectively managing room volume, leaving the building structure untouched with the approach—with all the benefits for rented and listed buildings. Unlike acoustic treatments, there’s no need to drill holes everywhere.

It’s true that electric energy is required, but the C214 is very modest in its power consumption. In quiet situations ~3.3VA were drawn, and in normal listening the AVAA C214 rarely jumped above 4VA (values phase compensated). Even in regions where electric energy has become super expensive (as is the case in Central Europe), one AVAA used 24/7 would add just $25 US to the energy bill per year. If you decide to switch on the AVAA only when you need it, this is not even a problem. On and off plops were so quiet I could not even measure it—well below 35dBSPL, at 1m distance is my estimate.

And every AVAA is shipped with a factory calibration chart (Photo 4), which states the absorbent capability over frequency. Compared to traditional passive absorbers, the active bass trap’s effective bandwidth is much wider and more evenly distributed.
 
Installation
On very few occasions have I had audio products in my hands that are so quickly installed for testing. The AVAA C214 does not require input signals, neither electric nor acoustic, a calibration run, or a lengthy installation procedure. Take it out of the box, connect the AC mains, switch it on, and it’s ready to go. Great!

The AC mains connector—a sturdy IEC C14—can be found on the back of the AVAA, where there is also an on/off switch. This switch can be hard to reach when the tube is in its final position in a corner. My wish for the next iteration is to have all LEDs, controls, and switches easier to reach, placed on one end of the tube.

The styling is restrained, low-key and will fit most rooms. It can be ordered in white or black finishes. The black of the provided AVAA test samples is only slightly reflective. An optional bracket allows mounting it on walls (Photo 5) or place it horizontally on any surface. When it comes to the overall appearance, I am fine with the design. It’s not something you would feel the need to hide, and when asked what it is—the LEDs on the front and back are a clear sign that this an electronic device—it becomes a conversation piece.
  When I unpacked the AVAA, my wife commented: “This looks like one of your strange special speakers or an air cleaner.” Other visitors immediately noticed the black tubes and asked what it was—most of them guessing (correctly) that it had “something to do with sound.”

How Does It Sound and Measure?
To put the AVAA C214 through its paces, I prepared two test scenarios. In Scenario 1, I equipped my small office with two AVAAs and tested it in several positions. In that room I have two Neumann KH310 Monitors and a KH750 Subwoofer working as primary speakers. At 11m2 (118ft2), the room has similar dimensions as those of many “B” and “C” studio and editing rooms. Some acoustic paneling is installed (free hanging Basotect panels on ceiling, skyline diffusors on the wall, and two Rockwool absorbers in the corner). The measured RT60 was already relatively low before I added the AVAAs.

In the listening tests, the AVAAs were not immediately audible when we turned them on. Their presence improved the measured sound stage in the low frequencies (Figure 2). The bass felt tidier, and instruments could be better discerned (Figure 3). Mixing and mastering jobs can be done faster and accurately, with less guesswork needed.
 
Scenario 2 had a bigger room with a footprint of 31m2 (~334ft2). The room was equipped with just three 2’ × 2’ diffusor panels to tame excessive flutter echoes. Immediately after switching the two AVAAs on, the room felt smaller and calmer. To a certain degree quieter (Figure 4 and Figure 5). The perceived effect was comparable to the moment when a constantly running refrigerator compressor switches off. It is when we switched it off that the AVAA’s effect suddenly became more noticeable.
  With the two AVAAs in the room, music playback was more fun, the bass was drier and more focused. Most important, listening for longer periods was possible without ear fatigue. PSI Audio explains that the removed energy in the low-frequency bands leads to less masking and explains the better soundstage reproduction and the more relaxed listening experience. That can be confirmed from my experience. The separation of instruments was clearer, single notes and micro dynamics were easier to inspect. The effect of better differentiation of signal sources embedded into the mix was more apparent in the bigger room (Figure 6 and Figure 7). I could not figure out a good way to measure these complex properties objectively—you must take my word for it. In praxis this meant I had more fun listening.
 
One of my earlier concerns before testing was, that the AVAA would lead to an over-dampened sound. This was not the case in my tests at all!

During evaluation, I performed a favorite room acoustics test of mine: Just have a conversation with another person outside, move in, and immediately resume the conversation in the treated room. If that feels wrong and the voice of your test person sounds vastly different, something must be off in the treated room. In that regard, the rooms felt easier when I tested them that way. Not in a way that the room disappeared, but the room felt less dominant. I tried to quantify that effect through STIPA measurements but could not measure a change. I suppose STIPA is not sensitive enough for changes in the low-frequency bands (its test signal falls in the octave band range from 125Hz to 8kHz).

For audio engineers, I see the potential to make their lives easier with these active bass traps. For the casual home listener, the same is applicable. You cannot go wrong using AVAAs. This is something that cannot be said about a lot of first-approach acoustic treatments.

App Control
One key difference between the studio-targeted AVAA C20, the first product of this type launched by PSI, and the more home-oriented new model is in the smartphone app to control the AVAA C214. PSI provided me with a beta release of the current app, which is set to be released to the public later this year, on iOS and Android. There is a QR code on the back of the AVAAs (Photo 6) that you can scan with the phone’s camera to start the pairing process. The pairing went smoothly and only took seconds for the app to locate the AVAAs in the Wi-Fi network. Once the pairing is done, the AVAAs will continue to only use the 2.4GHz band. Since most modern routers support dual-band operation, working with 2.4GHz and 5GHz devices at the same time, this should be no problem for many people. But keep in mind that for the initial setup the Wi-Fi has to be switched to a “2.4GHz mode,” or else the pairing will not work. I hope PSI will use a more modern microcontroller also with 5GHz support for connectivity in the future.
  When everything is set up, the app is used to activate/deactivate each AVAA C214 or AVAA C214 group and control the volume and processing status. When I increased the volume above 0dB amplification with the active bass traps placed in the corners, the AVAAs would feedback. I left them on the factory set level (Figure 8). The use of the app is optional. As of now, the app is not very useful, but PSI promises to include more features in the future, like firmware updates and metrics for optimal placement in the room.
  Conclusion
There are just a handful of audio products that invoke that “It’s magic!” impression for me. The AVAA C214 is one of these products. The digital controlled bass trap improves modal behavior of the room acoustics in a very effective way, with no side effects.

The AVAA is a no-brainer in almost every acoustic environment. In the studio, when used in combination with room correction, there is quick-win potential with less problems in the low frequencies at the mixing position. In a listening environment, its application is highly beneficial in any room.

At the time of writing the app is still not very useful. But I have confidence in PSI Audio’s product development strategy and in delivering on their promises.

I highly recommend the AVAAs. I never would have thought that removing modal bass energy with such a small device would be possible, and I am happy to report that this changed my mind. The only drawback I can think of is the price: EUR €3500/USD $3950 is not cheap. But when we factor in the cost of installing an equivalent amount of absorbers needed to achieve the same amount of improvement, this is a fair price. aX


PSI Audio AVAA C214 Digital Active Bass Trap Evaluation
Pros:

• Effective
• Little power consumption
• Saves time and space
• Ideal for rented spaces and makeshift recording rooms (both control rooms and live rooms) and even offices
• Ideal for inexperienced users
• No calibration required
• Very little to no acoustic side effects

Cons:

• On/Off switch in hard-to-reach position
• Price

Resources
European Patent EP3225038B1, “Low Frequency Active Acoustic Absorber by Acoustic Velocity Control Through Porous Resistive Layers,” granted September 5, 2018. Assigned to Relec SA, 1400 Yverdon-les-Bains (CH).
F. A. Everest and K. C. Pohlmann, Master Handbook of Acoustics, 7th Edition, McGraw Hill (2022), ISBN 9781260473599.
PSI Audio, www.psiaudio.swiss
F. Toole, Sound Reproduction – The Acoustics and Psychoacoustics of Loudspeakers and Rooms, 3rd Edition, Routledge (2018), ISBN 9781138921368.

This article was originally published in audioXpress, November 2024