Room Treatment Reviews

"Bass—the final frontier," declared Captain James T. Kirk. I have no doubt: The biggest problem in nearly every listening room is getting the bass to sound right. Today, we voyage to the frontier of bass response.

A Brief History of Time
Because my listening room is also a mastering room, it has to be as accurate as possible (see photo 1). The floor is a concrete slab, with solid-block wall construction and a cathedral ceiling 23' high at the rear—there are no floor-to-ceiling resonances. A bay window hidden behind the curtains disperses the lengthwise room mode by varying it between 18' and 20.6'. The curtains tighten the stereo image and damp the subtle resonant chamber that would otherwise color the sound.

As technology has improved, the system and treatments have gone through several iterations. At first I placed the subwoofers physically in line with the main speakers, and used the subs' own built-in low-pass filters. Then, only for the subs, I added a Meyer Sound analog equalizer to counteract some low-frequency anomalies while retaining the upper-frequency transparency.

Like all audiophiles, I wanted even more sonic perfection. Moving the subs to the front (footnote 1) corners improved the bottom, but caused timing incoherence between mains and subs. So I began to experiment with digital low-pass filtering, and discovered AudioVero's Acourate and AcourateConvolver software products, whose inventor, Dr. Ulrich BrÅggemann, became one of my mentors in the room-correction field. Of course, I was skeptical of this new approach, and spent a careful year switching back and forth between my pure analog system on its own and with the AcourateConvolver. I became thoroughly convinced that this digital correction system is transparent, does not detract from the warmth of my analog system, and has improved its imaging, coherence, depth, soundstage, purity of tone, and impact. So jump on in, the water's fine.

That being said: in principle, controlling bass frequencies with acoustic methods introduces less corrective EQ, and offers the potential for even better sound. Serendipitously, in September 2014, while giving a seminar in Yverdon, Switzerland for Swiss Radio, I met Alain Roux, founder and chief designer of high-end speaker manufacturer PSI Audio. He gave me a tour of his factory, including its impressive anechoic chamber, and showed me a prototype of an active bass trap he was developing and patenting: the result of three years of R&D with academic and industrial partners. PSI has been manufacturing active speakers for 25 years, and their proprietary analog technology, including phase compensation and adaptive output impedance, provided the technical foundation for Roux's bass trap, the AVAA C20, which began shipping in mid-2015. Intrigued by this development, I contacted PSI's CEO, Roger Roschnik, and made arrangements to listen to and test four AVAA C20s.

AVAA—the Final Frontier?
The AVAA C20 is a small, black, trapezoidal box that looks like a subwoofer and costs $1999. It's made of MDF and weighs a scant 28.6 lbs (13kg). (I can easily carry an AVAA C20; by contrast, a heavy, 4.25"-thick RealTraps MondoTrap requires two people to position!) It typically draws a mere 5W, or up to 50W when working at full capacity (exposed to 115dB SPL). The AVAA C20 can sit on the floor or a shelf; the most common and efficient location is on the floor in a corner, where many rooms exhibit their prime resonances (modes).

AVAA stands for Active Velocity Acoustic Absorber. The principle on which the AVAA C20 works is to convert an incoming pressure wave into a velocity wave and thus, effectively, absorb it. PSI calls the AVAA an anti-wall, and says it provides an absorbent volume 5 to 20 times its own size, depending on the wavelength encountered. To work its magic, the AVAA uses pure analog technology: a complex combination of a microphone, perforated membrane, velocity transducer, and amplifier with feedforward and feedback. Unlike its competitor, the active E-Trap from Bag End Loudspeakers (footnote 2), the AVAA doesn't need to be tuned, and will affect bass frequencies from 15 to 150Hz in proportion to their sound pressure level (SPL). It requires no adjustments other than proper placement. According to PSI, the AVAA C20 will absorb a pressure wave at any frequency within the range of 15–150Hz without affecting other frequencies. This is a remarkable claim—traditional traps need to be large and heavy, and it's very hard to make them frequency-selective. To meaningfully affect a 45Hz mode, a traditional passive absorber needs to be over 6' thick—¼ the wavelength of that frequency. Do these little boxes from PSI bend the laws of physics?

Because a listening room needs to be treated for problems at any frequencies, PSI stresses the need for supplementing bass trapping with treatments for unwanted reflections in the mids and highs, and optimizing decay times. Later, I'll talk in detail about my own room solutions.

Listening
The AVAA C20 samples arrived with a radio-frequency remote control made specifically for testing purposes—it isn't available commercially—and usable to turn off/on, from the listening position, single or multiple traps, in any combination: a very educational experience. The differences I heard with and without the AVAAs educated my ears about some weaknesses of my room that I'd never noticed: It's amazing what we can get used to, and never know how much better something can sound—until we hear it. Still, the AVAAs polished rather than "fixed" anything in my room, as nothing was seriously broken to begin with. To be fair, whenever I switched off the AVAA C20s, I also switched the AcourateConvolver to an old filter set geared to my room with just passive MondoTraps. But can an equalizer compensate for room resonances and time-domain artifacts? According to a controversial thread at Home Theater Shack, an equalizer can correct modal problems. I agree with the argument presented there. However, after a certain length of decay, the equalizer will be ineffective—thus a proper acoustic solution should perform even better.

Traditional trapping solutions are compromised: usually, they overdamp a room while trying to fix the bass—and ineffectively at that. The AVAA C20s set a new standard of excellence: most recordings I played benefited greatly, some only subtly—but I never heard any negative artifacts. I no longer felt the desire to boost the bass, because the missing frequencies had been filled in. The bass was fuller, more impacting and microdynamic, each note audible in front of a curtain of black, presented without a smeary resonant hangover. Without the AVAAs, music sounded defocused. Some examples:

One of my reference recordings is the Bombay Dub Orchestra's "Egypt by Air," from my mastering of their album 3 Cities (CD, Six Degrees). More than 75 Indian musicians participated in this album, playing acoustic instruments—plus the producers' deep bass synthesizer, which goes down to the center of the earth. I once played it for brilliant engineer Bruce Swedien, who was so impressed that he wouldn't give my CD back. With the AVAAs, the ambience and depth were improved. The electric bass and the deep bass synth sounded much tighter, and bass notes were better defined while losing no bloom or punch. A major improvement.

Another reference is "Fascinating Rhythm," from Dave Grusin's The Gershwin Connection (CD, GRP GRD-2005). This jazz album has a big, bangin' rhythm section and bass drum, with tight, impacting backbeats on snare, and lots of body. It's all about swinging and dynamics. When I switched off the AVAA C20s, I immediately noticed the low rhythm resonating from the rear corners, disturbing the clarity of the direct sound. I was amazed that I'd tuned out this problem—most of us are used to catching some low-frequency buildup in the rear of a listening room, but it shouldn't be there. Standing against the rear wall with the AVAAs switched back on, I noticed a phenomenon I've experienced in only a handful of the world's finest rooms: There was no bass boom—it was as if the wall had disappeared. Most impressive. Later, I discovered a Daft Punk record that somewhat resonated at the rear wall, but with less aggressive material, this was completely gone.

One of my favorites of the albums I've recorded for Chesky Records is the late Kenny Rankin's Because of You (CD, Chesky JD63). "Erienda" should have light but deep bass; with the AVAAs, David Finck's double-bass notes were properly damped—yet without the PSIs, they were sustained longer than the sound of David's own unamplified instrument.

"Walking on Sacred Ground," from Janis Ian's Breaking Silence (CD, Morgan Creek 2959-20023-2), was mastered by the late and greatly missed Doug Sax for maximum impact and dynamics. Overall, the AVAAs yielded a sound that was more enjoyable, involving, warm, and full than without them.

---

Footnote 1: We remind our readers that, in Stereophile parlance, the wall behind the main loudspeakers is the front wall, and the wall behind the listener's head is the rear wall—Copy Editor.

Footnote 2: See www.bagend.com/applications/active-bass-trap/.