PSI Audio AVAA C20 electronic bass trap Page 2

"Blackbird," from my mastering of Cassandra Wilson's Silver Pony (CD, Blue Note 6297522), recorded and produced by the great John Fischbach, has a warm, liquid quality and Reginald Veal's deep, luscious Fender bass. Herlin Riley's snare drum should knock you in the chest. Without the AVAAs, I could easily detect a disturbing overhang in the back of the room, and the bass, up through the midrange, lost focus, beauty, and definition.

"Do You Remember," Jill Scott's seminal hip-hop/neo-soul hit from Who Is Jill Scott? Words and Sounds, Vol.1 (CD, Hidden Beach 62137), was engineered conservatively in 2000, before the loudness wars got out of hand. With the AVAAs, its hip-hop bass and kick were loud, tight, and impacting. Without the AVAAs, a distinct bass overhang masked the clarity of the sound.

When I was about seven, my dad introduced me to Arthur Fiedler and the Boston Pops. I wore down a copy of Hi-Fi Fiedler (RCA Living Stereo), playing it on a V-M turntable and producing lasting memories. Fast forward to 2015—I discovered the album on HDtracks at 24-bit/88.2kHz and downloaded it fast as light. Despite the peak saturation, the sound is to die for; this Richard Mohr and Lewis Layton production, made between 1956 and 1960 in Boston's Symphony Hall, has a glorious bloom that was enhanced by the AVAAs, for a fuller, warmer, more authentic sound.

Other tracks exhibited similar improvements, including from David Chesky's The Body Acoustic (CD, Chesky JD274), and David Crosby's Croz (CD, Blue Castle 11421), both in 24/96 from HDtracks. And with the AVAAs, Buena Vista Social Club (World Circuit/Elektra Nonesuch), also in 24/96, sounded more distinct and focused all the way up to the midrange.

I'm hooked: The AVAA C20 is a revolutionary product that puts the final polish on the sound of a fine playback system. Attention, PSI Audio: I'm going to have to buy these—or else you'll have to tear them out of my hands!

Measurements and Placement
Each AVAA C20 comes with an individual certification (see fig.1) showing that it measures from ¼ to 1/8 the nominal 400 N-s/m3 impedance of free air in the frequency range of interest. Translation: It's better than air at absorbing sound. I have no way of verifying an AVAA's effective impedance, but I can measure its effects on frequency response, decay time, distortion, and waterfall response in the room.

Fig.1 Certification of a PSI Audio AVAA C20's acoustic impedance. In comparison, free air has a nominal acoustic impedance of 400 N-s/m3.

One object of acoustic treatments is to lower a room's response peaks and raise the dips without affecting anything else: a tall order. First, I measured my room's response without equalization—just a crossover and time correction implemented in AcourateConvolver. For ease of presentation, I measured these with Room EQ Wizard analysis software, with a 150ms window and psychoacoustic smoothing, the mike at the listening position. The black trace in fig.2 represents my measurement of the "naked" room, without PSI AVAA C20s or my six RealTraps MondoTraps.

To locate the resonant points in the room, I played the sinewave oscillator in Room EQ Wizard, changing its frequency by clicking on any point in the graph (a very nice feature). This room's problem frequencies include 29, 46, 70, 125, and 145Hz. (A dip at 125Hz is the Allison effect—ie, cancellations from adjacent boundaries.) Walking around the room, I could easily hear the maximum and minimum points. I couldn't detect any lengthwise standing waves—ie, any buildup between the front and rear walls: in my room, the primary modes are standing waves along the diagonals. The 29Hz sinewave built up in the corners and at the listening position. The 46Hz sinewave was strong in the corners and weak at the listening position (even when I played only the left or right channel, I could hear the 46Hz resonance in all four corners). The 70Hz mode built up at the front bay window and in the corners. The 145Hz mode manifested primarily as a resonance at the center front, near the floor.

Thus it was easy to find candidate locations for the AVAAs. Keep in mind that a frequency-response measurement is a small but important part of the picture. It quickly reveals the room modes, and permits comparisons of a room with and without treatments installed.

Because an active bass trap shouldn't be placed where it would interfere with a subwoofer, I put one AVAA C20 in each rear corner, then remeasured (graph not shown; see photo 2). Two AVAA C20s measurably improved the three lowest modes. Then I added a third, at center front. This yielded my prize trace, the orange one in fig.2: the effects of three AVAAs. Compare the orange (AVAA) and black (naked room) traces: They tell a dramatic story of a new technology at work, with an unprecedentedly surgical effect in the 20–50Hz range, which should have a salubrious effect on the sound. I foresee manufacturers toting these into boomy demo rooms at next year's Consumer Electronics Show!

Photo 2: PSI Audio AVAA C20 trap in rear right corner.

By comparison, the blue trace in fig.2 is the left speaker with six RealTraps MondoTraps: four behind the curtains, two on the front wall. Compare the performance: three AVAAs (orange trace) vs six MondoTraps (blue) vs no treatment (black). Notice that the Mondos seem to shift the center point of some modes, while the AVAAs precisely attack them. The AVAAs smoothly and effectively attenuate the 29Hz peak at the listening position by an amazing 2dB, which is 1.25dB better than six big Mondos. Just three AVAAs have effectively flattened the 25–55Hz range within 1dB—a most impressive performance. The AVAAs filled in the 45Hz dip by a significant 1.5dB, while six Mondos helped it by only 0.8dB. The AVAAs attenuated the 70Hz peak by 1.25dB, while six Mondos attenuated it by only 0.5dB.

Fig.2 Left speaker measured with Room EQ Wizard, no EQ applied, SPL at 1dB/division, frequency range of 10Hz–1kHz: no traps (black trace), six RealTraps MondoTraps (blue), three PSI Audio AVAA C20 traps (orange).

Notice how the AVAAs broadly improve the 90–110Hz range by about 1dB, including the dip near 120Hz, which the Mondos barely touched—although the Mondos did fill in the hole between 150 and 180Hz up to 1dB better than did the AVAAs. And above that, the AVAAs had barely any effect, up to and including a 300Hz dip that the Mondos improved by a subtle 0.5dB; the AVAAs were completely ineffective, as this is an octave above their stated range of operation. That said, a dip at about 550Hz was worsened by the Mondos, by a significant 1.25dB, while the AVAAs filled it in by about 0.8dB—even though this is far above their stated range of operation. Then again, the Mondos improved a 725Hz peak that the AVAAs ignored. Trapping is not a win-win scenario, but the AVAAs definitely did more overall good within their claimed range of effectiveness.

In the next step, using just three AVAAs, I measured and linearized using Acourate and AcourateConvolver. A first listen to some reference recordings revealed that removing the six MondoTraps had done wonders for the mids and highs, opening and livening up the sound in a mostly good way. But I felt that the bass and mids were now a bit too resonant, thickening the sound and making individual notes harder to distinguish. So I installed two customized passive traps, standing vertically in the bay window behind the curtains (photo 3).

Photo 3: Behind the curtains: AVAA C20 in center of bay window, flanked by two customized traps using 6" of Bonded Logic insulation. Note Dynamat damping sheets on windows.

What to do with the fourth AVAA C20? Placing it at the center rear subtly improved some modes, but worsened the modes at 30 and 70Hz. Moving it to the center front, stacked atop the one already there, helped some modes by a subtle 0.5dB without measurably or audibly hurting anything else. After listening to some more music, I decided that four AVAAs plus MondoTraps overdamped the room: it's great to have tight, even bass, but if it doesn't bloom correctly, it must be wrong. Three AVAA C20s were perfect for this room. (That said, after further listening, I realized that the upper mids weren't well defined—some slap echo in the corners was reducing the definition. So I reinstated the two MondoTraps on the front walls, visible in photo 1.)

The ultimate solution was a combination of three AVAAs and four Passive Traps instead of the previous six. Additionally, my room benefitted from: curtains; judiciously placed hard Sonex panels to remove some sidewall flutter echo and diffuse rear-wall reflections; and room correction (digital EQ, time correction, crossover) by AcourateConvolver. Passive traps 4" to 6" thick can help the mids and, to some degree, the bass, but too many can overdamp the upper mids and highs and make the system sound dead. PSI Audio's AVAA C20s helped me avoid that, um, trap.

PSI Audio
US distributor/dealer
ZenPro Audio, LLC
119 Towhee Circle, Orangeburg, SC 29118
(803) 937-6012

dalethorn's picture

That's an epic review. A lot of audiophiles who don't record or mix will want to try this, but I think they're going to need help. And I wonder what that help would cost. Great music tips too BTW.

Axiom05's picture

Interesting write-up on an interesting product. Please note that my following comments are based on my personal experience with bass traps and not from any professional or expert knowledge. Based on your frequency plots, one might conclude that, while audible, the effects from spending $6000 on active traps is quite minor and a poor value. As you have shown for both the active and passive bass traps, their effect in the frequency domain can seem insignificant (on the order of 1 - 2 dB or less). My experience with passive traps has been similar: I have not seen huge improvements in the frequency response (some frequencies get better and some get worse). However, I have found the true value of traps to be how they effect the time domain response. For me, reducing the "ringing" of the bass notes provides a significant improvement in the sound quality. Our ears seem to be less forgiving of time domain issues than of peaks in the frequency response. In other words, a peak in the bass region is more tolerable if the note isn't accompanied by a long reverberation time. It would have been interesting to see before and after waterfall plots of the product under review, particularly how effective they are at 29Hz. This is one of the potential strong points of this type of product as passive traps will have little effect at these lowest frequencies. However, it does seem apparent that any practical improvements using active traps will require more than one unit. Ouch!

Tim Link's picture

I too found this article interesting, especially the frequency response curves with and without the traps. We make and sell passive bass traps, and we get before and after measurements from our customers whenever we can. We too have found that often significant sonic improvements from the traps looks like very little change when looking at frequency response. To get a better idea of what's going on, we use a recording called MATT - Musical Articulation Test Tones. This will clearly show the difference when played in a room and recorded at the listening position. We run the recording through an analyzer that shows how fast the room is tracking the volume level at different frequencies.
What's also nice is that you can just listen to it and hear where the room is slurring the bass dynamics. This gives you a clear idea of whether or not the room could benefit from more bass damping.

drblank's picture

I don't know why he's comparing against the Mondo Traps. Those aren't that great of a product if you are trying to go after low frequencies under 100hz.

I also don't think that using subs is going to give the best attack and decay rates. I've talked to someone that has actually tested a variety of subs placed around the room at different heights and the attack and decay rates aren't as good as the products they compared them against. They compared them against diaphragmatic absorption cabinets. There are much better absorption products on the market other than the Real Traps or active subs for low frequency absorption.

I would be looking at companies that make Diaphramatic absorption products. They can be designed to really be effective towards the low frequencies under 100hz, but you need something that will effectively go down to the 30 to 50hz range as that's typically where you find most problems. Think in terms of the fundamental frequency of bass notes. A Piano, for example, goes down to about 27.5hz for the lowest note on a typical 88 note grand piano. The bottom two octaves on a piano are 110hz and below. The Mondo traps is just cheap building insulation wrapped in fabric, put in a metal frame and either hung or positioned around the room. They aren't low frequency absorption products. For that, you'll need diaphragmatic, membrane or Helmholtz resonators. Diaphragmatic being the most effective if designed properly.

Kal Rubinson's picture

Since the link for the BagEnd trap is dead, here's a link to my Stereophile review.