Room treatments in general, and bass traps specifically, are not the sexiest products in the audio pantheon. At least until you hear them. And even when you do hear them, it is quite possible that you don’t fully hear what they can do. Not through any fault of the treatments, but because bass traps generally have to be used in significant numbers or large sizes to be effective. Such installations are expensive and visually impactful. Unless, perhaps, you use active bass traps, like the PSI Audio AVAA under review here. 

What Problem Are We Trying To Solve? 

Okay, let’s back up a bit to invite everyone watching into the discussion. Your stereo system is set up in a room. In all homes, that room will have walls and a ceiling and a floor. There will be dimensions between those walls, where you might find that the front and rear walls are 20 feet apart and the side walls are 15 feet apart and the ceiling is 9 feet from the floor. You may have various openings to other rooms, but in any case, the room will have dimensions — be they simple or complex. This isn’t news. However, the key point may be less obvious, which is that those dimensions support resonances – frequency bands where the sound is emphasized.  

To extend the simple example I started with, a room that is 20 feet long will have a primary axial resonance at 28 hz. And it will have resonances around the major multiples of that number: 56 hz, 84 hz and 112 hz. It will also have resonances caused by the width. So, in our example, you would find resonances at 37 hz, 74 hz, 111 hz and so on. And the floor to ceiling dimension would set up resonances at 62 hz, 124 hz, 186 hz etc. These are just the axial modes, which are the simplest and generally most obvious.  

Why do we care? Well, we care because these resonances (or modes) will tend to show up as bass peaks in your room. And where you don’t have modes, you will sometimes have frequency response dips. These are more or less permanent features of your room. You can design a room to reduce the problem by improved spacing of the resonances. But you will still have resonances.  You can also place your speakers and your listening position to smooth out the results somewhat. This is a complex subject, and we will have some future videos addressing pieces of it.  

If you want to calculate the axial resonance modes in your room, you can easily do this with the calculator on your phone and a tape measure. You will have primary resonances at: 

1125/(2 x room dimension in feet)  

Calculate this for each major dimension (length, width, height). The result will be resonant frequencies measured in hz. You will also have resonances at 2x those frequencies and 3x and 4x etc. These formulas are most accurate with simple rectangular rooms. 

So, we have a series of resonances that tend to lead to bumps in frequency response. We should also note that since the room modes are resonant frequencies, there is likely to be a timing issue: the modes will “ring” or display “overhang” defined as the modal frequencies continuing to make sound for some number of milliseconds longer than desired, perhaps 100 milliseconds or more. 

We can address the peaks and the overhang by using bass absorbers (often called bass traps) or DSP. Bass absorbers are generally boxes or tubes with absorptive material or diaphragms sized to absorb bass. Which means they are relatively large. They are often placed in corners because the corner is a high-pressure zone for all axial modes. Ideally, the absorbers would be tuned to the modal frequencies we face, but practically they are often designed to absorb a range of bass frequencies.  

Bass problems, especially below 100-200 hz, are some of the bigger problems in high-end audio. If bass absorbers were a foolproof solution, I suppose this wouldn’t be the case, but bass absorbers face problems. You have to know you need them. You have to know how to use them. You have to pay for them. And you have to have enough of them.  

The PSI Audio AVAA 

The last issue, having enough bass traps, leads to another issue with visual impact. Which is to say, you might need 8 or 12 traps or more and these might be 3 foot by 16 inch columns or 2 foot by 2 foot by 18 inch triangular boxes. Each. In a smaller room, there may be the simple issue of where to place these. In a larger room you will probably need more traps and, in any event, they will be quite visible. That’s fine in a dedicated room, but in a regular living room it may be a non-starter.  

That’s where the PSI Audio AVAA bass traps come in. PSI Audio is a Swiss pro audio company and the AVAA was developed for use in recording studios but is now available for consumer use.  

The AVAA is an active bass trap. That means that, instead of just using absorptive material to control bass, the AVAA has drivers in a column that are driven by an internal amplifier. The AVAA column has a microphone in it, which “listens” to the bass in the room and feeds an out-of-phase signal to the AVAA amp and drivers to cancel some of the bass that reaches the AVAA. The key point is that the AVAA should be able to cancel more modal bass per unit volume than passive bass traps can. And, as a result, it should get the job done in rooms where large arrays of bass traps are impractical. The active aspect of the AVAA also makes it somewhat adjustable.  

Sound Quality 

Let’s see if all that works. 

I was supplied with two AVAA bass traps. As you can see, these are small columns about 25” high and 9” in diameter, which makes them pretty easy to place. I had one behind a chair and another adjacent to a floor lamp.  They are made of metal and seem quite solid.  

I used the AVAAs in two different rooms. In each case, I placed them in the available tri-corners of the room where the walls and floor come together. I set the AVAAs, using the PSI app, to 0 reference level and listened.  

My initial impression was that bass was better balanced and somewhat tighter than in the existing setup where I had spent hours dialing in speaker and listener location. The difference wasn’t dramatic, but it was a meaningful step in the right direction. Most audio progress, by the way, is in this “meaningful step” category, at least once you have a good basic system.  

I ran some measurements to see if the changes brought about by the AVAAs were noticeable. Here is a frequency response run from the listening position: 

The purple line is frequency response without AVAAs and the green line is with AVAAs. You can see that the AVAA has its effect primarily below 200 hz. Zooming in on that band we see this: 

Now we are looking at the purple line (without AVAA) and a red line (with AVAA). The bass peaks are lowered by 4-6 db, and the dips are lowered too, at least below 100 hz. Above 100 hz, the AVAA smooths things out slightly. The AVAA results look a little like turning down the bass (if you had a bass tone control, which you probably don’t). But on closer examination, the AVAA is a little different than a tone control would be, because it does more where the resonances and dips are and not as much in between. And then above 100 hz it seems to work in a different way.  

I moved the AVAAs to a different room with different dimensions and different speakers. Here is frequency response measured at the listening position from 20 to 400 hz: 

The green line is without AVAAs, the yellow line is with AVAAs. As before, the peak between 50 and 80 hz is reduced but this time the subsequent dip is also reduced. I don’t mean the level is reduced but that the dip is less deep and the overall curve is smoother. Some dips are caused by cancellation effects, and if the bass is reduced at those frequencies, the cancellation may be reduced.  

I also checked the spectrogram with and without the AVAAs. 

This is the spectrogram with the AVAAs. Without the AVAAs, the spectrogram looks like this: 

Spectrograms are harder to directly compare, but close examination shows the decay of the bass peaks is reduced in time by 75-100 milliseconds with the AVAAs. Part of this is simply due to reduced levels of course.  

Listening to a wide variety of music, with special emphasis on bass test tracks, I again felt the AVAAs made an immediately noticeable difference. The more important point was that the difference was always an improvement. With the AVAAs, bass was better balanced and more natural. This naturalness shows up with bass sounding more open and detailed, with a more accurate sense of air and harmonic definition.  

I should characterize the magnitude of the improvement wrought by the AVAAs. First of all, the bass isn’t suddenly rendered perfect. The general shape of the frequency response, with the existence of peaks and dips, isn’t transformed. But the improvement is bigger than what you often hear between different good amplifiers. That is a rough comparison, because the AVAA does all its work at low frequencies and amplifiers can affect the full frequency spectrum. But amplifiers make easily identifiable differences that can be important to accuracy. So does the AVAA, but it doesn’t just make changes, it does something that appears beneficial in almost every system I can imagine that doesn’t already have bass traps. It also is outside of the basic circuit from source to speaker, so you can get the AVAA benefits without sacrificing any of the hard-earned goodness in the rest of your stereo.  

Usability and Value Considerations 

The AVAA C214s cost $3950 each in the U.S. Note that you can use a single AVAA, but even then, the price is non-trivial for many listeners. On the other hand, you could easily spend more changing speakers or amplifiers or DACs or even wiring looms.  

To assess value relative to passive traps, I compared the AVAA to standard bass traps. I got this frequency response curve: 

I used four passive bass traps for the green line and just the two AVAAs for the yellow line. If this measurement is representative, it would appear that the passive bass traps do more of their work above 100 hz and the AVAAs are more effective below 100 hz. PSI advises that the AVAAs are designed to work below 160 hz, which is close to ideal for blending with broadband acoustic treatments. These treatments often work down to 200 or 300 hz at best. The PSI range is also the frequency range that requires the largest passive traps.  

Given how hard controlling room modes below 100 hz is, I also checked just the passive bass trap arrangement (dark green) against nothing (light green). 

My rough conclusion is that I would need 8 passive traps to do approximately what the two AVAAs do. The passive traps would cost somewhat less, but of course would be substantially larger. To put numbers on this, the volume of two AVAAs is about 4 cubic feet, while the volume of eight passive traps is about 32 cubic feet. In a larger dedicated room that might be fine, but I can imagine smaller rooms (say under 2000 cubic feet) or 3500 cubic foot living rooms where those passive bass traps would be a non-starter. It is also possible to mix passive and active traps if you really want to tackle tough problems. And I think a lot of audiophiles face tough bass modal problems.  

As a final note, the impact of the AVAAs can be turned up or down. There are 11 settings from 6 db more absorption than I used to 12 db less absorption. The drivers and amps don’t have unlimited headroom (there are warnings to tell you when you’re overdoing it); however, PSI has designed this for studio use so the maximum capability is for sound pressure levels up to 115 db. Therefore, it is likely that in smaller rooms, with appropriate listening levels, a single AVAA would be sufficient.  

I thought the AVAA C214s were impressively helpful. They are super easy to set up, very easy to hide, and probably do more than realistic numbers of passive treatments, albeit at a slightly higher cost. The big thing is that the bass sound quality with the AVAAs was noticeably improved in a way that may not be practical with any other technology.