Well...Wouldn't it be possible to just hook up a microphone to the input of an active subwoofer with the phase reversed and careful placement to produce the same results for a fraction of the price?
The PSA AVAA doesn't need any calibration and doesn't have any inputs or even DSP (it's 100% analogue), which makes me think it can't be that difficult.Well...
If you just put a mic on the "cancellation" sub you're just going to get some funky feedback, not cancelation of the other subs in the room.
What you need to do is characterize the response of Sub A (listening sub) at the position of Sub B and use DSP to invert that and play it through Sub B. You also need to account for the response of Sub B to get the response right. So you take your measurements with the mic, but I don't think you use it live, so to speak.
That said yeah you can totally DIY this if you set it up properly, and probably like @RayDunzl said, with relatively simple gear.
Something like that although I think it just reverses the polarity (shifting the phase affects different frequencies differently). It doesn't even have DSP so it has to be very simple. I think the key is the fact that it uses a sealed cabinet. If it was ported, it would be less effective at countering air pressure.Could the algorithm be:
* mic pickup the corner reflected sound at the back of the unit
* invert the signal (180* out of phase)
* bad limit the signal to <100Hz
* amplify and playback the signal at say 75% of original amplitude through a couple of bass drivers
?
The PSI Audio one has no DSP. Besides, I don't think delay is needed. The device is supposed to cancel out air pressure at the very location it sits, not at the listening position. This is why it needs no adjustments—you just shove it in a corner and it does its thing. If any delay is needed at all it probably would be small enough that you could implement it just by putting the microphone behind the cabinet (thereby creating some distance between the driver and the microphone).Maybe add some delay as well...?
Actually, I think the trick might just be precisely not to try to optimise anything for the listening position. The active bass trap just monitors air pressure locally and does the exact opposite so that the sum is always equal to atmospheric pressure. It's a basic negative feedback loop: when it senses high pressure pressure the driver retracts thereby creating extra space for air to rush into and literally making the room a little bigger (remember it's a sealed cabinet); when pressure dips the driver pushes out to increase it, making the room smaller.If one wanted to have a DSP-based version and add delay (on top of polarity inversion and low-pass and volume) then it could theoretically even be optimised to the listening position, couldn't it?
I'd love to but I don't have a sealed subwoofer. I was hoping someone might have attempted this already, or perhaps someone reading this who has the necessary bits could give it a shot. I'm planning a studio build and after plugging values into the porous absorber calculator I began to realise just how hard it is to absorb low end. Due to space constraints, the thickest I can go on absorbers is 16" and that robs me of valuable room space. Once the room is built and without the absorption in yet I might consider buying a couple of sealed subs to attempt this myself. I reckon I can swing a pair of subs and microphones for the equivalent of ~$1500, which is considerably cheaper than buying the PSI Audio product (about $5k for a pair). I could even build the preamp myself for a few bucks.I guess the next step is to give it a try and see what it does in practice
Intersting read. It says on p. 13:There's more going on inside them than it seems, I had 3 of AVAAs but I came back to passive resonators. Here's paper that shows the idea behind it, could be helpful to build one https://www.mbakustik.de/wp-content/uploads/article_aes.pdf
For active treatment I would rather look into Dirac ART when it's more available
So it's not just a speaker connected to an amp, but there is an internal feedback loop that monitors how the membrane moves in and out and ensures it keeps up i.e. a servo subwoofer.To end with, the power amplifier (6) uses a measurement (7) of the transducer (1) membrane velocity in a feedback loop in order that the membrane velocity matches the input signal of the amplifier.
Hmm, interesting. I'm sure there's got to be some filtering to linearize the response of this thing, but maybe your original idea would work better than I thought at first. If the frequency response of the mic and the sub aren't matched properly, you could still get feedback even if you invert the signal.The PSA AVAA doesn't need any calibration and doesn't have any inputs or even DSP (it's 100% analogue), which makes me think it can't be that difficult.