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Has anybody made DIY active bass traps using cheap subwoofers?
- Thread starter Scoox
- Start date
I've thought about it.
Using a miniDSP 2x4HD to control up to 4 subs, via delay and frequency response, could make it work, I'd think.
I actually have all that I'd need to do it, except the unoccupied floor space, so I never did.
Mostly I realized I don't seem to hear the fault I measure, so, didn't pursue it.
Using a miniDSP 2x4HD to control up to 4 subs, via delay and frequency response, could make it work, I'd think.
I actually have all that I'd need to do it, except the unoccupied floor space, so I never did.
Mostly I realized I don't seem to hear the fault I measure, so, didn't pursue it.
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.
- Thread Starter
- #4
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.
How does it work? Cancelation implies Helmholtz. The size of the room and the number of chambers is size dependant and band width
dependant. One chamber can cover many peaks within a band width BUT it can also ruin the sound if your not careful.
Active implies a electrical component, yes, no?
Chambers aren't difficult BUT to be adjustable, round tube that can be extended or compressed are more practical. Rectangle or square
boxes, not so much.
I've seen huge chambers in the corners of rooms that had different length and size tube extended into the room. It was pretty amazing
how easy it was to change a peak by extending a tube further into the room and taper the same tube with 2 or 3 smaller pipes or tubes.
just like 6, 12, 18, 24, 36, or 48db on a crossover. It looked like hell but it worked like heaven. Thin curtain would have worked wonders for
visuals and maybe a few lights and a pole for old what's her name.
Regards.
dependant. One chamber can cover many peaks within a band width BUT it can also ruin the sound if your not careful.
Active implies a electrical component, yes, no?
Chambers aren't difficult BUT to be adjustable, round tube that can be extended or compressed are more practical. Rectangle or square
boxes, not so much.
I've seen huge chambers in the corners of rooms that had different length and size tube extended into the room. It was pretty amazing
how easy it was to change a peak by extending a tube further into the room and taper the same tube with 2 or 3 smaller pipes or tubes.
just like 6, 12, 18, 24, 36, or 48db on a crossover. It looked like hell but it worked like heaven. Thin curtain would have worked wonders for
visuals and maybe a few lights and a pole for old what's her name.
Regards.
boxerfan88
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Could the algorithm be:
* mic pickup the corner reflected sound at the back of the unit
* invert the signal (180* out of phase)
* band limit the signal to <100Hz
* amplify and playback the signal at say 75% of original amplitude through a couple of bass drivers
?
* mic pickup the corner reflected sound at the back of the unit
* invert the signal (180* out of phase)
* band limit the signal to <100Hz
* amplify and playback the signal at say 75% of original amplitude through a couple of bass drivers
?
- Thread Starter
- #8
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.
- Thread Starter
- #9
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).
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Found a nice description on Polk Forum as well: Pass absorber
Guess most of us has the parts already to make a prototype.
Guess most of us has the parts already to make a prototype.
- Thread Starter
- #13
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.
- Thread Starter
- #15
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
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- Thread Starter
- #16
There was one thing I found intriguing about the PSI Audio product, which is that it only uses 70 W of power, which seems like... not much, considering most subwoofers take a lot more juice (my two 7360A's combined have a peak power consumption of 600 W!). Perhaps you don't need that much power to absorb sound. I guess all this device does is "get out of the way" when pressure is high (so it has air pressure working for it) and expanding when pressure is low (so it has the pressure dip outside the cabinet AND the extra pressure inside the sealed cabinet working for it).
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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
For active treatment I would rather look into Dirac ART when it's more available
- Thread Starter
- #18
Intersting read. It says on p. 13:
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.
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.
I think there's a big clue on the AVAA product page, which is that you need to place it "where the modes are". In that case I guess if you place it exactly at a peak, the job is simply to reduce the bass SPL by 50% at that location. (I think??) As long as your frequency responses line up, I guess it could be that simple.
In my mind, you would be placing the sub at an arbitrary location, more like how Dirac ART works. In that case, I think you would need to make significant adjustments to the response of the cancellation sub to make it do what you want.
Anyway, I am mostly talking out of my ear here - I think it's an interesting idea and I'd love to follow the thread if you actually try it.
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