Has anybody made DIY active bass traps using cheap subwoofers?

[Scoox]

In 2-3 weeks' time I might give it a try with a 15" sealed speaker and with DSP (with no servo unfortunately)

Nice! If you give it a go let us know. I think the servo part probably just makes the system more responsive? TBH I don't know what difference it makes, since the whole system is a feedback loop going through the microphone anyway. Perhaps the servo feedback loop tightens things a little more, but that's ok, the first step is to validate the theory.

The Yamaha NS-SW100 is a ported 10" sub that features active servo technology and it only costs $200. My Genelec subs also use 10" drivers so they are equivalent in size. A couple of these at the back of my room with a cheap microphone and preamp might do the trick.

 

[Scoox]

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.

My guess is that you can put it anywhere you like, but you want to put it where it's most effective. If you put it on a spot where max air pressure is half the max air pressure in the room, then you are only going to get 50% efficiency.

 

[ppataki]

Nice! If you give it a go let us know. I think the servo part probably just makes the system more responsive? TBH I don't know what difference it makes, since the whole system is a feedback loop going through the microphone anyway. Perhaps the servo feedback loop tightens things a little more, but that's ok, the first step is to validate the theory.

The Yamaha NS-SW100 is a ported 10" sub that features active servo technology and it only costs $200. My Genelec subs also use 10" drivers so they are equivalent in size. A couple of these at the back of my room with a cheap microphone and preamp might do the trick.

I will definitely try it sometime next month (I need to wait for my new amp to arrive first)
I will use one of these speakers (all my designs have sealed cabinets) for this test, will put it in the corner of my home office where I have this system
I will post here the measurements too

 

[Scoox]

I will definitely try it sometime next month (I need to wait for my new amp to arrive first)
I will use one of these speakers (all my designs have sealed cabinets) for this test, will put it in the corner of my home office where I have this system
I will post here the measurements too

Thanks. BTW that system looks classy AF.

 

[ppataki]

Hopefully next week I will have time with my neighbor to test this idea, I will report back with our findings/measurements

 

[Scoox]

Hopefully next week I will have time with my neighbor to test this idea, I will report back with our findings/measurements

Nice!

 

[kemmler3D]

Despite my dumb comments earlier in the thread I will be following with interest.

 

[OldHvyMec]

Despite my dumb comments earlier in the thread I will be following with interest.

Blame it on the medication!

Regards

 

[Andysu]

active bass traps , sigh i saw dozens of youtube videos that are all clickbait no actual showing this device actually doing anything , just , people blah , blah , blah showing themselves and hardly the unit , i lost interest in the subject now , unless ASR can show actual member doing something , shill youtube clickbait videos vex me

 

[ppataki]

OK, so let's see!

Let me first explain what you see in this photo:
- on the table I have my Sabaj A30A driving the SB Acoustics-based one-way loudspeakers (the laptop you see there is the source)
- in the foreground you can see the UMIK-1 measurement mic
- on top of the laptop there is an RME Babyface as the interface where we connected the mic (see in the next picture)
- on the top left edge of the table there is a Topping PA7 Plus driving the 15-inch Fane speaker in the corner (the amp is connected to the Babyface too)

DSP:
- we applied phase inversion
- we applied a low-pass filter at 100Hz
- we applied no delay

The mic was behind the 15-inch speaker in the corner (the book was needed so there was some distance between the speaker and the mic )

So basically what happens is that the mic 'hears' the signal from the main speakers (whatever that is in the corner), then the above DSP is applied to the signal which is output to the 15-inch speaker.
First I used the UMIK-1 to do RTA in REW to set the volume of the 'absorber' for the best possible frequency response
Then I made a sweep measurement with the UMIK-1 before turning on the 'absorber' and then after turning it on

See the results here:

There is a difference, but quite small.....
Moreover the absorber caused some unevenness above 70Hz

Overall I am happy that we could make this experiment happen - on the other hand I don't think this is the direction to pursue.... one would be better off with one or more subwoofer(s) to ameliorate the frequency response

I am attaching the mdat file so you can check out the other graphs too

 

[boxerfan88]

how does the measurement look like - if you were to rotate the setup 180 degrees?

• woofer facing the corner
• mic facing the room -- is now measuring the soundwaves from the room heading toward the corner

 

[ppataki]

how does the measurement look like - if you were to rotate the setup 180 degrees?

• woofer facing the corner
• mic facing the room -- is now measuring the soundwaves from the room heading toward the corner

Good idea but unfortunately I cannot test further since we dismantled the gear (the Babyface, the mic and the cables were from my neighbor....)
But hopefully somebody else will be able to test that

 

[kemmler3D]

Interesting that you were able to improve the null at 50hz by 3dB, but overall I agree not the result I would have been hoping for either. I think this kind of thing probably takes a lot of iteration and tuning. I appreciate you doing the experiment and posting results!

 

[John in CH]

There is a difference, but quite small.....
Moreover the absorber caused some unevenness above 70Hz

Overall I am happy that we could make this experiment happen - on the other hand I don't think this is the direction to pursue.... one would be better off with one or more subwoofer(s) to ameliorate the frequency response

I live in Switzerland and learned from a colleague about the AVAA system from PSI Audio. At the suggestion of him, I reviewed this article here in German (use DeepL or Google Translate) that speaks about AVAA construction. Most interesting is the employment of a perforated plate to disperse the pressure wave to increase speed / reduce pressure.

From the article (Translation from DeepL.com):
"Unscrewing the front grille of the AVAA reveals a curved perforated plate whose function is to convert sound pressure in the corner of the room in front of the perforated plate into speed behind the perforated plate. The process is vividly described by Jürgen Schröder in the online magazine LowBeats, who describes the process as a large water wave that hits a perforated wall at high pressure, causing the water to be pressed through the small openings at high speed but low pressure. The wave is therefore reflected much less strongly by the perforated wall than by a closed wall.

In the chamber behind the perforated plate there is a type of fleece as a flow resistor and a woofer that works with the front side on a closed volume and with the rear side in the volume behind the perforated plate. A microphone is mounted in front of the perforated plate, which is used to minimize the sound pressure with the help of an analogue control circuit of the woofer. PSI describes the effect of the AVAA as a “large sucking hole in the wall”. A similar “sound-absorbing” effect is known from conventional absorbers, which absorb more than the actual surface area would suggest. It is not possible to say at this point exactly how much this makes up. Likewise, the description of how the AVAA works remains somewhat speculative, as PSI is understandably keeping a lid on this."

So with front of woofer facing sealed enclosure and with the rear facing the perforated plate volume, it would appear a clever approach to reduce crosstalk between the woofer and the external microphone.

Zooming in on the photo from the article, the perforated plate does not appear something so special and could likely be procured online or from home/hobby supply store.

@ppataki - Do you think this information is inspiring enough to have another go at it?

With Friendly Regards from Switzerland...
- John

 

[ppataki]

I live in Switzerland and learned from a colleague about the AVAA system from PSI Audio. At the suggestion of him, I reviewed this article here in German (use DeepL or Google Translate) that speaks about AVAA construction. Most interesting is the employment of a perforated plate to disperse the pressure wave to increase speed / reduce pressure.

From the article (Translation from DeepL.com):
"Unscrewing the front grille of the AVAA reveals a curved perforated plate whose function is to convert sound pressure in the corner of the room in front of the perforated plate into speed behind the perforated plate. The process is vividly described by Jürgen Schröder in the online magazine LowBeats, who describes the process as a large water wave that hits a perforated wall at high pressure, causing the water to be pressed through the small openings at high speed but low pressure. The wave is therefore reflected much less strongly by the perforated wall than by a closed wall.

In the chamber behind the perforated plate there is a type of fleece as a flow resistor and a woofer that works with the front side on a closed volume and with the rear side in the volume behind the perforated plate. A microphone is mounted in front of the perforated plate, which is used to minimize the sound pressure with the help of an analogue control circuit of the woofer. PSI describes the effect of the AVAA as a “large sucking hole in the wall”. A similar “sound-absorbing” effect is known from conventional absorbers, which absorb more than the actual surface area would suggest. It is not possible to say at this point exactly how much this makes up. Likewise, the description of how the AVAA works remains somewhat speculative, as PSI is understandably keeping a lid on this."

So with front of woofer facing sealed enclosure and with the rear facing the perforated plate volume, it would appear a clever approach to reduce crosstalk between the woofer and the external microphone.

Zooming in on the photo from the article, the perforated plate does not appear something so special and could likely be procured online or from home/hobby supply store.

@ppataki - Do you think this information is inspiring enough to have another go at it?

With Friendly Regards from Switzerland...
- John

Currently I am busy with multiple coax speaker projects (can see them in the DIY section) and then I plan to have yet another sealed sub project then we shall see

 

[Scoox]

Currently I am busy with multiple coax speaker projects (can see them in the DIY section) and then I plan to have yet another sealed sub project then we shall see

Thanks for taking the time to do the experiment. I'm tempted to get a couple of AVAA 20 for my small studio but the price tag, I dunno, if I'm paying that much money I would need to understand what exactly it does, because I don't buy voodoo.

If no audio device is perfectly linear, and no speaker is perfectly matched to any microphone, and no two devices are exactly identical, then how does it work? Surely if you take one AVAA 20, put it in a corner and test with REW, the results would be different (or at least slightly different) than with a different AVAA 20 as they won't be exactly identical. I suppose the idea is to calibrate your monitors with the active traps already in place, as if they were part of the room, but some how I'm lead to think what they do is actually not that magical or precise. Does it even need any calibration after years of use or will it work forever? It has a built-in microphone for sensing sound, does it degrade over time (obviously yes) and how does that affect effectiveness? I really need to send PSI an email before shelling out 7k.

 

[ppataki]

Thanks for taking the time to do the experiment. I'm tempted to get a couple of AVAA 20 for my small studio but the price tag, I dunno, if I'm paying that much money I would need to understand what exactly it does, because I don't buy voodoo.

If no audio device is perfectly linear, and no speaker is perfectly matched to any microphone, and no two devices are exactly identical, then how does it work? Surely if you take one AVAA 20, put it in a corner and test with REW, the results would be different (or at least slightly different) than with a different AVAA 20 as they won't be exactly identical. I suppose the idea is to calibrate your monitors with the active traps already in place, as if they were part of the room, but some how I'm lead to think what they do is actually not that magical or precise. Does it even need any calibration after years of use or will it work forever? It has a built-in microphone for sensing sound, does it degrade over time (obviously yes) and how does that affect effectiveness? I really need to send PSI an email before shelling out 7k.

I really don't want to discourage you but after many years trying to fix bass issues in multiple rooms, I am totally convinced that the only way to really make that happen is through using one (or preferably more) subwoofer(s).
Strategically placed in the room with volume and time alignment

 

[Scoox]

I really don't want to discourage you but after many years trying to fix bass issues in multiple rooms, I am totally convinced that the only way to really make that happen is through using one (or preferably more) subwoofer(s).
Strategically placed in the room with volume and time alignment

I have two subwoofers (not yet in place though) but I'm not sure the GLM software will take work the way we mean here. This exercise would need careful manual calibration, I assume.

 

[ppataki]

I have two subwoofers (not yet in place though) but I'm not sure the GLM software will take work the way we mean here. This exercise would need careful manual calibration, I assume.

Yes, either manual, meaning volume calibration and time alignment set manually based on measurements
Or using some specialized software to help, like MSO or Dirac Live or similar

 

[Oddball]

Well, passive room treatment has been discussed already. Not familiar how effective GLM as never tinkered with it myself, but can say that Dirac ART (aka active room treatment) is really effective. There are couple of ART threads on this forum worth checking. Think most of the content is in the one below.

Active Room Treatment (ART) by Dirac

Having listened to the recording, I can say it's a fair approximation. I have, I've tried everything I can come up with (including independent adjustment of the subs). Nothing I've tried hints at any avenue for improvement in the character of the bass imposed by ART. I've approached my...

www.audiosciencereview.com