this is something i've been playing around for a while now & i would like to share. what do we need?
- a subwoofer (a nuber AW850 in my case which is capable of 19hz)
- a measuring microphone (behringer ecm8000)
- an equalizer (behringer dcx2496)

cost: ~500-600$ (of course, this could be decreased if you DIY e.g. the subwoofer)

the subwoofer is placed in the corner. the microphone records all frequencies in that corner. the subwoofer is just phase inverted & playing everything lowpass filtered that is recorded by the microphone. the equalizer is used for optimiztion. in that case, i've just boosted the 34hz with a high Q.

as you can see, in the room there was a big problem with a 34hz mode which could be solved. a conventional "passive" solution (e.g. passive helmholtz resonators) would have needed much more space (~1000+ liter)! another benefit is, that this active solution is tuneable easily by the EQ.

comments appreciated!

'

I can see that you made gains at 34 - but, in the process you also lost ground around 32 and 30........ this would not have happened with passive treatments -

PEQ is a good tool for fine tuning after room treatments - it is not (however) a suitable replacement for them..........

By the way - this is not an active resonator - it is active cancellation - and is used in a lot of different areas in acoustic design...... including HVAC and automobile design.......

Rod

These things are so neat. I was never aware of active cancellation until recently. Innovative for sure.

hm...if you look on 0ms, no ground is lost. its looking even better than before, since it is extended & not that sharp anymore. from my experience, this technique should have the same result as a very big passive helmholtz resonator (HR) tuned to 34hz. there is basically nothing much different between a passive HR and this one. from my experience with passive components, removing the "ringing" of the room always optimizes the frequency response @ 0ms.

maybe you missunderstood this point or i did not clarify. the subwoofer is not playing any "music", its only used for the cancelation process in the corner. the PEQ is *only* used to adjust the subwoofers signal (like "bringing it into resonance" -> then phase invert). this is just for optimization of the chancelation process. its *not* room equalization (in that case i would agree with you).

yes, this nothing new. i just call it active helmholtz resonator, since it is just like a HR on steroids.
i found it interesting that it works that great, is easy to setup, with low financial effort & space consumption. compared to passive helmholtz resonators, it could also be re-used (re-tuned).

thanks for you comments!

Please do me a favor and either overlay the data or send me the raw data so I don't have to keep popping form one pic to the next to compare.......

You can email me at [email protected] if you decide to send the raw data.

I stand corrected on the PEQ issue - I lost track there for a moment......


Thanks,

Rod

of course. here with overlay

There is definitely a deeper dip as I described......

Thanks for the overlay

Rod

i don't know which part of the figure you are referring to.
when i look at 29-33hz @0ms (you mentioned 30 and 32hz), the red area is filling up the dip that was previously there (with the blue area).

Sorry - (I don't have the data so I cannot check for myself) but there is a heavier blue line in both of the original plots you posted that is not in the merged plot - - now - when I plot things I do it so the lines show through the plot - in this manner I do not have to guess based on scale - however - based on scale that heavier line is 30Hz - and where that heavier line is in the original plots there is definitely a deeper dip........ however - if you wish to clarify for me at what frequencies the increased dip actually spans I would be happy to take your word for it -

Anyway you look at it - there is an increase in the low frequency dips that already existed - as well as the decrease you mention..... regardless of the frequencies involved.......... hence the blue above the red in the merged plot...... in roughly that area I pointed out.

Rod

sorry if this is daft...

is the microphone always connected and continuously active, or are you using it for measurements and running program material through the sub in reverse phase?

If the mic is always active, how do you keep it from feeding back? The sub and the mic are in the same corner correct? What is preamping the mic? I'm so confused.... but intrigued.

i see. i think you are referring to the area which i marked "green" in the figure below, right? this is around 27hz. if so, i think this is negligible (could be maybe just a small measurement effect since i measured only with 128k instead of 1M sequence length).
i was referring to the purple area, which i think is more important.

yes, the microphone which is connected to the subwoofer is always connected & running (otherwise it would not work). i'm using a different (calibrated) microphone to do the measurements.

the preamp for the mic connected to the sub is a behringer mic 800 with "tube emulation" deactivated & set everything else to "neutral".

feedback is avoided by:
- placing the subwoofer to the farthest corner with respect to the monitoring speakers
- placing the subwoofer on the ground, but the microphone near to the ceiling of the same corner
- good adjustment of the volume & the EQ
you just need to make sure, that it doesn't "self oscillate".

Great work karumba. Could you send the .mdat file to me by email please.
Address is on my websites below.
I have thought about this now and again over the years but never hit the simple solutions which seem to be the difference between working and not. e.g. the mic in the corner. Very interesting.
DD

thanks!
please find the .mdat file (REW) attached.

i just wonder wether a direct signal with delay and filtering might give better results than the microphone?

i've already tested that, but it doesn't work that good (but little improvement can be achieved). the reason is, that the direct signal doesn't contain the decay (=ringing of the room). a similar approach would be "double bass array".

Finally got around to testing karumba's idea myself.
Two speakers with serious LF down to 18Hz. Pointed into back corners.
A mic at a back corner ceiling. Parametric Eq boosting the frequency(s) of interest. Phase flipped on the mic pre.
Very tweaky but a promising result. I hope it encourages others to give this a go.



DD

wow...thats a really great result!
nice to see, that it works also in other rooms.

thanks for sharing!

This is nothing new, this type of “modal combat” techniques has been around for a long time:

http://mixonline.com/online_extras/E-Trap.pdf

correct. as far as i know, the IPR for that has been filed around 2003. the good thing is, that this can be achieved quite easily, cost effective & flexible by DIY. you just need a subwoofer, mic & a EQ. and this saves a looooot of space & is very effective (esp. for very low frequencies) as the measurments show.

Thanks for your kind and encouraging post Jens.

The mic is in the Bag End device.
Karumba's simple but enabling innovation is to move the mic elsewhere. This makes the DIY possible.
A friend acoustician is also experimenting, successfully I am told, with this notion.
He has found a great and extremely affordable source of 'set and leave' parametric eq.

DD

If one wants to learn more about this stuff, there’s a whole chapter (13) on topic in the excellent book; "Acoustic Absorbers and Diffusers":

Acoustic absorbers and diffusers: theory, design and application - Trevor J. Cox, Peter D'Antonio - Google Böcker

Is there some particular helpful point you are alluding to Jens?
Do they have a working example in the book, with test results? Or build or implementation details?

DD

Oh, sorry to have provided you and others with a good source in order to further increase the level of knowledge on topic. It won’t happen again …

jens, thanks for sharing. unfortunatly i couldn't read the whole chapter (maybe i'll buy the book), since some parts are not available. from what i've read, this shows a bit a different viewing angle, since the presented approaches are quite different from the one discussed in the topic.

also in the beginning of chapter 13 they write:
the cost & implementation difficulty of the approach i presented is absolutely no problem anymore. also the e-bag is - given the performance, the space needed & the reuse-aspect when moving to a different room (!) - not that expensive anymore. just imagine how much effort is needed to kill the decay of a 30hz mode in a larger room.

Well, I guess that depends how you see it but to me, it's far easier to just build a tuned trap. The result is also probably going to be better in total (assuming the trap was designed and placed correctly).

when you have full freedom & build a new studio from scratch, is fully agree. then there is no reason to go active.
but when space (for placing bass traps) is limited, active absorption could be very helpful (in addition to conventional acoustic modules).

Sometimes, yes. Agreed.

Hi karumba. Just playing around with REW, gradually learning how to drive it.
Your result was way better than mine, see below for a hyped up congratulations type of view!
My room is complex. Concrete, two alcoves. Three rooms in one really. Weird two close modes down around 30-37 Hz. I found the adjustments critically fine. Very tweaky.
Nevertheless I was happy with the result. If I were to install a system using a Sub or Subs I would certainly keep this technique in mind.
My good friend Jeff Hedback turned me on to these useful little gems.
Home | miniDSP
I neglected to test whether effect worked over a wide listening area, did you?


Best Regards, DD

dandan, really nice results. indeed, it needs a bit tweaking, but its worth it. i've also experimented with the phase shift functionality implemented in the behringer EQ. as far as i know it didn't end up with just 180°, but 175° or something like that. i don't know the reason, but maybe there was some small delay mains->sub vs mains->mic.

i've already seen the products from miniDSP...could be suitable for this task....if they would provide the boards with a simple mic-preamp incl. phantom power, this would be just perfect.

yes i did. it affects all areas in the room. i would be suprised if it didn't, since this is not much different from a passive helmholtz resonator. from my point of view the principle is the same. its just much "stronger" since it works due to applying "active interference".

i already had some thoughts of a special version without the need of a microphone, but by feeding the subwoofer with a DSP-processed signal of the original signal (which you are listening on your mains). that could be even more effective. i just need to solve some math problems before. maybe i'll describe my idea later in detail, since i anyway have no interest in making an IPR.