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  1. #1

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    Default Experimental Active Bass Absorber

    Back in the early 90's Nelson Pass developed a unique product called the Phantom Acoustics Shadow.It was basically an active bass trap of sorts that he claimed effectiveness for room modes in the 20-200hz range. It works basically by introducing a signal that is exactly 180 degrees out of phase with the room mode thus reducing it's amplitude.
    According to reviews it worked as claimed but was not a commercial sucess. Nelson was kind enough to reveal enough details (even answered some direct questions)about the design for one to make a working copy (or 4).
    For those interested here is a link to the patent.The Shadow utilized two 8" woofers at opposite ends of a sonotube type of enclosure.
    http://patft.uspto.gov/netacgi/nph-P...RS=PN/04899387

    While the dimensional ratio's of my room are very favourable and conducive to good bass response,and I do get tight detailed deep bass from my subs but I find the mid to upper bass a bit ill defined and in need of some attention.Instead of building very large corner traps I thought I would try this active approach first.I intend to build two units, one for each front corner of the room but I will stray from the Shadow design by using only a single 8" woofer per cabinet.This so as to keep them compact(approx.a cubic foot per) and WAF friendly.I have yet to decide if I will go for the mini Vandersteen look with black grille cloth sock and solid wood top and bottom or sono tube with solid wood top and bottom accent pieces?

    It's actually a very simple device basically consisting of a microphone element placed in close proximity to a woofer to povide a feedback signal.The signal is then preamplified, phase inverted and high and low pass filtered. Then it is amplified by a small chip amp that then drives a woofer.

    Pic#1 shows the 8" SEAS woofers I already have on hand.

    Pic #2 is the chip amp kit purchased from an Asian ebay source.It uses the TDA 2030's which are slightly lower power than the TDA2040's Nelson used in the Shadow. Amp power and quality are not important factors with this unit.

    Pic#3 is the tiny electret mic capsules.
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    Last edited by FTGV; 08-20-2010 at 12:41 AM.

  2. #2

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    Sounds very familiar :D

    Neat idea FTGV and I'd love to see the final result. You certainly have a great choice of driver.

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    Quote Originally Posted by dorokusai View Post
    Sounds very familiar :D
    Which part?

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    While it´s possible, I really doubt it would work. Not to be pessimistic, but active control of sound and vibration was my postgrad topic of research, and there are many variables in play.

    The fundamental idea of active control is to send a signal out of phase to destructively interfere with the original signal and thus being able to eliminate it. But complicated feedback controls are involved which can become unstable if not properly designed, think of the feedback noise when a mic is close to a loudspeaker.

    I have seen working setups of active noise and vibration control, both in the laboratory and in practice, but you need to be able to replicate many lab conditions to make them work properly.

    But could be interesting to see if this works though, good luck ;)
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    Quote Originally Posted by dfledezma View Post
    While it´s possible, I really doubt it would work. Not to be pessimistic, but active control of sound and vibration was my postgrad topic of research, and there are many variables in play.
    Thanks for the informed reply,did you read the patent?Also here is a link to a (subjective)review done back in 89 by Stereophile in which the reviewer indicates positive results. http://www.stereophile.com/roomtreatments/1289phantom/
    The fundamental idea of active control is to send a signal out of phase to destructively interfere with the original signal and thus being able to eliminate it.
    Yes thats exactly the principal involved.
    But complicated feedback controls are involved which can become unstable if not properly designed, think of the feedback noise when a mic is close to a loudspeaker.
    N Pass did speak about stability issues and apparently found that the inclusion of a single pole 100hz low pass filter and single pole 70hz high pass filter provided a useful increase in stability.Aswell care in level setting will be important.

    But could be interesting to see if this works though, good luck ;)
    Thanks I hope it does.
    Last edited by FTGV; 08-23-2010 at 11:56 PM.

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    Quote Originally Posted by FTGV View Post
    Thanks for the informed reply,did you read the patent?Also here is a link to a (subjective)review done back in 89 by Stereophile in which the reviewer indicates positive results. http://www.stereophile.com/roomtreatments/1289phantom/
    Yes thats exactly the principal involved.N Pass did speak about stability issues and apparently found that the inclusion of a single pole 100hz low pass filter and single pole 70hz high pass filter provided a useful increase in stability.Aswell care in level setting will be important.

    Thanks I hope it does.
    Please keep us informed about your results. I´m very interested in this topic ;)
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    Quote Originally Posted by dfledezma View Post
    Please keep us informed about your results.
    Will do
    I´m very interested in this topic ;)
    Have you heard of the Bag End E-Trap?If not you might find it interesting. http://www.bagend.com/ click on products on the left then the Professional Acoustics heading on the right hand side.
    Last edited by FTGV; 08-24-2010 at 12:15 AM.

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    How is this project progressing?

    I'm anxious to hear if you were able to duplicate the Phantom Acoustics Shadow because they a); had rave reviews and b); are no longer produced (last one 2007) and virtually impossible to find used.

    What are you using to fabricate the actual tube?
    Last edited by inspiredsports; 01-14-2012 at 11:50 PM.
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    It's been a slow process as other projects kept jumping in line but so far have the controller/amp about 90% complete.I' m just using MDF for the column as I wanted to veneer it to match my speakers.Sonotube would be ideal but doesn't veneer well.I don't know the specific's but it appears Monarchy Audio has atleast a partial DIY kit available.http://www.monarchyaudio.com/DIY_Other.htm
    Last edited by FTGV; 01-15-2012 at 12:11 AM.

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    spatial computer makes a black hole device that acts as an active acoustic treatment. It functions more as a bass canceller rather than an absorber. I actually will have one in my room for demo purposes here shortly. I also listened to a finished unit at Rmaf this past October. It was amazing how the device managed to tighten the bass and remove the sounds of the rear wall.

    Thanks for posting the info about kits. I will be of looking this thread for sure.

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    Quote Originally Posted by SolidSqual View Post
    spatial computer makes a black hole device that acts as an active acoustic treatment. It functions more as a bass canceller rather than an absorber.
    http://www.spatialcomputer.com/page9/page10/page10.html Interesting I was not aware of it's existence.From the description on their site it appears to be a new take on the Shadow unit as it's claimed to be effective over the same bandwidth and operates on the exact same principal.(I suspect they had a peek at the patent for the Shadow as well).The biggest difference appears to be the Black Holes use of a single larger higher excursion driver and bigger amp driving it.

    I will be interested in hearing how your home trial works out.Posting some pic's would be welcomed also.
    Last edited by FTGV; 01-16-2012 at 06:19 PM.

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    Clayton openly told me that he continued the work of Nelson Pass. Evidently the design of Mr. Pass was not as efficient or effective as Clayton's current iteration.

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    Quote Originally Posted by SolidSqual View Post
    spatial computer makes a black hole device that acts as an active acoustic treatment. It functions more as a bass canceller rather than an absorber. I actually will have one in my room for demo purposes here shortly. I also listened to a finished unit at Rmaf this past October. It was amazing how the device managed to tighten the bass and remove the sounds of the rear wall.

    Thanks for posting the info about kits. I will be of looking this thread for sure.
    I had GIK panels on my short list for a year before I could get them here at a price I could afford. The results were so dramatic that I'm convinced active cancellation is the next frontier, and my short list now has another line item.

    The information I've seen on the dual 8" driver Pass Phantom Acoustics version shows mode cancellation of up to 7db, the Black Hole claims up to 20db! I am really anxious to hear how your testing comes out.

    Question: Do you just use 1 Black Hole ??

    I did have a thought from a DIY perspective about duplicating the 8" dual driver Pass design. I wonder what upping the amp power and using 12" woofers in a 12" tube would do?
    Last edited by inspiredsports; 01-17-2012 at 10:20 AM.
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    Wow, naturally Spatial is going to show a "best-case scenario" graph, but their image below (red line uncompensated, black line with Black Hole turned on) sure paints a pretty picture!
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    Quote Originally Posted by inspiredsports View Post
    Question: Do you just use 1 Black Hole ??
    They recommend one unit placed at the rear of the room.

    . I wonder what upping the amp power and using 12" woofers in a 12" tube would do?
    The larger woofers would give it greater effectiveness at lower frequencies which would also require more power.

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    Heres the unfinished unit with the simple mic preamp/feedback circuits assembled on the perfboards and the little chip amp module in a Hammond aluminum box.I have most of the pieces of MDF cut for the columns ,just waiting for assembly.
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    Last edited by FTGV; 01-17-2012 at 07:00 PM.

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    Thanks fo reviving this very interesting thread (I never noticed it before). As usual, GV you have some unusual but so interesting threads! l Now, I wonder why going through the process of adding a mic instead of simply get the LFE to this small amp and inversion process? Is there any specific reasoning behind the use of an external mic? Wouldn't be more effective to invert/amplify the exact signal (LFE) that is going to the sub it self as IMHO, it should be the perfect signal required for the bass absorbing wanted effect at any given time (compared to the potential lack of proper perfect inversion the small mic could provide at times)? Just curious and eager to learn...


    I'll sure keep an eye open on this fine thread!
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    Quote Originally Posted by TECHNOKID View Post
    Thanks fo reviving this very interesting thread (I never noticed it before). As usual, GV you have some unusual but so interesting threads! l Now, I wonder why going through the process of adding a mic instead of simply get the LFE to this small amp and inversion process? Is there any specific reasoning behind the use of an external mic? Wouldn't be more effective to invert/amplify the exact signal (LFE) that is going to the sub it self as IMHO, it should be the perfect signal required for the bass absorbing wanted effect at any given time (compared to the potential lack of proper perfect inversion the small mic could provide at times)? Just curious and eager to learn...


    I'll sure keep an eye open on this fine thread!
    I think the key is to get the "live", "room pressurized" waveform with the mic specifically BECAUSE IT ISN'T THE LFE, it's the signal as distorted by the modes of the room in which the sound is being reproduced.

    I think it's the room's contribution that is being controlled. Isn't it the DIFFERENCE between the LFE and room distorted bass that is being cancelled?

    I think if you cancelled the exact LFE, wouldn't you have zero db of bass ??
    Last edited by inspiredsports; 01-17-2012 at 08:46 PM.
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    Thanks for the reply, makes sense...
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    Quote Originally Posted by TECHNOKID View Post
    Is there any specific reasoning behind the use of an external mic?
    Yes,the mic has to be located were it can detect an increase in pressure created by room modes which is usually greatest in room corners.

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    Thanks for the answer GV, will keep an interested eye open to your progresses and comments on this project!
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    Quote Originally Posted by inspiredsports View Post
    I think it's the room's contribution that is being controlled. Isn't it the DIFFERENCE between the LFE and room distorted bass that is being cancelled?
    Eaxtly it is the excess amplitude created by room modes that you are trying reduce by introducing an inverse copy of the signal detected by the mic.

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    Quote Originally Posted by TECHNOKID View Post
    Thanks for the answer GV, will keep an interested eye open to your progresses and comments on this project!
    Your welcome,and I hope to have it up a running soon.

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    Quote Originally Posted by SolidSqual View Post
    . I actually will have one in my room for demo purposes here shortly.
    Just curious if you are in posession of one yet?

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    what an interesting concept

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    I was anxious get one of these up and running so decided to re-purpose a small unused 10"subwoofer to test the electronics and see if it would actually work as intended.In Pass's patent paper he indicates the mic module needs to be in close proximity to the woofer cone to insure stability in the feedback loop.To accomplish this I assembled the mic element ,wire leads and RCA connector into a piece of .25"plastic water pipe.This was then epoxy'ed to a metal L bracket which was then mounted on the bottom of the enclosure.This allowed proper mic placement and required no modification to the enclosure and is easily removed without any cosmetic damage.

    The electronics consist of a 400watt Class D amplifier and the mic preamp/feedback circuit in a separate enclosure.It's a real rats nest but this is just a test the theory set up to see if it's worth the effort to build a pair of dedicated units. Upon first power up the the mic and feedback circuits worked perfectly and without any extraneous noises.

    As for it's effectiveness on room modes initial impressions are positive (atleast on material containing deep bass) but I need to do more experimentation with placement and possibly some tweaking of the feedback circuit before I get a real handle on it's worth.
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    Hey Fred, nice work! I like the mic bracket. Did you throw your acoustic treatments away? I imagine the frequency range correction is dependant on the un-sub's specs?
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    Quote Originally Posted by SCompRacer View Post
    I like the mic bracket.
    It was all I could come up with while wandering through Home Depot looking for ideas
    Did you throw your acoustic treatments away?
    Not yet, I think they would compliment each other since so far it seems to be most effective in a narrow range that is likely below the effective range of all but the largest corner mounted panel bass traps,with the panels absorbtion kicking in above that.
    I imagine the frequency range correction is dependant on the un-sub's specs?
    It's mostly dependant upon the filtering in the feedback circuit but a larger woofer will be more efficient a lower frequencies.
    Last edited by FTGV; 03-10-2012 at 09:05 AM.

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