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11-20-2016

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skushino
Seattle, WA
Posts 93
Joined on 07-07-2004

Post #:	1
Post ID:	22857
Reply to:	22857

Infinite Baffle Implementation and "Compression Sub-bass"

I'm of the opinion that listening room aesthetics play a big role in enjoying music at home. This is self-evident, but over the many years playing with audio, my listening room has become incrementally cluttered with "audio detritus". It's ugly and cluttered. I'm working to get stuff out of the room. I believe this is also an opportunity to improve some aspects of my playback.

Let's start with sub-bass, because those cabinets are the largest objects in the room. I'm considering replacing the tapped horns with an IB design. Why IB? The design is elegant: similar to a sealed cabinet sound, without the box, without the heroic effort needed for massive volume and bracing, without room clutter, with much higher efficiency, and potentially better bass sound. Bottom line, an IB may sound good (won't know until I build), while being simple to implement in comparison to alternatives. The sole downside I anticipate is no positioning flexibility after I cut holes in the floor.

So far so good. My idea is to build two manifolds with 2 x 18" low Qts drivers each, mounted opposed to each other to offset movement caused by vibrations. I know where I plan to cut holes based on years of listening in this room. One left and one right manifold positioned in the floor at the boundary with the side walls.

The IB "guidelines" online suggest a manifold opening greater than or equal to the total Sd. In the case of 18" drivers, Sd = 192 in^2, implying an opening ~ 400 in^2. What would the effect be if the manifold opening was smaller, say only ~200 in^2? In the CD world, this would be analogous to a CD with 2:1 compression ratio. I'm wondering if mildly "undersizing" the manifold opening results in benefits normally associated with horns and compression drivers. After all, we already do this on our midbass horns, using a conventional direct radiator driver in a compression environment. At sub bass frequencies, I wonder if using the 18" drivers in a compression environment, but without a horn / waveguide, makes good sense. Bass is omnidirectional.

Anyhow, many IBs are built with undersized manifold openings and supposedly sound fine. I'm considering building this with purpose, to capture dynamics usually associated with compression drivers.

Anyone reading with first-hand experience to support or shoot down?

11-22-2016

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Paul S
San Diego, California, USA
Posts 2,650
Joined on 10-12-2006

Post #:	2
Post ID:	22863
Reply to:	22857

Basic IB

Scott, I tend to think of IB in simple terms, basically, literally, as an "infinite baffle", the idea being that there is no load other than "open air" on either side of the driver diaphragm, and there is no "baffle step"/acoustic short between the two sides of the diaphragm, because the two sides "never see each other". IMO, the idea of using IB is to clearly hear the driver's "voice", for whatever that might be worth, and the "right" driver will be able to articulate with consistency through its range. Given my simplistic notion of IB (at lower frequencies), I don't understand/accept the specific ideas of differential loading you are talking about. I have experimented with "wave guides" that are really just curved baffles, with some "benefits" down to lower MF. True LF gain from a driver requires a horn, or "adding" from a vent from a "controlled environment. Whatever, I do not think of IB as a way to "push" LF from a given driver. FWIW, I don't think one gains much from thick-coned, long-throw LF machines in an IB enclosure, so why dedicate the space for it? OTOH, since it all has to be tested, anyway, and I have had my share of pleasant surprises in hi-fi, I will add that there's no way to be sure until you try it.

Best regards,
Paul S

12-04-2016

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rowuk

Germany
Posts 454
Joined on 07-05-2012

Post #:	3
Post ID:	22870
Reply to:	22857

Is it really IB?

I read an interesting post by Martin King, the MathCAD wizard that created some sheets to predict driver behaviour in different types of boxes. He had 3 groups of speaker enclosures: boxes so small that standing wave resonances (sealed and vented) are not significant, then large boxes where there are significant resonances in the box and the third was horns where there is very much controlled resonance in the pass band.
I would suspect that IB is not the panacea that it seems to be.

Whenever I feel blue, I start breathing again.

12-04-2016

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Paul S
San Diego, California, USA
Posts 2,650
Joined on 10-12-2006

Post #:	4
Post ID:	22872
Reply to:	22870

Particular Driver + Objectives

Well, there are standing waves, and there's SPL, on and on, and there are plenty of good arguments against any implementation. Not many drivers today that would be a fit for IB, which might include ways to damp s.w. resonance, such as interior "curtains" and/or "random ventilation". But first one needs the "right driver" and the "right speaker system" to put it in.

Best regards,
Paul S

12-12-2016

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Scott L
Posts 17
Joined on 02-26-2008

Post #:	5
Post ID:	22879
Reply to:	22863

Infinite Baffle Bass Installation.

Gentlemen:

I assure you, Infinite Baffle bass simply has no peer. Yes, you do need specialized drivers. The Infinite baffle bass concept is so old, it's rarely even considered anymore, plus, you are indeed married to the installation.

12-13-2016

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Paul S
San Diego, California, USA
Posts 2,650
Joined on 10-12-2006

Post #:	6
Post ID:	22880
Reply to:	22879

Loading the Drivers

Scott, I'm guessing the installation shown in the photo makes lots of LF, but how does it provide balanced, "free air" loads on both sides of the drivers?

Best regards,
Paul S

12-13-2016

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skushino
Seattle, WA
Posts 93
Joined on 07-07-2004

Post #:	7
Post ID:	22881
Reply to:	22880

Update

Guys, my attention has been elsewhere the past couple of weeks.

The catalyst for the original post was to go beyond the usual "speakers in a room" approach, and use the room itself as an integral part of the sound. IB seemed like an attractive way to make this happen. The IB "rule of thumb", manifold exhaust area >= Total Sd, made me wonder about any potential benefits resulting from purposely undersizing the opening to load the drivers with mild compression.

My original idea was a baffle style IB, similar to the picture from ScottL. But many users report that wall manifolds require heroic bracing to mitigate flex. The manifold implementation side-steps around this problem by balancing the physical driver movements. The sole downside I see with a manifold is managing the exhaust area. This led to my idea that a purposely undersized exhaust could potentially result in more pleasing bass. My manifold placement uses the floor / wall boundary to support the bass output as well. Sound? Well this is one of those crazy audio projects that will be impossible to predict the outcome until after building.

My woodworker is visiting on Thu to discuss the build. I expect the manifold dimensions will be defined by building and implementation considerations, like spacing between the floor joists, and bracing. Will update further.

12-13-2016

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Scott L
Posts 17
Joined on 02-26-2008

Post #:	8
Post ID:	22882
Reply to:	22881

The bigger view

skushino wrote:
Guys, my attention has been elsewhere the past couple of weeks.

The catalyst for the original post was to go beyond the usual "speakers in a room" approach, and use the room itself as an integral part of the sound. IB seemed like an attractive way to make this happen. The IB "rule of thumb", manifold exhaust area >= Total Sd, made me wonder about any potential benefits resulting from purposely undersizing the opening to load the drivers with mild compression.

My original idea was a baffle style IB, similar to the picture from ScottL. But many users report that wall manifolds require heroic bracing to mitigate flex. The manifold implementation side-steps around this problem by balancing the physical driver movements. The sole downside I see with a manifold is managing the exhaust area. This led to my idea that a purposely undersized exhaust could potentially result in more pleasing bass. My manifold placement uses the floor / wall boundary to support the bass output as well. Sound? Well this is one of those crazy audio projects that will be impossible to predict the outcome until after building.

My woodworker is visiting on Thu to discuss the build. I expect the manifold dimensions will be defined by building and implementation considerations, like spacing between the floor joists, and bracing. Will update further.

Paul S wrote:
Scott, I'm guessing the installation shown in the photo makes lots of LF, but how does it provide balanced, "free air" loads on both sides of the drivers?

Best regards,
Paul S

It's difficult to see in the picture, but this IB build was done in a corner, a "strut out", behind which was an existing closet of about 500 cubic feet.
The 4 pairs of 15 inch drivers are force coupled at the driver frames AND magnet-to-magnet. The pvc magnet couplers have holes drilled into them, to the same area as the vented pole pieces. In this case, YES, the manifold exit is slightly greater than 8x SD, but I have also done push-pull, slot loaded bass where the slot exit (manifold) is 2:1 (iow 1/2). For some reason I can NOT see the very pictures that I attach, but obviously, you guys do. So, here please see the entire system. The IB handles bass below 60Hz only. Of special note: there is NOTHING between the loudspeakers. I don't need to parade my gear for the whole world to see as if I were some gaudy want-to-be, living in Las Vegas. Besides, anything stacked between the loudspeakers ruins
the image/sound stage.

12-13-2016

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Paul S
San Diego, California, USA
Posts 2,650
Joined on 10-12-2006

Post #:	9
Post ID:	22883
Reply to:	22882

"IB" ?

Interesting discussion, and nice installation, but I still don't understand how this elaborate set-up unloads both sides of the driver cones, apart from free air?

Scott L, when you click on the "link" you've created in the dialog box, check at the bottom (or elsewhere) on your screen for a small box that allows you to click to open the photo(s).

Best regards,
Paul S

12-13-2016

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Scott L
Posts 17
Joined on 02-26-2008

Post #:	10
Post ID:	22884
Reply to:	22883

Infinitly Baffled

Hi Paul,

I'm really not sure how else to explain it, but essentially it is "free air". The driver does not become influenced by a defined amount of "trapped air" in an enclosure, thus, it pretty much maintains it's free air specs. In my case, the drivers Qts is .707 and the Free air resonance is 17Hz. The room's acoustics becomes the final arbiter of all things, as in, my room will only support bass down to about 24Hz. 19x22x8 feet. I sit 13 feet away from the front edge of the large round, red horns. The QUALITY of the bass is uncanny. Limitless, so to speak. There is no box sound. There only exists the pressures required to develop the "note". The push-pull, slot load mid bass, because of the pressure load (2:1, and in keeping with the thread topic) exhibits a forward thrusted
pressure envelope, that is akin to a mid bass horn, just more compact in size. It matches quite well in it's wavelaunch characteristics, to that of the horn right above it. Because both the low bass, and mid-bass, employ reaction-forced-vibration cancelling, the otherwise wasted energy that happens in a conventional enclosure DOES NOT happen here. The 60Hz low pass to the IB sub coincides with the wavelength to room size; you simply can not hear where the low bass is coming from--it loads the whole room. I have had visitors walk right up the the IB manifold and place their hand on the side-to-side support strut and ask why it is they DO NOT feel any vibration? They obviously missed the chapter in science class that covered Sir Isaac Newton.

Thanks for the inquiry !

12-13-2016

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Paul S
San Diego, California, USA
Posts 2,650
Joined on 10-12-2006

Post #:	11
Post ID:	22885
Reply to:	22884

180

Thanks, Scott L. I get the math, and no doubt hearing is believing. Still, I do know for certain that "a driver" can be influenced by a defined amount of trapped air in an enclosure, at least as far as the sound that comes from the driver, and sound can be tailored just by changing the volume or density of air on one side of a driver; so I will have to meditate further to understand how compression and rarefaction do not factor here, notwithstanding equal/opposite "cancellation". Again, I have never played with trying to get infrasonics from IB, and opposed-driver subs I have heard got way down there in much smaller enclosures.

Best regards,
Paul S

12-14-2016

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Scott L
Posts 17
Joined on 02-26-2008

Post #:	12
Post ID:	22886
Reply to:	22885

GO BIG !!

Paul S wrote:
Thanks, Scott L. I get the math, and no doubt hearing is believing. Still, I do know for certain that "a driver" can be influenced by a defined amount of trapped air in an enclosure, at least as far as the sound that comes from the driver, and sound can be tailored just by changing the volume or density of air on one side of a driver; so I will have to meditate further to understand how compression and rarefaction do not factor here, notwithstanding equal/opposite "cancellation". Again, I have never played with trying to get infrasonics from IB, and opposed-driver subs I have heard got way down there in much smaller enclosures.

Best regards,
Paul S

Paul,

     You have touched upon some very important aspects here. I'm glad you helped me remember !     In slot loaded systems, where the bass drivers are mounted push-pull, and in close proximity to each other, the parameters DO INDEED change, by somewhere in the 10-12 % range. As an example, in my mid-bass ppsl manifold, the drivers free air resonance changed from 46Hz, down to 40Hz; and the Qts raised from .35 up to .39

With respect to smaller bass enclosures --- sure, they may measure as if they go deeper, and probably do trigger that response on a meter, but, what do they sound like ?     I know that there is a whale of a difference because I have heard both. The much lower efficiency/high mass drivers need considerably more power to "push" them, and correspondingly sound so sluggish as compare to the effortlessness of a low mass/higher efficiency driver in a much larger enclosure. Guess you'll just have to trust me on this, and hopefully one day you will be able to hear this for yourself, in a fine music playback system !

                               best regards,
                                                    Scott L.

12-14-2016

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Paul S
San Diego, California, USA
Posts 2,650
Joined on 10-12-2006

Post #:	13
Post ID:	22887
Reply to:	22886

Magic

Scott L., I am not stuck on IB, but I am pretty well stuck on lighter-coned drivers that I develop for tonality, apropos. I actually have several "light", 30" cones and giant motors sitting around that I aim to try in large enclosures, "someday". But with IB I am really talking LF rather than ULF, and it would indeed be a leap of faith for me to allot so much space for ULF, which - at this point - I tend to think of as +/- a parlor trick, at least in a typical residence. The electrical curves alone make my head spin, and the "lengths of the original waves" seem to preclude "verisimilitude". Sure, I'd love to hear significant improvements at ULF, and I'd cover some distance for that opportunity.

FYI, there is already a ULF thread (or 2) started at GSC. Now I wonder if this is properly posted, since it's not about horns.

Best regards,
Paul S

12-15-2016

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skushino
Seattle, WA
Posts 93
Joined on 07-07-2004

Post #:	14
Post ID:	22888
Reply to:	22887

More

ScottL,

It looks like the two pics you posted are of different systems, the first being slot loaded PP, and the second (full-system) showing a "traditional" IB. You also seem to be confirming my intuitive hunch of potential sound benefits and horn matching using mild compression loading, or exhaust area < Sd. I want to explore further.
Can you share further about your design inputs / criteria?

For instance, the drivers I'll use are 4 x Aura 1808, Qts = 0.19, Fs = 24 Hz, 2 each in separate L + R manifolds. These numbers differ from traditional IB drivers, but I'm not discouraged from auditioning them in an IB configuration.

Also, what guidance, if any, wrt: exhaust area? Your comment about observing rising Q and lower Fs using compression are encouraging.

12-15-2016

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Paul S
San Diego, California, USA
Posts 2,650
Joined on 10-12-2006

Post #:	15
Post ID:	22889
Reply to:	22888

Fs

Scott, is it your specific aim to configure an enclosure to "effectively" lower driver Fs? While this, along with an exponential increase in power, is a proven strategy for generating ULF, it seems like this approach creates more of a sealed/"IB" hybrid that forfeits "true IB" "ease" for more (U)LF. This is not to say it certainly matters at ULF; just wondering.

Best regards,
Paul S

12-15-2016

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Scott L
Posts 17
Joined on 02-26-2008

Post #:	16
Post ID:	22890
Reply to:	22888

It's just one system

skushino wrote:
ScottL,

It looks like the two pics you posted are of different systems, the first being slot loaded PP, and the second (full-system) showing a "traditional" IB. You also seem to be confirming my intuitive hunch of potential sound benefits and horn matching using mild compression loading, or exhaust area < Sd. I want to explore further.
Can you share further about your design inputs / criteria?

For instance, the drivers I'll use are 4 x Aura 1808, Qts = 0.19, Fs = 24 Hz, 2 each in separate L + R manifolds. These numbers differ from traditional IB drivers, but I'm not discouraged from auditioning them in an IB configuration.

Also, what guidance, if any, wrt: exhaust area? Your comment about observing rising Q and lower Fs using compression are encouraging.

It's just one system. I'll attempt to attach a picture here, showing the left hand side. The I.B. bass sits just behind the main speaker system.
The I.B. is just for low bass (ULF) and it is a push-push, reaction-forced vibration cancelling where the plenum housing has an exit area GREATER than 8xSD. The IB bass runs everything UNDER 60Hz.

The mid-bass is push-pull, slot load, and it sits under the big red horn. The slot exit is 1xSD, Which is 1/2 the combined SD considering it is 2 drivers per channel. I'll post another picture in the next follow-up. Mid-bass covers 60-250Hz.

For your Aura drivers, the Qts is much too low for an infinite baffle installation. I am assuming the published specs are also the actual specs. If you mount them similar to they way I have my ppsl, you should acquire a net result increase in Qts (exactly how much, we can not be sure of?) If you are serious about your installation, you can always build the manifold first, mount the drivers, and measure the specs. The next step would be to decide how large of an air space you will need, and if you wish to e.q the response, or use a bass reflex. I do realize that in some circles bass reflex would never be considered.

Do you have the aura drivers sold through Parts express, or those that were sold through North Creek Audio? Do you have the other t/s values as well ?
I am especially looking for Qms and Vas.

12-15-2016

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Scott L
Posts 17
Joined on 02-26-2008

Post #:	17
Post ID:	22891
Reply to:	22889

More info

Paul S wrote:
Scott, is it your specific aim to configure an enclosure to "effectively" lower driver Fs? While this, along with an exponential increase in power, is a proven strategy for generating ULF, it seems like this approach creates more of a sealed/"IB" hybrid that forfeits "true IB" "ease" for more (U)LF. This is not to say it certainly matters at ULF; just wondering.

Best regards,
Paul S

To be quite honest, as far as the Fs of the drivers used in the Infinite baffle, I felt they were already low enough, at 17 Hz.

Where I discovered the change in drivers specs was in the PUSH-PULL, slot load configuration, which is what I did for my mid-bass.
I used a pair (per side) of the Eminence Definimax 4012HO, and the fs dropped from 46 hz, down to 40hz, after measuring the pair, mounted in the plenum.

Power handling and use is not really much of a concern in either the IB bass, or the mid-bass. Both systems are extremely efficient. Having said that,
in both cases there is a tremendous amount of power reserves, as the amplifiers in each case are capable of providing just a tad bit less than the maximum power handling. But as an example of actual power usage, on really LOUD music, the mid bass averages 4 watts per channel, and the IB bass sometimes uses 1. As in 1 watt. I've never really taxed the system, but it will play back the Sheffield Track & Drum solo at live performance levels,
with an extreme amount of ease. In that case, the power levels do go up considerably, like 4 watts for the IB SUB and 16 watts mid-bass.

In the attached picture, please look close and you can see how the one driver in nestled just inside the other driver. This is a close up of the push-pull, slot load mid-bass.

And, just one last note for clarification; Infinite Baffle bass builds are to be done totally sealed.
The front and back waves of the driver(s) shall NEVER meet.

12-15-2016

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Paul S
San Diego, California, USA
Posts 2,650
Joined on 10-12-2006

Post #:	18
Post ID:	22892
Reply to:	22891

Sealed Vs...

Yes, for practical reasons, one side of an IB driver is usually "wrapped and sealed" to deliver lower frequencies from the other side, since it is generally impractical to actually make a baffle large enough to prevent sonic shorts. However, at higher frequencies, both sides might be open, depending on the driver, the size of the baffle, and what one desires to get from it. Whether or not this is actually, technically IB is moot, as far as I'm concerned. I don't know much about the drivers you cite, but most LF drivers feature a response that tails off as the frequency drops, and - of course - this is where multiple drivers come in, to augment flagging response, using a network and appropriate power to yield the desired plot, especially vs. horns. Like you (I suppose), I prefer the sound of LF from larger diaphragms and shorter throws vs. the opposite. Again, I still think of hi-fi ULF as a mysterious thing, build and try, see/hear if it works. At this time I have no practical reference, apart from live music. Since I won't be getting that at home, it comes down to what I can actually do in my house. I don't know your musical tastes or playback expectations, but it looks like you have a nice solution.

Best regards,
Paul S

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