Obi's IB (1 Viewer)

[Greg Monfort]

FWIW, technically, it need not even be a large sealed cab. The definition of IB is that the out-of-phase rear waves can't in any way combine with the in-phase waves radiating from the front, so a flat baffle suspended up in the air outdoors that's acoustically large WRT the driver's lowest frequencies is the theoretically perfect IB.

For a 5Hz sub though, this means a heavy >72ft dia or square baffle, so the only viable alternative for HIFI use (unless you live somewhere you can enjoy yours year round outdoors) is an enclosed area with enough volume to keep from loading the driver with any appreciable acoustic resistance.

GM

 

[Robert George]

Jeff:

I understand completely your hesitance to accept the idea that this design actually works. I admit the entire time I was building this thing, I was thinking "this is never gonna work". Imagine my delight that it not only worked, but far beyond my highest hopes.

 

[Kerry Hackney]

I was wondering, if a person has a drop ceiling in their HT, could the plenum above the tiles be used for an IB install or is there not enought isolation from the back wave. Could be pretty trick to cut a 2x2 panel to lay in the grid with a Tempest, or 3 or 4 of them for that matter.

 

[Brian Bunge]

Greg,

A bit off topic, but I was wondering, what is your formula for determining the necessary length between the driver and the floor for the legs of a downfiring subwoofer?

Thanks,

Brian

 

[ThomasW]

Oh gees Greg, let's get REAL technical
Kerry
The rear wave will blow through a drop tile ceilng. There's not enough solid structure to isolate it. Also it will vibrate the tiles and dust will rain down everytime you turn it up

 

[Greg Monfort]

TW,

Yeah, well, I did it to clarify because I sometimes use IBs (often called open baffles) to mount ~fullrange drivers and/or tweeters rather than the super large sealed cabs they would otherwise require.

BB,

I use the reasoning that the max 'slot' area between the perimeter of the baffle/floor be < Vd = (2*Xsus*Sd) of the driver (I normally use Xmax-1.5*Xmax since Xsus isn't usually readily available) to ensure free flow up to this point, with some acoustic loading above it to help protect it if pushed harder.

Never having done any elaborate testing about all I can say about its validity is that I haven't measured any obvious impedance increases until the driver's pushed, nor heard any 'vent' noise.

Anyway, as an example, the Tempest Xmax = ~0.646" and Sd = ~120.74in^2, or Vd = ~156in^3. If it's used in a 24"o.d. tube, then the perimeter is 24*pi = ~75.4", so 156/75.4 = ~2.07" min. between the baffle/floor, though if carpeted and only using legs I would make this to the top of the pile. I'm not a fan of just legs though, much preferring a relatively massive base to attach the tube to.

GM

 

[TerryC]

Glad you like it!!!!

How would you say it compares to the SVS setup and did you ever try the SVS's in that location before the install?

 

[Robert George]

Terry:

I did experiment with placement with my SVS subs a couple of times. At one point, I tried one sub by each of the mains and that was a very nice sounding placement, but I couldn't keep the subs in this configuration due to placement problems on the left side. The stacked configuration on the right side was the best I could come up with and that happens to be only a couple of feet from where the IB is now.

Comparing the performance of the IB to the dual SVS is not easy. The SVS did seem to go a little lower and were a bit more efficient in that I had to increase the sub level to get the same SPL out of the IB. However, the IB seems to have more "range" for lack of a better term. In other words, the IB plays more accurately from low levels to very loud levels than the conventional subs. I am getting more balanced performance for music while still getting serious impact for movies.

It's really difficult to describe the difference except that it is a noticable difference. Very different sound between the IB and a conventional sub.

 

[Jay W]

Very nice project Obi, additionally the asthetics of the room just keep getting better and better .

 

[ColinM]

Can you get a convincing 'shock wave' sensation with an explosion? How about a nice, tight kick drum?

Last question, I promise!

 

[ThomasW]

Colin

As with any sub 'there's no replacement for displacement'. So if the goal is to rock the house, IBs will do that very well. Generally speaking it's a good idea to have at least 2-15"s or 4-12" to get that job done.

The Qtc of the IB is the Qts of the drivers. As a result there's almost no 'boom' with an IB, there is a huge deep 'roar' instead. So yes VERY tight and detailed kick drum. Obi's system will be an interesting mix of ported and IB sonic signatures.

 

[Parker Clack]

Obi:

Have you thought about doing the same thing to the left side (when facing the monitor) of your system so you would have 4 subs total? Do you think that this might help "balance" out the sub bass and you might need less EQ?

Parker

 

[Robert George]

Parker:

I did look briefly at the left side, however, there are things on the other side of the wall that would not be easily moved (like my CD rack). Also, that is the side of the closet area that houses all the electronics so I would not really want that kind of acoustic pressure rattling my DVD and CD players. I know ThomasW recommends 2 15's or 4 12's, but as far as my preference goes, these 2 12's do an excellent job. Any more bass would be too much for me.

As it is now, the bass does not seem "unbalanced". With a crossover point of 80Hz, the subs are not producing anything that is directional.

Colin:

Thomas is correct. There is no "boom" at all to this design. Just deep, tight bass. Plenty of impact when needed yet still very defined with music.

 

[David Giles]

Ahhh!! This is killing me! I've really been trying to convince myself I don't need to build an IB. But Obi, you're just making it more tempting with every post.

I don't think I'd be happy knowing that I had given up anything on the low end. So I'd probably have to go the overkill route and just go ahead and build the 4-tempest manifold. Sounds like that would "rock the house" as well as my SVS's do now, AND give me the sublime (pun intended) bass you guys are talking about.

BUT.....MUST..........RESIST

 

[Andrew Pratt]

David dual tempests should have a healthy displacement advantage to Obi's dual SVS drivers so you may not need 4 to start. What I'm going to be building very shortly is a box that will start off with two tempests with room add two more of the other sides if I feel the need later.

 

[John Sturge]

Niiiccceee... Nice job Robert. Hey, how were you able to get your hands on a Sumo amp, [email protected] the American distributor said: "SUMO is no longer available. We only offer a repair service." The website for the Sumo Products Group was dead too.
Were you just lucky to get one, or am I just looking in all the wrong places?

 

[David Giles]

What I'm going to be building very shortly is a box that will start off with two tempests with room add two more of the other sides if I feel the need later.

Yeah, that's probably what I should do. I guess I just figure it'd be so much easier to do it all up front.

A few questions:

What size is your home theater room? What kind of sub(s) do you have now? Are you going to attic-mount your IB? If so, I sure look forward to hearing your comments. I'm still worrying about the amount of bass transmitted throughout the rest of the house (not to mention OUTSIDE the house).

Thanks Andrew!

David

 

[Robert George]

John:
Unfortunately, Sumo is no longer building amps. Damn shame, too, as these were some terrific amps (as you obviously know).
The Andromeda IIb that I have was bought used from a friend of mine. The IIb was a factory upgraded model II that was sold just before the Andromeda III was released. The Andomeda III was the last of the Andromeda line that was produced, and IMO, the best they made. Internally, the IIb is a model III.
Good luck finding one. I browse Audiogon from time to time and almost never see any Sumo products listed, much less this model amp. Actually, I had mine listed for a week before I decided to replace the Samson pro amp powering my subs with the Sumo. It isn't for sale anymore .

 

[Tom-K]

How do you keep airleaks from happening. I was wondering if I could do this, but does the wall have to be airtight? what about cutouts for televisions? component racks etc? My separate room also has two doors at the other end does all this keep an i/b from working? Thanks.

 

[ThomasW]

Large cutouts need to be sealed. That's what Obi did behind his equipment racks and RPTV. Small openings like wall outlet boxes aren't an issue.

The physical distance between the front wave and the rear wave 'leak' will be the deciding factor if there are cancellation issues.