"Infinite baffle" question

My brother wants to put a subwoofer(s) in his basement listening room. He has a storage room next to this room that is about 600 cu.ft. so he's thinking about using this storager room as his subwoofer "cabinet".
Can he effectively use this storage room to get optimum performance from a quality sub driver? What advantages are there in such a large "cabinet", and what driver is best suited to this "infinite baffle" approach? Is there an optimal size where anything larger hurts the woofer's response?

Thanks,

Tom

From my experience with IB's the Adire Audio Shirva and Tempest work great. I havent done one with anything else. I dont know how well a JBL would do it. I cant tell you much other than I've used those drivers and it kicks ass.... i dont really know much about them

Go to any box design program and put in your driver of choice.

Choose "Closed Box" as the design.

Change the volume and watch what happens. :D

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Originally Posted by Zilch

Go to any box design program and put in your driver of choice.

Choose "Closed Box" as the design.

Change the volume and watch what happens. :D

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I don't have a box design program. Maybe you could tell us "what happens"?
Thanks,

Tom

OK. As much in a nutshell as it's possible to simplify:

Any woofer in a sealed enclosure is "looking for" a certain damping (Q) range to work properly. There is a range which you can get a combination of good resonance damping (freedom from boominess, peaky frequency response, etc) and maximum woofer bass extension (plays down to lower frequencies before "rolling off"). In engineering parlance, this range would be described as having a Qtc (woofer in enclosure damping) of between .500 and .707... which describe a critically-damped (maximally clean transient response, ie best balance between lack of ringing and not having a slowing of the initial transient "rise" at the start of notes) and what's known as a Butterworth alignment (which is the highest Q before the damping gets so low that the response starts to peak up higher than average level near woofer resonance- an alignment where the woofer has MAXIMIZED low-frequency extension before the 3dB down point on the bottom (F3 point).

In a standard sealed box, the woofer generally has a medium or low Q of its own, and the enclosure "loads" the driver strongly, raising the total Q of the system to somewhere in this range. However, in an infinite baffle, the box is SO large, as to NOT have much effect on the driver damping... so the driver needs to START OUT with a HIGHER Q... so that in the end, you wind up with something in the optimal range described above.

So, in short, drivers that work well in IB setups, will generally have a Qtc of HIGHER than .4, so that they don't require much loading to raist that to above .500. In fact, some of them will have Qtc ratings of as high as .6... though, those tend to require HUGE volumes, and can be a little "uncontrolled" sounding in most "normal" applications.

Some drivers specifically made for infinite baffle, include the Dayton IB series drivers from Parts Express... available in several sizes. If you look at the specs that PE indicates for those, you'll see specs in a typical range of what you would want for an IB driver, from any manufacturer...

Now, there IS one trick, that can be used to RAISE the Q of a typical driver, so that it's usable in IB- take a dual-voice-coil woofer and ONLY use ONE voice coil (leave the other one OPEN, ie DO NOT connect it). This will cut the magnetic strength constant (Bl product) of the driver in half, which will roughly double (usually between 75% and 110 higher than if the driver was used "normally", ie, with both coils connected). This will oftentimes result in drivers with REALLY LOW f3 points (ie, near or sometimes below 20 Hz) in room, at the expense of a loss of efficiency for any one driver. However, in an IB... if the space is huge, it's possible just to use MULTIPLE drivers in an array, and get back the lost efficiency... as well as increasing output capacity and reducing distortion (each driver has to move less for the same level of sound)...

Regards,
Gordon.

18 2245's:jawdrop:

Hello,

Here is a good place to go for IB:

http://f20.parsimony.net/forum36475/

In general IB is all about Xmax so you will want multiple drivers.

IB is when the enclosed volume is more than about 4 times Vas.

The Q of the system will be the driver Q and the rolloff is like a sealed box so there may be a synergy with room gain at the low end (so you don't need much eq).

Tom

Hey Tom that's some sweet stuff. Nice link. Now I'm wondering... :hmm:

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Originally Posted by JuniorJBL

18 2245's:jawdrop:

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KILLER suggestion!!. I read an article some time back, I forget which magazine, where this guy did this very thing except he used more cubic feet, if you can imagine that,he cut a slot in his floor that vented the bass up into his HT room above. I think he used (12-14) 15" Dayton Titanic subs. Picture a rectangle, cut out of the floor, for the vent, between 2 floor joists, I believe the vent was about 1 foot x 3foot. He used 1" thick MDF to fashion the baffles and oriented them so that the baffles were tied to the joists that "framed" the vent. He then took MDF and enclosed those baffle/panels ends to form a box that reached from floor to ceiling. The back of the woofers were open to the basement, which had fiberglass batting and sheetrock. This guy got some very big 16-20 hz SPL's (duh!). So I've seen it done and I'll search for the source of the article to see if I can get you the info. Great idea there, you gotta love somebody that is ready and willing to rock that hard!!. BTW, I remember this guy powered his subs with Crown Marco Reference amplifiers too, really wild. Good luck with this cool project, Steve G.

edit: Here is a start, try this link for some idea's http://home.new.rr.com/trumpetb/audio/rab009.html and I also want to suggest the wonderful AURA 18" subs with the Neo/ Radial motor structure if you don't use JBL's. Check this MONSTER SUB out too! http://www.royaldevice.com/custom.htm The Royal Device ;)