of the Infinitely Baffled"
IB Subwoofer (aka IB subs)- FAQ Pages
Generic 'how-to' guide for designing, building,
installing, and fine-tuning your IB subwoofer
page is under construction and will be continuously under construction.
Expect frequent changes, updates, and as usual lots of typo's and poor grammar ;^)
Here's a pic of Mark M, helping his brother Michael install his IB. Hopefully this picture and the one's below will help everyone get into the right mood to build their IB.......
below shockabuku's bat barn installation, and feets don't fail me now... :^)
Below a lighter moment in IB building, Ryan S. 2 thumbs up .... ;^)
Harold and family checking out their line array
Scott's line array
William Cowan admiring his handiwork
Frederik's Dual Tempest IB
Al installing Artie K's IB on a hot Florida day
Steve C's line array
jimcant's line array in Oz
One of Mazeroth (aka Christian B) dual RSS390 IBs
Chadci 4 Mach 5 MJ18 manifold
Line array* vs manifold.....?
Provided they can be properly located, sufficiently stiff/rigid line arrays offer the best performance. The main benefit to a line array is having an unrestricted opening, the size of the driver for each driver. This is an optimal situation. Other than their sheer physical size there are two primary drawbacks to line arrays. First is they don't cancel the mechanical vibrations like a manifold. Second is we've seen numerous instances where this lack of mechanical cancellation causes a significant amount of wall movement. When wall flex occurs it cancels output from the IB. So unless you can sufficiently stiffen the wall or ceiling to avoid flexing go with a manifold.
The baffle for a line array should be built from no less than a 3/4" layer of MDF, laminated to a 3/4" layer of OSB, or two layers of 3/4" OSB or plywood. If large heavy woofers are being used, I recommend oak 1"X2"s be glued on edge between each pair of drivers and the entire baffle perimeter be stiffened with 2"X4"s or similar. If possible the entire baffle assembly should be braced in several locating against the structure of the building. All this will help stiffen and add strength to the baffle, and keep wall flex to a minimum (something very important when using a line array).
Manifolds are compact, a lot of drivers can be packed into a relatively small area. The ability to mount the drivers in opposition provides a high degree of cancellation of the mechanical forces from the drivers (note this does not mean there's less acoustical vibration transferred to the room from the sound waves themselves). The downside to a manifold is that the outlets are never as large as the multiple openings in a line array. This results in a small decrease in sound quality.
The fundamental rules for manifolds are 1) make the outlet as large as possible, and 2) keep the manifold as short as possible.
(Below is info also found on the main FAQ page)
Our recommendations for the size of the outlet are derived from those used to calculate the spacing between a down-firing sub and it's base plate. Fortunately Bill 'Collo' has a nice webpage for calculating this spacing. He also has a quick reference guide I've reproduced below.
As an example we see a pair of high excursion 18"s would have a recommended opening 18" dia X 12" wide. With an IB the height of the opening is often larger than the diameter of the driver, and the width is usually that of a stud-bay or the joist spacing. So we're within the recommendations seen in the graphic.
Invariably someone asks "how small can the opening be?" It's best to avoid having an opening smaller than 50% of that from the calculations.
* Some people are confused by the use of the term 'line array' with regard to IB subs. 3-4 low frequency drivers mounted in a line do not make a true line array/line source loudspeaker. We use this term for descriptive purposes only. _______________________________________________________________________________________________________________________________________________________________________________________________
Line array options
Line arrays aren't limited to a
single strip of drivers. One can make faceted arrays and greatly increase the
number of drivers in a given space.
Here's the same idea but originally thought up for a ceiling mount
Manifold, 'innie' vs 'outie'.....?
Most people building manifolds build a hidden or 'innie' unit with an exposed grill or grate to cover the opening. As noted above these do represent a compromise in absolute sound quality, unless the opening into the room is as big as the sum of the Sd of all the drivers.
Enter into the picture the "outie' manifold. This concept was invented by a creative person who couldn't access the crawlspace below his HT but still wanted an IB. The 'outie' manifold looks to the listener like a standard box sub. It's a finished box in the listening room, and all the drivers are visible. It vents into a space blow the box. For example.....
The one additional benefit to an 'outie' design, is that like the line array, it offers an optimal sized opening for each driver. As a result little dynamic compression will take place
A few more FAQ's....
How many drivers do I need?
Why isn't there a formula for calculating how many drivers to use?
I see all the 4-15" driver IB's in the gallery. I have a single ported 15" sub and it shakes the room. Are you going to recommend 4-15"s for my IB?
These are all basically the same question. People need to understand that every IB is a custom design, and a custom installation. Also people have different performance expectations, and no two 3000 cu ft rooms are the identical. That's why there are no formulas for calculating the number of drivers needed for a given volume of space.
As explained on the main FAQ page, the output level of any box sub is limited by the thermal heating of the voice coil. IB subs are limited by the excursion limits of the suspension. As a result a single 15" in a ported or seal box can play quite a bit louder (be driven harder) than a single 15" mounted in an IB. So depending on the room, the listener, and other variables, it can take 2 to 4 times more drivers in an IB, to equal the output of a box sub.
Also I will always error on the side of recommending too many drivers (= too much displacement). The reason for this is that it basically guarantees no one will be disappointed after they've gone to all the time and trouble to create their IB. Finally having a high amount of displacement, guarantees the workload on the drivers is low, and as a result so is the distortion.
But I can't afford 4 drivers, I can only afford buy 2.
Because of the relative uncertainly regarding how many drivers are needed/wanted, I recommend designing and building the manifold so it's pre-made to accept additional drivers. This means making the cutouts for the additional drivers, then closing off those holes before installing the manifold. This is extra work up front. But it's much easier to do this in your shop prior to installing the manifold, than it is to do in the attic, basement, or crawl space, after the fact.
Mike Keith followed this advice with the manifold he built for his 18" Avalanche drivers. He started with a pair of Ave 18"s, and is now quickly able to add additional drivers should the need arise. Since the pictures are fairly large I put them on THIS webpage.
Designing, Building, Installing and Wiring a manifold
Location, location, location....part 1
The only place I have for an IB is in the back of the room is that okay?
Not recommended and here's why. A note of music consists of a 'fundamental' and multiple 'overtones' or 'harmonics'. The harmonics are much higher in frequency compared to the fundamental. As a result if the IB is behind the listener, that's where the fundamentals are being reproduced. The harmonics are then played by the mains. This results in a smearing or muddying of the stereo image.
To illustrate this point, below is the frequency response from the "E" string (42Hz) from a electric bass guitar. One can clearly see how high in frequency the harmonics go compared to the 42Hz fundamental.
As an aside, this plot also indicates why subwoofers with the ability to play very low frequencies are important........
Location, location, location....part 2
The first thing to do is to go up into the attic, or climb into the crawl space, or go where ever you intend to install the IB. Look at the structural elements. Make sure you have an unobstructed area to install your manifold. If there are structural elements in the way, one must make a decision as to whether or not they can be moved. If they can't be relocated, then another position should be chosen for the installation. If there are any questions regarding the relocation of structural elements, a structural engineer should be consulted. DON'T start chopping away at a load bearing wall, an engineered truss system, floor or ceiling joists, or any other structural element unless you fully understand the implications of that action!
Unless one is planning on relocating structural elements of the ceiling, the manifold should be sized to fit the existing joist spacing. Most joists are 16" OC this gives 14.5" of open space. Some newer homes will have a engineered truss system with 24" OC spacing. This allows for a much bigger opening, depending on how the trusses are constructed.
Below is an example of something that shouldn't not occur. Never notch structural elements. A manifold is not a structural element and shouldn't be treated as such. Cutting these notches means the two rafters are now incapable of supporting that section of the roof.
BE SURE to take measurements in the exact position where you intend to place your box. There is a surprising amount of variability in the construction of a house. As a result, one can't trust that the spacing of joists in one area will be the same in another, nor that the joists themselves will even be parallel.
If there's any doubt about the manifold or drivers fitting in the space, use of a template is recommended. Below is an example of a template for a box containing 2 -18" drivers on each side.
In the picture below note that there's an electrical wire (white line) running through the space where the manifold will be located. It will be necessary to relocate this wire. Sometimes there's enough slack that the wire can simply be moved to out of the way. If there isn't sufficient slack, the wire must be spliced, and a pair of junction boxes installed one at each end of the splice. The use of junction boxes for splicing is part of the electrical code requirements.
If one is using the existing joists with 16" OC spacing, the box should be designed so that it sits on top of the joists. This provides the largest possible opening into the room below.
When the manifold is sitting on top of the joists, the builder must fabricate blocks to close off the space between the joists. This is moat easily accomplished using the same sized material as is used for the construction of the joists. So short blocks of 2"X4", or 2"X6", 2"X8", etc., should be glued and screwed between the joists. Obviously this construction should be airtight.
Designing and building the box
How big should I make my manifold/outlet?
This is a situation where bigger is almost always better. If you don't want to make structural changes, then you must work within the existing framing. This means adapting the design of the manifold. For example, let's start off with standard 16" OC joist spacing. This limits the opening to 14.5" in one direction. One can make a 16" cube and mount 4-15"s in it. This is absolutely the smallest opening possible without detrimental bandpass effects occurring. If possible, a better choice is a rectangular manifold, that's external dimensions are 17.5" X 32", with a corresponding 14.5" X 30" opening.
If you're wondering
about the effect of the particular opening size, model a bandpass simulation in
Unibox or WinISD Pro using your opening . This will tell the frequency where the
bandpass effect will occurs.
What's the recommend construction method for a manifold?
The recommended construction for a manifold is a outer layer of 3/4" MFD, laminated to an inner layer of 3/4" OSB. MDF machines nicely. Laminating MDF to OSB provides strength and creates an inner surface that will grip 'T-nuts' or hurricane nuts. Using these specialty nuts makes mounting the drivers much easier.
Why not just use 2 layers of MDF?
MDF has basically zero inherent strength. Manifolds have large cutouts for the drivers. As a result it's a good idea to stiffen the box using a laminated combination of MDF and OSB.
Below is an example of a manifold
made with a laminated sandwich of MDF and OSB.
Note that the seams of this box have been sealed with a caulking compund
IMO the easiest way to fabricate a manifold like this is to first construct the outer box. Then cut, glue and slide in the inner pieces. I use a brad nailer to hold the inner pieces in place while the glue dries. This method give a little 'wiggle-room' with regard to the accuracy of the cuts for the inner pieces.
The decision as to the shape of the box is basically determined by the available space. Generally speaking the larger the box and the larger the opening, the better the sound quality. There is no sound quality difference between a rectangular box and a cube if they are the same size. So choose the shape that fits your needs.
If possible, the manifold should be only high (tall) enough for a single row of drivers. Deep manifolds do work, but shorter the height the better the sound quality.
For example; my big IB is 42" tall excluding the height of the floor joists.
That height is a compromise I had to make, given where I needed to place the manifold. However 42" height means that the sound wave from the top row of woofers is reaching the listener before the sound from the bottom woofers.
I use the Jasper jigs, one mounted on each of 3 different routers, each router containing a different bit (easier than changing bits). I cut the insets first with a 3/4" flat bottom carbide tipped bit. The driver cutouts are then made with a 1/4" diameter, solid carbide, spiral cut, up-twist bit. Most of the home center stores sell these bits. If this hole cutting technique seems confusing I've detailed it with pictures in this LINK
Occasionally the 1/4" spiral cut bits aren't quite long enough to penetrate1.5" thick material. A work around for this problem is to use a 1/2" to 1/4" router bit adapter (provided your router will accept 1/2" shaft bits). Use of the adapter will allows 1/4" bits to extended lower when placed into the router
A note about circle cutting jigs. There are at least 1/2 dozen brands on the market. I use the Jasper jigs because they're quick, easy to use, and very handy when making a bunch of cuts, since it's not necessary to re-measure and adjust anything between cuts (there's a Jasper model 200 attached to the router in the above picture) . Frequently people baulk at paying $40 for a piece of plastic with holes drilled in it.
Home-made or purchased beam pivot jigs will also work. The primary issue with them is changing settings requires trial and error tests to ensure accuracy
If you're front mounting
drivers, it's best to cut insets (grooves) in the MDF to accept the front of the woofers. This
ensures an airtight seal, and assists in aligning the drivers in the openings
when mounting the drivers.
Attaching the manifold
The Simpson 'Tie-Plates' are great for attaching your manifold to the joists.
These are available in most home center stores. If you look at my 12 driver IB, or the Mini-Me IB, you can see how these plates in use. Drywall screws work fine with these plates.
I recommend a thick layer of acrylic caulk, or closed cell neoprene foam tape (aka camper or topper tape) to provide an airtight seal between the manifold and the joists.
People think they can 'isolate' the manifold from the structure of the house, using a flexible connection between the manifold and the joists. 'Been there, tried that, and it doesn't work.' So don't waste your time.
Gimme a brace......
If a rectangular box is used, one should add stiffening braces to the middle of the longest sides. Since most people are using 15" or 18" drivers, the large driver cutouts weaken the structure. As a result dimensions longer than 24" should be braced.
Here are two examples of bracing. Either design is fine.
I recommend the use of some form of clamp to secure the woofers to the box. The placement of the standard mounting screws in the frame of the woofer, leaves very little wood between the opening cutout for the driver, and the holes for the mounting screws. Using clamps avoids problems with tear out of this area. Clamps also make mounting and dismounting the drivers much faster and easier. (a good thing in a hot attic!)
For smaller and lighter weight woofers, common clamps for 1/2" electrical conduit can be used, if the end is ground to a point. Below is the hardware used to mount the drivers for my small IB with 4-12" drivers. The washers are called 'bonded' washers, they have a rubber layer bonded to a steel layer
Below is a close up of one of my
home-made clamps. It's positioned so
that it engages the standard mounting hole in the frame
Parts Express sells a grill cover
metal clamp kit (p/n 260-352) that will work for mounting some woofers
NOTE: the clamps sold by retailers are 'generic' and unfortunately this means they may not fit all drivers.
For larger heavier woofers, real speaker clamps can be used. The pro-sound speaker manufacturers make die-cast aluminum clamps that are specifically suited for this job. These work best for rear mounting the driver after removing the rubber trim ring. The little lip on the front of the clamps grabs the turned up lip on cast frame woofers.
Unfortunately these cast pro-sound clamps are difficult to find and slightly pricey (~$1.95 ea). Here's a LINK to one supplier. One can also use larger diameter conduit or pipe clamps for larger heavier woofers. NOTE: check the construction of the frames of your woofers to make sure the cast clamps will work.
For lighter weight smaller woofers 12", 4 clamps are sufficient. For big heavy woofers, 15"s or 18"s, 6 or 8 clamps are recommended
NOTE: DO NOT
buy the plastic clamps used for mounting grill covers to mount your woofers. These are
completely inadequate for the task at hand.....
A nice and inexpensive DIY clamp can be seen in this forum POST
Nicktf made some simple but effect clamps using only blocks of wood.
Click HERE to go to page 2
to the main FAQ page
LINK to the IB sub forum