Originally Posted by JimWilson
At one point I had done some pretty in depth research on H frames, W frames and the like because I was going to commission a subwoofer of my own design. I wasn't able to find a partner for the project though (I have zero skills as a craftsman) so I let the concept die. I still find all the less standard alignments fascinating - isobaric, push/pull, bandpass, et al - but the road less traveled has always been a draw for me, so it's really not surprising.
Because of my research though I find it curious they designated that design an OB, because based upon what I gathered it wouldn't fit within the classic definition. An open back subwoofer is not the same a true OB design, so from my perspective that unit is more like an H frame tower.
Originally Posted by enricoclaudio
I'm not an expert on OB but the design in question may work as an OB only if the driver is mounted on the middle of the frame depth, then it would be a H frame OB design. With the drivers mounted up front would be a U frame OB design but with that depth it may couldn't work as an OB subwoofer correctly..
I took another look at the pic of the stack of six H-frames, and realized where the confusion is coming from. The pic makes each frame look like it has a baffle at the front of each of them. They don't; what looks like a baffle (the black square with a round hole cut into it) is in actuality a grille frame covered in cloth, just to give the H-frame a finished look---it can, and often is, left off. The baffle is in truth mounted at the exact center of the frame; if the frame is 14" deep (typical), the baffle is 7" back from the front of the frame (ergo, it is also 7" from the rear).
Jim, the reason there in no actual difference (except one--read on) between a classic Open Baffle and an H-frame is that all the frame is doing is increasing the distance between the front and rear of the driver (simulating a wider flat Open Baffle), to prevent cancellation. The H-frame is so named because when viewed from above, it's structure is that of an H. I'll try to make a diagram of an H-frame as viewed from above, to make it more clear:
Okay, not bad (ignore the dots between x's---the AVS Forum system wouldn't allow me to leave the space blank!). Can you tell I'm no computer whiz?! Any way, this is how the H-frame looks when viewed from above, the two vertical rows of x's representing the two sides, the single horizontal row the baffle. If the above H-frame has 14" as both it's width and depth (typical H-frame dimensions for 12" woofers), what the H-frame does acoustically is increase the distance between the two sides of the baffle by the number of inches in the path between the two, if you see what I mean. In the case of a frame with the 14" dimension, that equals 7" (from the front side of the baffle to the front edge of the side), plus 14" (the distance from the front edge of a side to it's back edge), plus 7" (the distance from the back edge of a side to the back side of the baffle), for a total of 28". For the purpose of contrasting the H-frame with a simple flat OB baffle (see below), let's add another 13" to account for the 6.5" distance from the center of each woofer to the edge of the baffle it is mounted on (6.5" for the front of the woofer, 6.5" for the rear, hence 13"). We now have an effective path length between the center of the woofer's front to the center of it's rear totaling 41" (28" + 13"). Are ya with me so far?
What the 14" H-frame does is make the woofer think it's on an Open Baffle 41" wide (the wider a flat Open Baffle is, the greater it's output and bandwidth. A wider baffle lowers the frequency at which the out-of phase front and rear output of each woofer cancel each other). The distance between the center of the front side of a 12" woofer mounted on a simple flat Open Baffle 41" wide and the center of it's rear side is, obviously, 41" (20.5" from the center of the front of the woofer to the edge of the baffle, around the edge of the flat baffle and another 20.5" to the center of the woofer's rear side. A flat, unbraced piece of 3/4" MDF (or even Baltic Birch plywood) 41" wide flexes/vibrates like crazy when pummeled by the movement of two high-output 12" woofers, is very resonant, producing lots of sound of it's own. This we do NOT want! Which brings up the second thing the H-frame does, something Danny Richie feels is very important in obtaining the best sound from the GR Research/Rythmik OB/Dipole sub: It greatly reduces the resonance that the simple flat panel OB baffle possesses. The two sides of the H-frame allow the baffle to be only 12.5" wide rather than 41", which alone helps a lot. But the sides also stiffen the baffle, preventing it from flexing and to therefore resonate less. The horizontal brace in the center of the frame does the same, as do the tops and bottom of the frame (not included in the diagram above. The top and bottom can be any size you want---flush with the sides, or extending past them, as in the stack of six pictured a few pages back). Ric Schultz at EVS (Electronic Visionary Systems) is offering his custom-made version of the GR Research/Rythmik OB/Dipole sub, which he has chosen to make as a simple flat-panel OB rather than as an H-frame. He too wants to insure the baffle is as non-resonant as possible, doing so by making it of three sheets of 3/4" MDF glued together, for a baffle 2.25" thick! There is more than one way to skin a cat. I'm not sure what Ric has chosen as the width of the baffle, however. I'm sure you can specify any you want.
There is one other difference between the H-frame and the simple flat-panel Open Baffle: the cavities created by and within the H-frame structure does result in a cavity resonance, which is a design penalty of the H-frame not shared with the flat-panel OB. The resonant frequency of the cavities within the frame can be computed (it is a function of the cavity dimensions), the resonance being an acceptable trade-off as long as the frequency is kept above the band pass of the woofer system. The deeper the H-frame, the lower the frequency of the cavity resonance. Also, the deeper it is, the more output it creates. Therefore, an H-frame should be constructed with how high in frequency the sub is going to be played (the low-pass x/o frequency being as far below the cavity resonant frequency as possible, conversely the cavity resonance as far above the low-pass x/o frequency as possible), and how much output and low frequency bandwidth is desired, in mind. Danny recommends a frame depth of 14" as a good compromise for use up to 300Hz (the top of the sub's range), such a depth providing a good balance between the cavity resonance frequency of the frame with the frame's capability to support low-frequency output and bandwidth as much as possible. As is usual in design, there are trade-offs which need to be weighed against each other, compromises to be made for the greater good.