H-frame OB


In my ongoing search for more impact I had imagined a rather convoluted idea for a large Open baffle. One which would use the AV stage as a horizontal baffle. A large slot made at the back of the stage would allow the array sub-baffle to breathe down through the stage floor to the room below. The slot would have to be of adequate size to allow free air flow from the rear of the four 15" drivers. Or the baffle would not function as intended.

Cancellation would only occur when the opposite phase, pressure waves met 5' away from the slot. At the open stairwell in front of my seat.

I quite liked this idea because it offered a reasonable size of baffle without the usual IB heat loss to the great outdoors. Not quite infinite but there was still hope of true Open Baffle sound quality with a very well extended response.

Then ThomasW, the vastly experienced guru of the IB Cult forum, suggested an H-frame OB instead.

I had no idea OBs could reach very low until I started doing my OB homework online. It seems the H-frame design loads the driver  cone(s). Pushing the free air resonant frequency downwards. All I wanted to do was fill the deep trough between 150 and 200Hz. Infrasonic response was not necessary.

A couple of hours of to-ing and fro-ing to the shed provided a working model. I borrowed two cut-down baffles from former IB projects. Then added 4" x 1" rough sawn plank frames front and back to hold two baffles together. This made a solid, but butt ugly, 2 x 15" H-frame OB to play with.

The narrow frames should help push the baffle frequency up out of harm's way. There was absolutely no point in investing quality materials in an experimental model. I had a beech strip, kitchen worktop waiting in the wings if something prettier was found worthwhile. The test subject was placed behind and midway between the right main speaker and the LCD TV. 

For test purposes I used the SVS Bash amp from my old 16-46PCI/NSD cylinder. I have no idea whatsoever if this amp has any response shaping. After series wiring and checking for polarity with a 9 V battery I ran a few REW sweeps through two of the older AEIB15 drivers sitting in the baffle. These drivers are vinyl coned with a measured Fs of 32Hz. The rest of the parameters of these non-spec drivers are likely to remain a total mystery.

This is the result of setting the SVS 12dB/octave crossover to its 120Hz maximum. Measured at the listening position with the Galaxy 140 SPL meter. No EQ is possible using this amp unless I insert the BFD in the signal leads. There is really no point in this when I can try the EP2500/BFD/CX2310 tomorrow. Though I seriously doubt I need remotely so much power. If I can pull down that wide hump from 30-50Hz, with the BFD, then that peak at 150Hz could be a useful trough filler. 

 The waterfall graph of the above curve. While the rest of the curve is tidy, 50Hz is real mess!

Now the result of bypassing the crossover on the SVS Bash amp. Rather inexplicably the hump has narrowed and a deep trough bottomed out at 90Hz. The complete opposite of what would be expected. Had I simply swapped the graphs, one could be easily forgiven for believing the curves matched the original descriptions. Very strange indeed!

Note how I have stretched the graph limits to 300Hz to capture the 235Hz baffle resonance. Quite a remarkably high figure for a baffle large enough to house two 15" drivers. Commercial designs usually use a couple of 10" drivers in an H-frame for the bass section of full range OBs like the Orion. The best designs use notch filtration to remove the peak.

As can be seen in this waterfall graph an OB is rather well damped. This is despite the awful response curve. There is some ringing at 50Hz which may be electrical rather than mechanical. I have noticed a 50Hz hump in quite a number of my REW waterfall traces.

I tried playing a Mike Oldfield CD through the OB and brick-sized Sandberg computer speakers. I was immediately impressed with the power of the OB. It seems an OB can be quite efficient. I was measuring up around 100dB(C) with the SPL meter at 1 metre with the SVS gain at halfway. Cone excursion was still quite small. The "jangly" sound I had experienced with small, naked baffles in an earlier trial was thankfully absent.

I may try a nearfield sweep tomorrow to see how well these drivers behave. It might prove whether the SVS amp has any response shaping. The OB sounds quite unlike the IB. With a lot more upper bass present. Hardly surprising really. When a 120Hz or no crossover at all is superimposed on the response. The IB has a nominal 80Hz roll off. Could SVS have applied a notch filter to remove the natural pipe resonances from the cylinder?

As promised: A series of nearfield sweeps about 4" from the upper driver dustcap. The blue curve is with the 120Hz crossover engaged. The highest curves were all nearfield without the crossover switched in. All with good agreement between them. Only the green curve is taken at the listening position with the 120Hz crossover engaged. The unfiltered nearfield curves suggest these speakers are fine and the SVS amp as flat as matters. The 30-50Hz hump may be the drivers natural free air resonance being damped by the baffle.

The SVS amp is incredibly sensitive to gain level with nearfield measurements. Merely touching the knob would change the level by 20dB. Most of the nearfield sweeps were all but inaudible. Later I matched the resulting curves at 80dB for easy comparison.

There must be some hum at 50Hz. The Creative "Live" Sound card sockets are worn out. Even the weight of the mini-jacks and very thin cables is enough to cause a break in the stereo signal when listening to the computer speakers.  

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