H-frame OB

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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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More button pressing

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The recent drama on the Lionsgate intro has been bothering me ever since. It sounds as if the bass has been turned up from normal. Certainly as if something had changed. I just couldn't put my finger on it. All the films we had watched since were very bassy. I stared at the bars on the mixer and BFD but there was no sign of obvious excess.

Inevitably a window had to be found to run REW to see what was actually happening. The response was rather worse than I remembered but only a couple of dB hot compared with the speakers. The old drivers are still getting maximum boost at 20Hz. (+16dB 120B/W) The newer drivers have no filters running at all.

I found the same problem as before with the mute buttons on the Behringer CX2310 active crossover. It took a few attempts to get them to switch on by rapidly tapping the button. Until then I could not rely on them.

This is the waterfall plot of all 8 drivers and speakers combined. Somehow I failed to save the simple SPL curve.

A bit of a mess with a deep valley between IB and speakers. No wonder it sounds bassy even though the sub isn't running hot. I checked up to 500Hz and the speakers extended on upwards at much the same level.

Here are the overlaid curves for all 8 IB drivers, single sets of four drivers, with and without speakers. The blue line is with the speakers set out of phase with the IB. The heavy violet line the in-phase response. It's a toss up between having a wide valley or an abyssal trench! Quite a mess with either option. It is no wonder there isn't much impact on drums or plucked bass! Though there is a free 10Hz hump to provide some deep infrasonics.

Time for a complete rethink. Only the upper (newer) set of 4 IB drivers causes the valley. I really ought to put these drivers in the bottom half of the IB manifold. Then use the older drivers in some other alignment between the speakers. There is a fair bit of room under the 45 degree sloping ceiling for boxes. Though I'd prefer a large baffle or IB.

Beneath the stage is a wide open space leading to the rest of the L-shaped, living room. The ceiling below the stage is covered with plasterboard tiles. These need replacing with T&G pine boards to match the rest of the living room ceiling.

One would have to be blind to miss the obvious chance to use the floor of the stage as a deep baffle for the older drivers. True, the baffle would only be about 6' deep from front to back. Before it reaches edge of the stair well. But, the older drivers need not dig very deep and won't anyway with their 32Hz Fs. So I can forget the massive +16dB boost and let the drivers behave as big woofers. Let them roll off naturally.

My idea is to mount the four old IB15 drivers vertically on a thick baffle as far back on the stage as possible. It would be sealed all round or even made into an open bottomed, triangular box sealed to the floor. The rear of the baffle "box" would exhaust straight down through a wide slot in the ceiling below. Probably covered with a "camouflage" cloth grill. A sort of understated IB, if you like. Certainly much bigger than any normal, standing, open baffle!

So I should be finally able to reinforce the missing bass in the deep valley between the real IB and the main speakers. The reduction by four drivers will allow more room to breathe for the four remaining drivers in the IB enclosure. I can chop the manifold in half to return to my former layout. 

The problem is getting permission from my wife to launch into further bouts of DIY after years of putting up with it. Taking down the old ceiling tiles would make a bit of a mess. Though I planned to make a heavy poly tent to work inside.This would keep the dust inside. Instead of spreading right through the house. The tiles could be bagged for removal outside.

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Getting in a flap!

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You all know that Lionsgate film intro with all the shiny gear wheels? :-)



Well, anyway, Lionsgate nearly broke the house! I had started the hired DVD disk and turned the volume somewhere around the usual setting.  There had a been a couple of wimpy trailers...when all of a sudden the Lionsgate intro started. The entire house shook like a leaf! The walls and ceiling creaked and vibrated incredibly violently! Of course I quickly turned it down...

We were about to watch "Abduction" on an ordinary, hired DVD. I chose Dolby 5.1 rather than DTS. It wasn't long before we realised that the film makers  took their bass rather seriously. Every so often there would be a loud roar and the curtains across the double doors to the IB enclosure would waft 6" in and out! I'd never seen this happen before! It became a habit with frequent excursions of the full length, heavy velvet. Great fun! The first motorbike scene sounded as if the bike was right in the room!

The film was panned on IMDB with a 4.3 score. It's true that there were weak moments. Not least Sigourny Weaver's wooden acting!

The scenes between the two young stars were much too "teenage movie/Moonlight" and completely superfluous. (except for teenagers perhaps?) We all had the human biology lesson. We do get the picture!

There were moments in the story where the suspension of disbelief was hanging by a burning thread.

The action plot still managed to hold the excitement for us. We would happily have given the film an overall mid-5s... except for the incredibly weak ending. Which limply burst the rapidly deflating balloon.

Bass-wise this film has to be a firm 8 or even a 9!

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