A wild hair of an open baffle idea

[BudP]

Hi all,

I have been thinking about an open baffle EnABL project for a few months now. It seemed the natural thing to do, what with most of the arguing having calmed down and life having gotten boring once again.

Back a couple of years ago, I was horse traded a pair of Lowther PM6A, 16 ohm, silver voice coil drivers. Each of us thinks he got the best part of the deal, so it was a good trade. I have been thinking about what to do with these great big sunflowers for all of this time. I looked at Bushorns, Azura horns, back wave horn cabinets, multi way weird and conventional cabinets and just could not get excited about any of them.

I kept remembering how truly musical and engaging the last pair of EnABL'd Lowthers, also PM6A drivers, had sounded, just sitting nude on my test stands. In addition I worked on a pair of Emmi Beta 8 drivers for Gary Pimm and once they were as tamed as I could get them, I was struck by how articulate they were in the mid bass and down. But I really did not want to have to fight my way through those Raleigh wave jungles again.

Another person, John Hasquin, asked me if I had treated any Emmi Delta Pro 8 drivers with EnaBL. I hadn't even heard of them, but on exploring, I found a person willing to sell me a pair of 16 ohm drivers. They have the same motor as the Beta 8, a cast basket and a cone from a company Emmi purchased a few years ago. John says they are very very good, in one of his recent tapped horn enclosure experiments and I do rather trust his opinions.

So, I have a Lowther, useful with 7 watts and a Delta Pro 8, useful with 225 watts. I do not expect to ever listen to either at more than about 100 db spl, so I have a lot of spare capacity to devote to matching and equalization.

And in comes another of those interesting moments in life. Panomaniac and Gary Pimm, both, had been on about how good a Berringer DCX 24/96 was, after you replaced the op amps on input and output, with high quality transformers. Rene' Jaeger then opined that the DAC's ought to be replaced also, with units that do not have an embedded op amp.

I, on another hand, had been experimenting with transformer volume controls, with 80% nickle core, full isolation, a small gap to flatten the permeability curve and 24 steps, with a primary tap to allow half steps between, ala Dave Slagle's (Intact Audio) excellent tapped autoformer's.

Hmm I thought, how about the TVC's on the outputs, to allow the DCX to run at full bit rate and control the volume to set levels for a tri amp mode, for Lowther, Delta 8 and a sub woofer? And of course, a TVC as master gain for the input. Oh boy, this was way beyond my capabilities, but those who know how didn't seem worried at all.

So, now for that pesky bottom end. Gary Pimm had already demonstrated just how good a John K cardioid bass set up could be. But he is using 15 inch pro sound woofers and a Crown monster amp, along with significant EQ from the Berringer to generate 16 Hz at 110 dB spl.

Not going to happen in my home, somewhat unfortunately. However, could I get a pair of low efficiency Tang Band W8-740 P woofers per side, in a V mounting, with a 54 inch cardioid path length, to provide decent bass, to maybe 103 db spl? They are only 84 db sensitive, but they will absorb 150 watts of power and Sure just brought out some ridiculously cheap 200 watt Class D amps.

So, here is the meat of the problem. How to provide enough baffle for the Delta 8's to work to around 80 to 100 Hz, but still have a cardioid pattern at the low frequencies and also match the nude Lowthers radiation pattern at about 300 Hz?

Below is my first cut at a design. I know it looks finished, but this is just the way I go about things. I lay out my thoughts in solid modeling, with as much attention to all of the competing needs as I can muster, and then begin to tear away at it, trying to optimize the model. This does give me plans at the end and a good idea of where I am journeying to. For scale, the top of the Lowther is 46 inches off of the floor and the baffle is 11 1/2 inches wide. The funny looking swoops in the bass section openings come from Carlson speakers from long ago. The damping material is cut cotton wall insulation (chopped Jeans actually) and comes highly recommended for this application.

My hope is that some of you, who actually know the pit falls of OB design, will chime in and point out my excesses and short comings. As always, all comments are welcome.

Bud

[gainphile]

Can you share us what your plan for the xo points?

For the lowther driver, nude = small circular baffle, and this may result in "perfect" dipole null and peaks. The distance should be irregular (like a rectangle) so the response is smoothed out. But the only way to find out is measure it.

[BudP]

At the moment near 80 Hz from the sub to Delta Pro 8 and 300 Hz from the Delta 8 to the Lowther. I am not fixed upon these frequencies but they seem to fit the drivers in question.

The previously treated Lowther PM6A was an exceptional sounding Omni driver and the pair were in Jon VerHalen's small OB, A/B comparison, at the 2008 RMAF, between EnABL'd and non EnABL'd drivers. Audible characteristics are very smooth, with broad high frequency dispersion. I realize this is counter to orthodox understanding, but it is what I found with the drivers at hand when operating nude. Greater than 60 degrees swept for high frequencies will be more or less wasted in any case. The Lowther mounting frame is a loose square in shape and this may be helpful.

I have attached the data sheets for the Delta 8 driver. I have no data sheet for the Lowther and the Tang Band sheet is too large for my access level to attach to this thread.

Bud

[Rudolf]

This is really ‘out of the box’ thinking, Bud.
And a really intercontinental driver choice too.

There are just two points to discuss from my side:

I would prefer a single central back spine to the double one that you feature. The two spines will cause some reflections between them and will partly block the radiation to the back sides, because each of them is not symmetrical to the source.

I have no experience with Karlson slots, so I cannot comment whether they will work. But I know how they are supposed to function. They won’t work the way you have positioned them. The slots are meant as terminations of a pipe, transmission line or however you call it. They have to be ‘in line’ with that pipe (the U frame), but not perpendicular, as you did them. I once saw a U-frame design on diyaudio with the slots at the right position, but can’t remember who did it (Gary Pimm?).

Rudolf

[pedroskova]

The Beta 8's, with an Fs of ~70 Hz and a qts of 0.22, will (on your proposed minimal baffle), drop like a stone at around 200Hz (or higher). You could apply a lot of EQ, but it would make more sense to go with a more conventional U-frame bass module and take advantage of the natural high pass characteristics that the Beta 8's are presenting you (by crossing in that region).

If you want the minimalist look, (but don't want to deal with the choppy nulls and peaks) maybe you should look into clear acrylic baffles(?).

[Sjef]

Hi Bud,

I have been experimenting a lot with naked driver dipoles in the past. I know it is a very tempting idea because naked drivers always sound the least coloured and faster then baffle mounted drivers. I have done this with 8" drivers as well and from my experiments I can give you a couple of hints.

First of all, the Lowther will never work cleanly to 300Hz, even with eq it's not going to work out right. It does work but that's about all, do not expect the very best quality of it at the end. They will work pretty good however from 600-800Hz on up, this way the driver is used almost completely above it's pistonic range frequency and becomes a true dipole. Below about 500-600Hz Hz the response will drop like a stone anayway. I also thought I good get away with drivers used this way but then I realsied that I was listening to the drivers on the test bench when I liked them so much baffleless. In other words, I was listening to them pretty close range, once you start listening to it at greater distance the behaviour is completely different, all of the great fantastic midbass htat I have heard on the testbench has gone completely.

Same for the delta pro 8, response will drop from 300Hz and has to be compensated for. Since it's a low q driver my estimate is that it will work from max 120-150Hz on up, with lots of eq. No way to get a good clean 80Hz from them on a baffle like this, unless you listen very close range at 50 cm from the drivers or so.

Offcoarse all drivers can be eq'ed down to the desired crossover frequencies but it will start to sound compressed with a wrong power response and integrating the driver in a proper way is going to be a hell of a job.

I would look more at a low frequency solution from fo to 200-250Hz (also pretty hard to accomplish in dipole or cardioid) the Delta pro 8 from 200-250 to 600-800Hz and the lowther from 600-800|Hz on up depending on filter slopes, room positioning, etc etc

just my 2 cents

[BudP]

Thanks for all of the information.

So, it sounds as though a set of wings, angled to minimize reflections and deep enough to allow a reasonable amount of power below 300 Hz will be needed for the Delta pro 8. Gary Pimm has done this so I can ask him about his choices.

I was providing that bullet shape to encourage the same high frequency radiation from the Delta 8 as the Lowther will have, when nude. I do not yet see a way to encourage that with wings, unless a swept angle to the wings will work to give me a sliding frequency response due to differing path length. I don't imagine that would work though.

I will say that I find all of the treated 8 inch drivers to have audible bass down to about 80 Hz. I suppose that this must be from room and floor gain, as the drivers are only 30" off of the floor and the same distance from a brick back wall. This with a listening distance of about 6 feet and 20 degrees off axis, in a room with very good damping, 16 X 18 feet, with a stair well for the last 3 of the 18 feet.

So now, some more thinking. Again, thanks for the comments from all of you, very helpful indeed.

Bud

[kenpeter]

Originally Posted by Rudolf

I have no experience with Karlson slots, so I cannot comment whether they will work. But I know how they are supposed to function. They won’t work the way you have positioned them. The slots are meant as terminations of a pipe, transmission line or however you call it. They have to be ‘in line’ with that pipe (the U frame), but not perpendicular, as you did them. I once saw a U-frame design on diyaudio with the slots at the right position, but can’t remember who did it (Gary Pimm?).

Rudolf

The exponential slot in back is perhaps just a funny shaped vent...
But the one in front looks to be a functional Koupler of some sort.
At least the reverb and dispersion aspects would be functional.
But may be above the cutoff of your crossover anyways...

I don't think bass is significantly enhanced by Karlson impedance
transformation, unless you pony up somehow with missing 2/3 of
the line. Doesn't have to be achieved entirely by length, JEK often
abused an extra resonant chamber to delay the phase and achieve
near full effect.

You might just have a 6th order bandpass box there, however...
And you can't argue with the style and appearance.

But the drivers for the sub.... KappaPro10 would have been far
superior to TBW8-740 in the frequency range you are likely to
achieve in such small boxes with such large vents.

[Alex from Oz]

Originally Posted by BudP

So, here is the meat of the problem. How to provide enough baffle for the Delta 8's to work to around 80 to 100 Hz, but still have a cardioid pattern at the low frequencies and also match the nude Lowthers radiation pattern at about 300 Hz?

G'day Bud,

I have a novel idea for this - you've got mail.

Cheers,

Alex

[BudP]

Hi all,

Well, after many other design paths I have returned to the original, with the following change.

The Delta 8 is now firing into an non parallel wall, open back box baffle, with significant levels of damping, using chopped cotton rag pressed into sheets. With some 33 inches of physical linear distance, from outer cone edge, back to front, I am assuming a roll off point beginig at about 130 Hz.

If you think about this design keep in mind that I am after peaks of 103 dB from 35 Hz to 18 kHz, with average spl down in the mid 80's. And, all crossover and frequency response / spl EQ will occur in the digital domain, at 24/96, with absolute spl to be determined with stepped transformer attenuators that are isolation style, not an autoformer, and with micro gaps in the distributed gap core structure, using 80% nickle.

The Lowther amp will indeed be a push pull SS voltage and current gain driver section, using a Pimm CCS for input splitter. This signal to be combined in a PP to SE interstage, driving a 300 B with plate choke and level three SE OPT, with only a 16 ohm secondary and 48% nickle core.

The Lowthers are now EnaBL'd and are capable of very even output to 300 Hz, nude on a pedestal, though there has to be some floor gain involved, and the stepped attenuator transformers are finished winding. And now begins the connection of hundreds of coil wires to appropriate lugs on the Electro Switch two wafer switches.

Wonder if I will live long enough to finish this?

Bud