Questions about OB Array build

[ALF-]

Hi guys.

I need to hear some words of opinion and perhaps some words of advice (although I confess that I am notoriously bad at taking advice - possibly a by-product reaction against a vocation where I am called on to give much advice).

 

Now, these questions may seem naive and foolish to you - especially coming from a grey-beard who has been building and tweaking speakers since about 1967 … but I have always built and adapted the recipes of others - I am not a particularly original audio chef, and I I am a clergyman / theologian and former plant researcher - induction and impedance and such are just not part of my conscious mind.

 

I have had a long love affair with Open Baffles … and now I desire more.

I have ordered 8 Wild Burro Audio ‘Betsy’ 8†full-range drivers that I intend to make into a pair of (short) full range open-baffle arrays … bass will be helped out by my 2 per side Hawthorne Augies in OB.

I have read good things (and some criticisms) of Visaton B200 OB Arrays, and I am intending to try similar but cheaper … I’m not particularly worried about comb filtering - the 4 close together don’t have a very great difference when it comes to distance to ear in listening position … I’ll probably use a bullet tweeter (or a horn loaded compression tweeter) to fill in the high top end.
I’m getting old, and am starting to hate cutting holes, so while I don’t mind experimenting, I’d like to keep the hole sawing to a minimum.

 

Now - stupid questions.

1 … does the array have to be completely vertical - this is what I’m intending, but the thought does occur that perhaps as a space saver and source concentrator I could have the 4 arranged in a close square of 2X2, or even a diamond with a tweeter squeezed in the middle.

 

2 … now this sounds really dumb, but wiring them, is it better to have 2 in series paralleled, or 2 paralleled in series? Does this change in any way the sum of the strange load individual drivers see when having an identical fellow in series with it? (I am reluctant to ask my tube amp to attempt a 2ohm load)

 

3 … is a rearward facing tweeter going to be of any benefit to the OB? I tried a rear upward pointing tweeter with a completely different open baffle a few years back, but … well … I dunno - my mental jury is still out … I had a rebuild.

 

Any constructive thoughts or experiences will be appreciated - but please - don’t hit me with too much theory -I use my ears rather than conceptions I build from reading theory - over the past 45 years or so I have found that practice doesn’t always obey the theory - speakers often don’t seem to read the papers.

 

Thanks fellas - cheers! ALF.

 

[leo]

As a clergyman I do hope you have read Swedenborg.Do go ror the "4 square" arrangement,as it keeps the upper closer to the lower and you can sit closer and not lose the sense of intergration.I wired the 2 pairs in series and then paralled them In my case it was beneficial as I was using a second order C/O and wanted the option of using 2 different cap.to each pair,one crossing lower and the other higher.With 1 tweeter,you know your reality with 2 you begin to scratch your head.Leo.

[ALF-]

G'day Leo

People don't usually say things to me like "I do hope you have read Swedenborg" ... but I have read a bit, but can't remember much - if I recall, he was a bit more of a mathematical bent than I am (I'm just plain bent).

I was thinking much as you've said re the distance between upper and lower.

Since posting my question, I have been contemplating using a 5th driver and trying them as a Bessel Array ... then with a single tweeter that I can switch in or out as needs be.

I hadn't considered Xovers.

The concepts flow slower these days.

Thanks for your reply - you have made me think.

ALF

[leo]

OK,the divine purpose of your original post was to re-introduce you to the teachings of Swedenborg.Begin by reading "Heaven and Hell" that should be available free online.It speaks directly,via the Lord to the rational mind.This on-off switch you mentioned is a good idea so you may make an instant comparason between two options.Stay with the 4 square configuration.I love it.Remember,you only need 2 of the bass/mid drivers meeting the tweeter at the C/O point.The other 2 can be crossed over lower so that any erratic spikes they produce will not be added to the vital midranges as an addition to what may already be there.This is why I used a second order network for my OB speakers.Leo.

[ALF-]

Thanks Leo ... It's available freer on my bookshelves.

I'm absorbing information, and soon, with glacial speed, I will act.

Thanks for the advice.

[b.d]

Hi Alf. I remember you from my Hawthorn days, glad to see you still experimenting. OB line array is one thing I still want to try myself. If its not too much cutting (go rough 'n ready with a jigsaw before finalizing design and routing out nicely) I'd be interested in trying the 4-square arrangement myself, or should I say hearing what you think of it relative to the short line array. You'd give up some benefit of an array for other possible benefits.

 

I used to run a single B200 OB with a dipole Neo3 tweeter above, it did help but was not critical. An array of B200 might lead to a narrower vertical sweet spot and offer a better reason to add a supertweeter, but then the sensitivity of the tweeter is going to be too low to match if you were going for a 4.5 way passive...would require active biamping or a much more efficient dipole tweeter...but I can't think of any suitable. I think though that if you run the B200s full range and feel the need for more top octave energy or dispersion (Eq might suffice for the former), that a single monopole bullet/CD tweeter would suffice. 

 

Cheers

B

[Newman]

Hi Alf

 

1. I think a vertical line will be better than a square. A square will have lots of lobing to the sides, and this affects the overall sound even if you point the speakers right at your face. I would not suggest a front tweeter, same reason, plus I'm guessing the Betsy can reach as high as you can hear.

 

2. No difference, technically.

 

3. A rear firing tweeter is not a bad idea, but probably not essential IMHO. Maybe with its own amp, so you can easily dial in its level.

 

Good luck with the fun project!

[b.d]

I've had a number of large widerange drivers with supertweeter, trying the tweeters front, rear and upward firing. Lobing is scary in theory and also when measuring at any single point in space, but I find with XO >10k is to me inaudible to me one everything but pink noise.

 

Going square you will have horizontal lobing yes, but even lobing exapmles at their most extreme (with C2C distances of greater than one wavelength, e.g. 12" Bastanis Prometheus (MT) vs Bastanis Apollo (MTM)) I found a bit of a non-issue in practice. On the plus side of the square arrangement you'd have much less group delay...and point-source-like symmetrical radiation patterns..But even group delay, in worst case examples of ESL and arrays, it seems to not be as much of an issue as might be predicted on paper.

 

I could be wrong, and square might suck for reasons I havent guessed at, just saying if you got the time and timber we'd be glad if you did your bit for science and gave them both a go

[ALF-]

Thanks fellas ... I appreciate your input.

While I'm in the thinking mode - another dumb question ...

I have a couple Visaton BG 20 Full Ranger speakers in a box somewhere in the cellar ... if I used this as the 5th driver in a Bessel arrangement with 4 Betsys - the out of phase driver , would the fact that it has a mismatch in qts (and a bit) in its efficiency be a terrible issue?

 

 

Or - I could use the B200 with the Betsys ............. ?

Edited November 11, 2013 by ALF

[ALF-]

Oh - and thanks for the separate amp idea - I have a chip amp somewhere that could be conscripted into service ................

[henry218]

hi alf,

have a read of jim griffin's white paper

http://audioroundtable.com/misc/nflawp.pdf

for OB, expect to do some EQ , so measurement and dsp based crossover is a must.

cheers

henry

[gainphile]

Acoustically OBs are the second most complex speakers to build (the 1st being cardioids), so do utilise DSP power to assist!

[sfdoddsy]

If you are hoping to get the benefits of a line array, then yes, you need to do the woofers in a line.

 

The whole point of an array is to not clump the drivers together, but to rather to create a source that creates a 'line' of sound. Ideally from floor to ceiling, but certainly high enough to cover the area from the floor to above your head.

 

The Griffin paper linked to above has the recommendations for distance between drivers, but from memory 4 x 8 inchers would be marginal.

 

As for Open Baffle/Dipole, the real benefits are lower in frequency, from the mids down, due the difficulty of actually creating a true full-range dipole speaker.

 

Basically, the higher in frequency you go, the narrower the baffle needs to be to have a dipole effect with back to back tweeters, or indeed true dipole tweeters like some ribbons. Eventually, it needs to be narrower than any available tweeter. Which is kind of tricky.

 

Once you get above 4-5Khz or so, you won't get a dipole effect (ie nulls to the side). You can get a bipole effect, which some like. Hence there is a great deal of debate about rear facing facing tweeters for OBs. 

 

I'm not familiar with the drivers you mention, but I assume by 'full range' you plan on crossing them to the tweeter pretty high.

 

In a regular OB, given they are 8 inches drivers mounted on a baffle, you'll lose the dipole effect around 1khz and they will start beaming. Check Gainphile's site for examples.

 

Horn loading or waveguides are probably most effective in narrowing the radiation pattern of the tweeter to match that of the mids.

 

This doesn't mean you won't get the other advantages of an open baffle. It just won't be a dipole if that is what you're after.

 

But you are right. It's all theory and I'd recommend sucking and seeing. Before going commercial with my current speakers (which are line sources), I played with a DIY array using the BG Radia ribbon midrange/tweeter and four 8 inch bass/mid drivers. It sounded good, but not as good as my previous open baffles. But it was worth trying.

 

It's easy to do a test baffle with four 8 inch holes in some mdf. You'll probably need to EQ the bottom end.

 

 

Just go for it.

[gainphile]

Just go for it.

 

 

Yes... and remember the difference between science and mucking around is to write it down

[b.d]

If you are hoping to get the benefits of a line array, then yes, you need to do the woofers in a line.

 

The whole point of an array is to not clump the drivers together, but to rather to create a source that creates a 'line' of sound. Ideally from floor to ceiling, but certainly high enough to cover the area from the floor to above your head.

 

The Griffin paper linked to above has the recommendations for distance between drivers, but from memory 4 x 8 inchers would be marginal.

 

As for Open Baffle/Dipole, the real benefits are lower in frequency, from the mids down, due the difficulty of actually creating a true full-range dipole speaker.

 

Basically, the higher in frequency you go, the narrower the baffle needs to be to have a dipole effect with back to back tweeters, or indeed true dipole tweeters like some ribbons. Eventually, it needs to be narrower than any available tweeter. Which is kind of tricky.

 

Once you get above 4-5Khz or so, you won't get a dipole effect (ie nulls to the side). You can get a bipole effect, which some like. Hence there is a great deal of debate about rear facing facing tweeters for OBs. 

 

 

Sorry for the tangents Alf, but wanted to question to myself and ask the other guys what the benefits of short arrays are…I mean I understand the benefit of the 3db vs 6db attenuation with distanced of full range line-sources, and the infinite line-source effect of going floor to ceiling, but having line-source behaviour over only half the frequency range seems potentially worse than non at all, i.e you have the problem of the sound becoming unbalanced with greater distances from the speaker as the LF and HF attenuate at different rates, as well as increased vertical beaming in the HF(can be a benefit, if you only listen seated)…

 

Also, I've questioned to myself the importance of aiming for proper dipole behaviour above the mentioned 4-5k…to my mind the lateral phase-based cancellation in the super HF being less of a benefit due to the fact that lateral attenuation is increasingly happening just by virtue of increased beaming, and because this phase cancelation to the sides seems less significant to me that the improvement to the power response of bipolar radiation, i.e the improvement to the power response of having the whole front wall of the room reflect back balanced sound rather than the hugely unbalanced spectrum a monopole puts out. 

 

But however you arrange the drivers, I suspect you might find some pleasing effect just by virtue of moving that much more air, I mean there seems to my ears a common quality that I like of both full-range front horns and big ESLs that I suspect is just by virtue of their impedance coupling to the air, which you don't get from single smaller cones.

 

p.s. Alf, I remember reading somewhere Danny Richie of GR Research trying a quad arrangement of his Neo-10s in OB and liking it, though would have been a lower xo point to tweeter 

[henry218]

danny richie designed the OB using stacked neo8 on its wide side with neo3 tweeter in MTM config.

 

[b.d]

They talk here about an experimental square quad design, could be wrong in how I picture it

http://www.audiocircle.com/index.php?topic=85662.200

Mixing horizontal with vertacle panels would have radiation issues beyond what four cones would so it's not directly relevant, but quads have been done before like in the pretty acclaimed Legacy Whisper.

[ALF-]

I will suck it and see - Like the tourist ad said, "If you never never go, you'll never never know".

(CAUTION - Rambling and waffling alert!)

It's all about the internal personal relationships between discontent, sanity, and a need for diversion. I am on call and working usually 7 days a week from dawn until bedtime, so I spend a good deal of time in a semi-hallucinatory state of mind - the potential for really strange ideas is very great ... I look on this as a gift. I'd absolutely hate to be sane!!!

- you guys are all into audio, so I am certain you know what I mean ... we're all mad!

I don't really need another set of speakers and my exIsting WMW OBs sound pretty darn good - they should keep me happy into retirement...

A second system would be nice, mainly as displacement activity (there is a potential pun in there) and I have ordered these 8 cheap full range 8" guys on a whim, so I'm feeling obliged to do something with them.

A carton of Neo10s would be a nice alternative, but not on a clergyman's stipend.

I am curious about options, and "My name is ALF and I'm an Open-baffle-oholic".... That's where the questions about short arrays, squares, and diamonds in OB come from.

I don't have the woodworking skills to be a horn builder, and I don't like the sound of most I have heard, and I prefer playing with OBs rather than boxes.... If I'd kept all the OBs I have had over the past decade and a half I'd need a warehouse to store them. (I say this as a bloke who hates cutting holes).

What I certainly don't need is 4 separate new single-driver speaker stereo pairs ... Might be fun, but where will I put them? - If I could get someone to give me half a dozen more hand built amplifiers as a late birthday gift I certainly could find a space for them, but I won't plan on that (but, if you are reading this Earle Weston, you know my address).

I have to confess that I am not feeling confident that the short array OB idea will be a dazzling aural epiphany experience.

If there are thoughts about novel alternatives for my current displacement diversionary project, don't hesitate to let me know.

It's time to get up so I will have mercy and stop typing at you via my little iPad.

Happy Thursday Bros

ALF

[ALF-]

They talk here about an experimental square quad design, could be wrong in how I picture it

http://www.audiocircle.com/index.php?topic=85662.200

Mixing horizontal with vertacle panels would have radiation issues beyond what four cones would so it's not directly relevant, but quads have been done before like in the pretty acclaimed Legacy Whisper.

I can imagine what the Missus would say if I came home with a set of Legacy Whispers! ( but I don't think they'd fit in the Citroen)

ALF

[b.d]

hallucinating is underrated yes.

I'd try the line, and if there's any potential I'd be temped to say try 4 more per side...but could get a tad expensive. Keep us abreast of your progress though, I'll be watching with interest.

Cheers

B

[ALF-]

Well, the drivers haven't come yet - still on back order in the States.

But, having been thinking, talking and mucking around, I think it likely that I will abandon this hare-brained idea ... I will keep 4 of the drivers to play with, and will probably offer the other 4 for sale here for you guys to play with as well.

I'll keep youse posted

Cheers

ALF

[leo]

Yep,keeping the 4 seems like a good idea to me.

[ALF-]

There has been some delay - a couple month's audiophoolic drought complicated by the addition of a hectic Advent, Christmas, New Year, lots of funerals, two weeks off during a heat-wave, a flare up of arthritis, and a death in the family ... in other words, my life as it usually is, except of course for the death of Joy's father - we laid his remains in earth last week.

 

I have abandoned the idea of a full-blown array ... but I have been experimenting --- all sorts of gaffer taped, cardboarded and butchered permutations. 

At the moment I am listening to a pair of old padded baffles with some sundry extra bits of wood screwed on to them as extensions (and yes - there are drivers in the baffles ... 3 each).

The solid state amp is driving (in each baffle) two Betsy drivers full range - they are in series (!), yes I don't like drivers in series - so I did it ... the Betsys sit atop of the PSI coax speakers - the tweeter disengaged and the crossover removed and replaced with a simple coil cutting them in at 500Hz @ 6dB. The mid/bass section of the PSI is so fast and articulate that it makes what surely must be one of the best drivers covering that range (for open baffle, that is) I have ever heard - they are so musical it maketh the jaw to drop - verily! 

The bass is impressive for such a narrow baffle. The midrange is engaging and very 'real'. The treble lacks nothing to my aged ears, but may well benefit from a super-tweeter coming in quite high ... probably >12K.

 

Like I said, I have abandoned the array idea as a moment of madness, but I am very happy with this arrangement, even in their baffles of rags and patches.

 

Oh- and despite what the logic of my wallet might tell me, I think I like the cheap Betsy sound over the expensive Visaton B200 sound ... and it has a whizzer cone and I have always regarded them as a wee bit suss.

 

The beauty of this silly set up is that I can very easily listen to the PSI coaxially full range by simply swapping a few wires, so I have in effect 3 systems rather than 2 ... noice!