Martin King's MLTL-48 for jx92

[keladrin]

Hi,

I am impressed with this design, having built it last year and I am seriously thinkimg about building a similar one but wish to include 2 drivers instead of 1 for a far-field application. What changes are recommended for this configuration? Would doubling the internal volume suffice?

[Scottmoose]

You mean GMs? http://www.ejjordan.co.uk/diy/index.html

Yes, doubling the CSA of the line should suffice, no other mods needed. However, if you're going to do this, I'd suggest running bipole. Two FR drivers on the same baffle is begging for trouble.

[keladrin]

Hi Scottmoose,

Please explain - is the touble you refer to to do with the back-pressure on the drivers?

[Scottmoose]

Nope, it's the inevitable comb-filtering & attenuation that bothers me.

OK, if you place two drivers producing the same frequency on the same baffle, unpleasent things happen. The outputs will interact with each other, starting at a frequency determined by the distance the two driver-centres are apart (so, if you mounted 2 JX92s units as close together as possible, this would be 2.45KHz). When this happens, everything above this frequency will suffer from progressive attentuation. Worse, while you can Eq that flat, there's nothing you can do about the severe comb-filter effect / lobing that occurs (basically, the response will become increasingly ragged).

[GM]

Originally posted by Scottmoose

Yes, doubling the CSA of the line should suffice, no other mods needed. However, if you're going to do this, I'd suggest running bipole. Two FR drivers on the same baffle is begging for trouble.

Greets!

Not quite , for a given alignment you have to also either double the number of vents or use an equivalent area single one which will be a little shorter due to being more mechanically efficient (less friction), otherwise it will be tuned lower and the vent may audibly 'chuff' at higher power.

Maybe not , it depends....... Consider we angle the drivers to focus on some point in the distance. The outputs will sum to create a well defined, but relatively small 'sweet spot' at this point in space/time, so if the focal point is far enough away that the angle is sufficiently shallow there's no obvious comb filtering. How far away this point is where dual drivers on a flat baffle sounds as one over an acceptably wide arc depends on the line's height, early reflections (if any) in our acute hearing BW and of course the individual's hearing acuity.

Another thing to consider is acoustic energy rolls off with increasing distance and frequency, so the further away you are the more tonally balanced a rising response speaker is. This means in the 92's case it may not need to be toe'd in as much if we're talking a really big room, which allows for some unconventional baffle layouts to be considered.

Regardless, bipole is great for typical listening distances if the rear driver is rolled off to just being an acoustic baffle step filter, but in large areas/distances with wide BW point drivers you need all the acoustic energy focussed towards the audience for max tuning flexibility.

GM

[Scottmoose]

Blast. Forgot to mention the vent. Thanks Greg.

I'm obviously not having a good day today. I'd forgotton the rising response of the Jordan too. Good point re forward energy for the nearfield. I did think about possibly focusing the drivers (I was looking at foucsed arrays only yesterday, which is kind of ironic), but with only two the options are a bit more limited. Still, worth exploring anyway if it's done right.

[keladrin]

So what we are saying is that at a listening distance of say 3m + and the drivers a few cm apart (the distance between drivers small compared with the difference between the distances of each driver from the ears - the drivers are either side of ear height), comb filtering will not really be noticeable by many people, unless they move closer to the speakers (assuming flat baffle), while at closer distance there is some attenuation noticeable and nodes apparent as one moves up/down.

Bearing in mind in stereo mode there will be a certain degree of comb filtering anyway if the treble is coming from 2 separated sources and you move off-centre, and this is very apparent even at a few metres, because of the wide separation.

Then what about in single driver mode when the driver is toed in as recommended - are you not listening through two ears, each at a different distance from the driver so there will be comb fitering and attenuation created in the listener?

[Scottmoose]

No, but with careful positioning & angling of the drivers (not necessarily a flat baffle) it can be reduced or eliminated.

In stereo? Perhaps in the centre image. I think you might have meant mono? That certainly has an issue with combing, & why some 78-fiends have reverted to mono setups. OB with a co-ax is particularty favoured I believe.

[planet10]

Originally posted by keladrin
Then what about in single driver mode when the driver is toed in as recommended - are you not listening through two ears, each at a different distance from the driver so there will be comb fitering and attenuation created in the listener?

This is a natural situation for the ear/brain and is how we can tell where a sound is coming from.

dave

[GM]

Originally posted by keladrin
So what we are saying is that at a listening distance of say 3m + and the drivers a few cm apart........

Bearing in mind in stereo mode there will be a certain degree of comb filtering anyway.........

Correct, though per my caveats, I don't know if 3m is far enough away for most folks, I never did much in the way of experimenting with arrays except 'infinite'. About all I can say is that two stacked Altec 511 horns (10.62" c-t-c spacing) was unlistenable to me back in my youth above a few kHz at ~17.5 ft IIRC and still a bit 'phasey' sounding ~46 ft away at the other end of the room, but these 'throw' the sound a ways, extending its nearfield, and are more phase coherent in their passband than a wide BW point source driver, so the distance can't easily be scaled AFAIK.

Bozak did a vertical tweeter array using four 2" cone units, but I didn't like it as much as his earlier virtual multi-cell tweeter horn, which used eight 2" cone units, even though from an acoustic phase POV the array had to be far superior, so go figure. Bottom line is neither were very coherent to me when used as stereo pairs, though the virtual horn was better IMO for mono, which it had been designed/optimized for, so no surprise there.

Right, stereo is nothing more than mass quantities of complementary comb filtering and why it doesn't take much to audibly degrade it.