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Old 7th October 2007, 08:15 AM   #11
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Quote:
Originally posted by Scottmoose
No, but with careful positioning & angling of the drivers (not necessarily a flat baffle) it can be reduced or eliminated.


With just 2 drivers perhaps the baffle can be flat because they can be equi-distant from the ears (one above, one below listening line), while with more than 2 drivers there will always be some at different distances (in theory)?
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Old 7th October 2007, 09:29 AM   #12
Nardis is offline Nardis  United Kingdom
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Interesting idea. I like the idea of moving a bit more air without adding a different driver and more xover. I have added a Fountek ribbon to my 48inch MLTL using a xover based on Jim Griffin's

I use pentagonal cabinets and a quick crunch on my spreadsheet says that the cabinet dimensions will not have to be unacceptably greater to double the internal CSA. while keeping to a narrow baffle and with a good WAF.

Would I be able to use the 2 Jordans + ribbon in a D'Appolito arrangement?
Presumably there would be an efficiency gain?
What would happen to bass extension?

Nardis
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Old 7th October 2007, 01:42 PM   #13
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I assume bass extension would not be significantly better but there would be a large efficiency gain (and probably no need for baffle-step correction). Also better dynamics and punch I assume, and higher power handling capability. Come to think of it, apart from the horrendous expense (nearly double) it seems a good mod for a serious system.
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Old 7th October 2007, 04:39 PM   #14
Colin is offline Colin  United Kingdom
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You could roll off one of the JX92s above 250Hz or so if both facing forward.

If running them parallel, a 3 ohm resistor in series is recommended (by the Aurousal.co.uk site, which has a short section on using two of their JX92 speakers per side). The JX92S are 4.5 ohm.

Question for Scott or GM - presumably, if using two drivers, you centre them about the recommended distance from the top of the line?
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Old 7th October 2007, 04:54 PM   #15
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Yep. Centre taken from the point where the two drames touch.
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Old 7th October 2007, 08:53 PM   #16
GM is offline GM  United States
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Quote:
Originally posted by Nardis
Interesting idea. I like the idea of moving a bit more air without adding a different driver and more xover. I have added a Fountek ribbon to my 48inch MLTL using a xover based on Jim Griffin's

I use pentagonal cabinets and a quick crunch on my spreadsheet says that the cabinet dimensions will not have to be unacceptably greater to double the internal CSA. while keeping to a narrow baffle and with a good WAF.

Would I be able to use the 2 Jordans + ribbon in a D'Appolito arrangement?
Presumably there would be an efficiency gain?
What would happen to bass extension?

Nardis
Greets!

Right, 2*CSA = ~W*1.4142, ~D*1.4142, but I don't know how to figure a simple calc for a pentagon (assuming it can be done).

Anyway, this is a very popular layout, but like any D'Appolito alignment it will have more off-axis issues than just a two-way, so must be either XO'd low/steep enough to not have to toe them in so much for best performance or use a fairly complex XO to the desired FR without stressing the ribbon.

Bass extension is what you make of it. Obviously, using the same tuning as a single driver yields the same tonal balance except theoretically no need for any BSC filtering. Tune it lower and you trade some LF 'weight' for a lower cut-off, which is what I did with my dual driver cabs since they are backed into the corners.

Acoustic efficiency should be +3 dB down low, but due to the need to keep the nominal impedance high enough for most amps it will be mostly negated by a lower voltage sensitivity, so the net gain will probably be minimal for relatively high extra cost.

GM
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Old 8th October 2007, 01:06 AM   #17
GM is offline GM  United States
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Ran across some math today that defines the transition from near to far field of point source drivers. It doesn't jive with my ears, so as always YMMV:

l = d^2/(c/f)

where:

l = distance from speaker
d = center-to-center (ctc) spacing
c = speed of sound (SoS)
f = frequency of interest

Since the distance increases with increasing ctc and/or frequency, 20 kHz is the practical limit, or only ~45" for the JX92S.
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Old 8th October 2007, 11:18 AM   #18
Nardis is offline Nardis  United Kingdom
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Quote:
Originally posted by GM


Greets!

Right, 2*CSA = ~W*1.4142, ~D*1.4142, but I don't know how to figure a simple calc for a pentagon (assuming it can be done).

GM
The answer is there is no simple calc that I've found - hence the need to write a spreadsheet which divides the pantagon into triangles and rectangles.

Here's a PDF of the cross section in 18mm of a pentagon for twin JX92s.
I'm wondering if I want to do it all again with the second driver. I hadn't figured the need to cancel out the gain in efficiency by the need to beef up the impedance. But at least this time I know what the assembly process mistakes are liable to be.

Your original CSA translated into metric at 19381 mm2.
In inches the twin design works out at exterior measurements
Baffle 6.1" Sides 8.4" Rear panels 6.3"
This keeps the narrowest possible baffle. Its widest point is about 12" and it's about 10.5 deep.

It needs a good table saw.
Attached Files
File Type: pdf twinjx92pentagon.pdf (70.1 KB, 47 views)
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Old 25th October 2007, 08:38 AM   #19
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Quote:
Originally posted by GM


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.

GM
Hi GM,

Sorry for the delay - just to clarify, do you mean a doubling of port area will suffice (same length) or do you have to double length also. My tuning prog says you have to double both for same tuning (with doube volume) but this is for BR so may not apply?
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Old 25th October 2007, 03:02 PM   #20
GM is offline GM  United States
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Greets!

Your program is wrong if it doubles the vent length every time you double the drivers, Vb and number of equal area vents for any vented alignment. In this case the vent length stays the same. Think about it, what's the difference between this and bolting two identical vented speakers together?

Indeed, there's a good argument for doing it this way since two drivers typically have different specs, so won't electrically equally share the available power causing one to excurse more than the other.

GM
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