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Old 15th February 2009, 11:05 AM   #1
Tyimo is offline Tyimo  Hungary
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Default Two drivers loading the same horn.

Hi!

What is the theory for two drivers loading the same horn?
( for example the Lowther Academy )
Is it need a 2 times bigger horn ( throat and mouth) or just a simple sized horn?

Greets:

Tyimo
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Old 15th February 2009, 04:11 PM   #2
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Hi Tyimo,

Well, since nobody has answered, yet, and speaking strictly as a newbie, I'll hazard a wild guess. Most likely all of what I'm about to say is wrong, wrong, wrong.

In adding a second driver, presumably the chamber size would double.

The thoat, mouth, length and flare are all mathematically related to each other. If you know three, you can derive the fourth. So you can't just double the throat while keeping the rest the same. (But you could cut a new throat somwhere further up the horn, but that changes the length as well).

Having said all that, when calculating a horn, you tend to size the throat as a ratio to Sd, with the old rule of thumb being throat = .3 to .7 of Sd. Adding the second driver doubles Sd, so if you were designing from scratch, presumably you'd double the chamber, double the throat, keep the mouth the same, and change either the length or the flare (and then solve for either length or flare, which ever is unknown). Don't put any faith in what I'm writing, as I'm merely repeating what I recall reading.

I have learned that designing a BLH is like trying to hit a bullet with a bullet -- there are so many mathematically "valid" variations but finding one that will sound good will take months of an expert's time to design, and even then, the final result's sonic quality still has so many external variables (driver tweaks, amp, room etc.)

Tough stuff, way over my head! Tyimo, did you ever build a Replikon-style horn as you were planning?
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Old 15th February 2009, 04:15 PM   #3
GM is offline GM  United States
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Greets!

The simple answer is 2x all CSA dims and chamber Vbs, but multiple drivers can be used to achieve specific design goals other than peak SPL, witness the truncated theater bass horns of days gone by, tapped horns, etc..

GM
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Old 15th February 2009, 04:22 PM   #4
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Hi GM, I like your answer better.
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Old 15th February 2009, 04:32 PM   #5
Tyimo is offline Tyimo  Hungary
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Hi!
Thanks!
The problem is that I heard that basically a given horn can load with any amount of membrane area (say 2-3 driver) if we want to reduce cone movement and to get less distortion......

What do you think??

Tyimo
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Old 15th February 2009, 06:34 PM   #6
GM is offline GM  United States
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Greets!

It's true that to design the horn proper it doesn't use any driver specs per se, IOW all you need to design it is a low and high corner frequency (FL6, FH6) which defines its throat and mouth area with its expansion rate defining its axial length. Finding the right (FL6, FH6) for a given driver for a given app is where the math and to some extent the 'art' of horn design gets complicated, so while a driver's Sd isn't a design criteria per se, it becomes a big deal where compression ratios and/or desired HF BW and/or sheer size is concerned.

Bottom line, design a horn for one driver and add a second one results in the same kind of reduced performance that occurs when you do it in a typical sealed or vented alignment. Also, WRT horns, cram multiple drivers into the throat of a horn designed for an audibly different HF BW than they require and you can wind up with a poorly performing system and why so many mix n' match systems that litter the 'net are such poor performers that folks like me respond to queries of '......will XXX driver work in YYY horn?' with 'only if measured driver specs are reasonably close'.

GM
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Old 16th February 2009, 04:19 AM   #7
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Hi GM, may I ask a question? If one doubled the CSA, the throat and mouth would double, while the length and flare stay the same. Is it correct to say that:

(a) the double-sized mouth would now be capable of going down another octave, but
(b) the design would be limited by the the horn's length (and thus wouldn't necessarily go down any lower)?

In other words, if the original Am was tuned to 50hz, and the length was set to a quarter wavelength of 50Hz, doubling the mouth would basically have no effect on how low the horn could go? You'd have to double the length as well? Sorry for the newbie nature of the question.

@Tyimo, did you ever build a Replikon-style horn?
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Old 16th February 2009, 05:30 AM   #8
GM is offline GM  United States
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Greets!

?? Of course you can, I can always decline if I prefer not to.

Right, the only thing that changes is the bigger throat's cut-off is lower. If you double the axial length also, then output continues only a half octave lower due to doubling mouth area:

frequency = ~13560"/((SQRT(area/Pi))*2*Pi)

or

area = (((~13560"/frequency)/Pi)/2)^2*Pi

GM
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Old 16th February 2009, 05:39 AM   #9
Tyimo is offline Tyimo  Hungary
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Hi!
What doest it mean CSA ?

Tyimo
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Old 16th February 2009, 05:48 AM   #10
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Hi Tyimo, CSA = Cross-sectional area.

GM, thank your sir!
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