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Old 17th March 2010, 04:37 PM   #11
GM is offline GM  United States
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Quote:
Originally Posted by Bill poster View Post
Ok thanks, you know your stuff and explain in a way that's understandable to a novice!

If I do an omni it will end up looking too much like the Duevels..........

FINAL question (he says); back folded horn loading- if the line is long enough to reach 30 -35 hz or so, is there a stipulation on how wide/area the mouth needs to be?
You're welcome!

How does a novice know when someone 'knows their stuff', he asks rhetorically?

Exactly!

Yes, it depends on what in-room F3 you want, i.e at what point do you want horn loading (1/2 WL) to roll off into TL loading (1/4 WL).

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Old 17th March 2010, 06:54 PM   #12
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You're welcome!

How does a novice know when someone 'knows their stuff', he asks rhetorically?

Exactly!

Yes, it depends on what in-room F3 you want, i.e at what point do you want horn loading (1/2 WL) to roll off into TL loading (1/4 WL).

GM
True. I just trust you.

Lost me on the F3 horn to transmission line bit... and if the front horn is tuned for 180 hz is there any way to reduce the length of throat front to back? I am thinking No
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Old 17th March 2010, 09:46 PM   #13
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OK, just so you understand I'm anything but infallible.

For the purposes of this conversation, horns are 1/2 WL resonators with a 1/4 WL fundamental whereas a TL is strictly a 1/4 WL resonator: Resonances of open air columns

From this we see that as the horn unloads due to not being big enough in net box volume (Vb) to support 1/2 WL resonances it decays into a 1/4 WL one down to its pipe cut-off (Fp), causing ripples in its HF BW. Anyway, I imagine this is all be explained in-depth in one or more of MJK's various technical papers.

For a given FLH mouth cut-off, the only way to make it shorter is to truncate the throat with the attendant rolling off of LF gain BW, so to get it back, attach a bigger driver with the right specs to it or in the case of this '40's era Westrex cinema horn, four of the same size/type:

GM
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Old 18th March 2010, 08:35 AM   #14
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last things first: I believe you mean the effective circumference, ergo less than a WL of the lowest desired horn loaded frequency..........
Hi GM,
I don't understand what the difference between our two postings is.
Could you explain why my phraseology is wrong or misleading?
And more precisely explain exactly what the message in your posting should be telling me?
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Old 18th March 2010, 03:36 PM   #15
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Greets!

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in free space the circumference of the circular mouth should be greater than the wavelength of the lowest frequency.
I interpreted this as meaning that the physical circumference of a horn in 4pi (free) space must be greater than 1 WL in circumference of the lowest frequency to support it, a commonly held theory at one time and often repeated as irrefutable fact over the ensuing decades, but the reality is that it need only be large enough to support it acoustically, so its circumference (area actually) can be somewhat less due to the rather elastic boundary conditions (for lack of a better term) near/at the terminus (mouth).

Not being formally schooled on the subject or having made the kind of exact measurements available today to determine either a theoretically correct or real world exact circumference, about the only way I know how to explain it is to visualize a sound 'bubble' as being basically analogous to the creation of a bubble by blowing against a thin film of soap suspended in a circular wand (AKA 'bubble ring' or whatever the technical name is of what is supplied with a child's bubble blowing kit).

A soap bubble at rest then can be construed as a sound frequency at its 180 deg zero crossing, so as the bubble is expanded to half its size equates to its compression (positive/+) mode and if blown from the other side, its rarefaction (negative/-) mode only a sound 'bubble' is flip-flopping at ~1130 ft/sec (~344.42 m/sec).

In the real world though, the resonant membrane is just exciting air molecules 'stuck' to it, so obviously a sound bubble somewhat larger than it will break loose same as our soap bubble does and it's this difference between the ring's smaller circumference referenced to the soap bubble's that sets the horn's circumference, not the WL's circumference.

I originally worked out a 'close enough' rule-of-thumb to satisfy me from this and by comparing Altec's and others various horn designs to whatever measurements I could find, but sometime after joining the Sound Practices mailing list (AKA 'joelist'), someone posted a D.B. Keele? AES paper reference on the subject that has the math solution, but haven't ever gotten around to finding/reading it, so don't know how mine compares to theory.

So, do you still believe we said fundamentally the same thing? If not, then please elaborate on what point you meant to convey that I misunderstood you to say.

TIA,

GM
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Old 18th March 2010, 03:54 PM   #16
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I suspect this is the Keele paper you were refering to Greg? http://www.xlrtechs.com/dbkeele.com/...uth%20Size.pdf Not sure how he's managed it, but it looks like he's got all, or the majority, of his AES papers up on his website now.
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Old 18th March 2010, 04:34 PM   #17
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I guess, I can't seem to find the old joelist message. Anyway, good to know about his site, thanks! I'll DL/read the paper a bit later. The rainy weather's let up for the moment, so got to go back to getting the shop functioning again.

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Old 19th March 2010, 02:55 PM   #18
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I suspect this is the Keele paper you were refering to Greg?
Yes, that's it, thanks!

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Old 19th March 2010, 03:10 PM   #19
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Hey Bill,
Actually there is a way to lessen the frontal area of a horn and that is by mirroring against the sidewall. In the picture you see Tractrix mids with Lowther drivers. Mirroring is mentioned and described in the UK Tractrix patent from 1928.
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Old 19th March 2010, 03:27 PM   #20
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Correct, Andrew pointed it out in post #8, though as always a picture is worth a thousand words.

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