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Old 9th January 2013, 11:02 PM   #11
brianco is offline brianco  Ireland
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Originally Posted by Don Hills View Post
Has anyone thought about, or done some work on, horn mouths that are not symmetric? Where one side of the mouth extends further than the other? Is the effective mouth defined by a line drawn across from one side to the other, or is the effective mouth somewhere further back in the horn? For example, on a line drawn from the shortest edge to the nearest point on the other wall? A rather extreme example is given in the Cornu horn threads, such as the plan in this post:



Related to this, has anyone noticed whether the presence of a surface (floor or wall) against one side of the mouth of a horn significantly affects the tuning (apart from the SPL reinforcement)?
Be Yamamura - back in the 90s - designed several such horns and they were very, very good. He made them from cork! The ones I heard were front loading Lowther drivers, and I had some at home for a while. I had a pair which front loaded a smaller driver and lived happily with these and a prototype stereo amp from the same designer. [The drivers were also rear loading a conventional horn]

The pic shows the first prototype....much bigger ones followed!!
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Old 10th January 2013, 04:34 AM   #12
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...
Well, itís the acoustical axial path-length exit angle, so as the hornís output begins collapsing [beaming] in the HF, this is its center-line, but due to horn damping and local boundary conditions its acoustical output is falling, so not necessarily its Ďline of maximum SPLí.
Ah. That's what I meant, I just said it poorly. Thanks.

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I assume the Cornu horn is a foreshortened [truncated] one ...
Iíve never built one, but assume that itís necessary to damp down any HF coming out of them to keep from having four phantom tweeters around its perimeter. Then again, it may add some interesting ambiance. ...
Yes indeed, they are truncated. The builders have found it necessary to add significant damping in the driver chamber and sometimes in the throats of the horns. The damping has proven effective in damping the resonances and suppressing much of the HF.

I can see how the differing path length of a curved horn may matter for a HF horn, but I don't see that it will greatly affect the performance of a truncated horn intended for bass reinforcement, where you can bend, fold, spindle and mutilate the path without greatly affecting the performance in the lowest octave or so. The length matters as much as the profile, which is where we came in - how to determine the effective length of a horn with an asymmetric mouth.
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Old 10th January 2013, 04:45 AM   #13
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Originally Posted by brianco View Post
Be Yamamura - back in the 90s - designed several such horns and they were very, very good. ...
I see what you mean about the asymmetric mouth on the front horn. The effective length can probably be estimated quite accurately, according to GM:
Quote:
Its acoustical axis, path-length is in general defined by the mean of the effective short and long side of the horn, same as cutting a pipe off at an angle or carving a Karlson slot out along its length.
The problem with determining the effective length of the Cornu horn is: what's the effective length of the longer, curved side of the mouth? At what point does it cease to be considered as part of the horn mouth and begin to be a "boundary condition"?
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Old 10th January 2013, 05:58 AM   #14
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Originally Posted by Don Hills View Post
The problem with determining the effective length of the Cornu horn is: what's the effective length of the longer, curved side of the mouth? At what point does it cease to be considered as part of the horn mouth and begin to be a "boundary condition"?
My 1st guess would be where the normal to the long curve intersects the corner of the square + an end correction.

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Old 10th January 2013, 07:10 AM   #15
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That's the way I usually do it. But why would we need an end correction on a 1/4 wave resonator? We do it on Helmholtz ports, but we still calculate the port resonances using the physical length.
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Old 10th January 2013, 08:01 AM   #16
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MJK has an end correction in his model.

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Old 10th January 2013, 10:39 PM   #17
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MJK has an end correction in his model.

dave
Guess I'll have to measure the resonances of a piece of PVC pipe and compare against the calculated values...
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Old 10th January 2013, 11:52 PM   #18
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My 1st guess would be where the normal to the long curve intersects the corner of the square + an end correction.

dave
Correct: "As such, each mouth abruptly ends at the four corners plus however much the pipe end correction of each acoustically lengthens/expands them."

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