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Old 9th January 2013, 09:21 AM   #1
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Default Asymmetric Horn Mouths

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:

Quote:
Originally Posted by xrk971 View 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)?
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Old 9th January 2013, 09:59 AM   #2
xrk971 is offline xrk971  United States
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Here is an example of what Don is referring to.
Ever think of building a Cornu Spiral horn? Now you can!

The attached image shows one of the first spiral horns I made which was inspired by the Cornu plan as I heavily modified the paths and spacing. Also attached is a plan I calculated using the true Cornu spiral for a 24 inch horn.

Last edited by xrk971; 9th January 2013 at 10:11 AM.
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Old 9th January 2013, 12:31 PM   #3
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ive often wondered the same, and also what the effect would be in a mitred cut on the end of a straight pipe. Such as a vent with a mitred or tangential cut on the outer end, or a pipe TL with a tangential open end.
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Old 9th January 2013, 02:27 PM   #4
GM is offline GM  United States
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Yes, Bell Labs/W.E. did extensive testing [including anechoic] in the earliest days of sound system design on virtually every aspect of it and why there’s been so little truly new technology and/or improvement over the decades except in the materials and manufacturing sectors.

Of course local boundary conditions affect the horn’s design, performance!

Think of the horn mouth as being a large point source ‘driver’ with an effective ‘Sd’ [acoustical mouth area] of its physical ‘piston’ area plus its local boundary conditions, so adding boundaries increases both the horn’s effective ‘Sd’ and acoustical path-length and why a horn can be truncated quite a bit if folded up into a corner like the Klipschhorn.

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. Note the dashed line painted on this early W.E. folded horn to aid the installers in ‘pointing’ it at the desired ‘sweet spot’.

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Old 9th January 2013, 09:33 PM   #5
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Thanks for the insight, GM (can I call you G?)
I would classify those horns as having symmetrical mouths. If the dotted line is meant to indicate the "line of maximum SPL", it looks to me as if the effective mouth at HF is somewhat back in the horn.

Looking at xrk971's diagrams, the situation is complex. Each horn has a "short, straight" side and a "long, curved" side. At some point the "long, curved" side ceases to become a horn side and becomes a "local boundary condition". the question is, what's that point?

There's a further complication in that these horns are designed to be mounted on a wall, which adds another boundary condition, but I'm conveniently ignoring that for now...

I'll code up a model for the ripple tank simulator. I'll also see if this can be modeled in ABEC, I've been waiting for a reason to learn it.

Last edited by Don Hills; 9th January 2013 at 09:37 PM.
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Old 9th January 2013, 09:50 PM   #6
jrenkin is offline jrenkin  United States
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Those are some fun looking horns!

The mouths appear symmetric, but they are not really. What the real issue GM is pointing out is not the symmetry of the mouth, it is the symmetry of length of the different sides of the horn. Which is the same as the symmetry of the horn mouth. In this fancy curled horn that has an "inner" short wall and an "outer" long wall, same as an "asymmetric" mouth. It is the measured path not the appearance of the mouth that matters.
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Old 9th January 2013, 10:11 PM   #7
xrk971 is offline xrk971  United States
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Jrenkin,
The inner wall (curved one) is actually the long wall. These horns are a lot of fun, and they sound great. Very easy to build if you are interested in trying it out. All the 1 dim models utilize a central average axial distance with corresponding cross sectional area. In the context of the 1 dim model, that is all that matters. Of course, that will not accurately model what is happening near the mouth when it expands fast like this, a 2 dim time varying model is needed. Now we are getting into computational fluid dynamics.
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Old 9th January 2013, 10:18 PM   #8
jrenkin is offline jrenkin  United States
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Quote:
Originally Posted by xrk971 View Post
Jrenkin,
The inner wall (curved one) is actually the long wall. These horns are a lot of fun, and they sound great. Very easy to build if you are interested in trying it out. All the 1 dim models utilize a central average axial distance with corresponding cross sectional area. In the context of the 1 dim model, that is all that matters. Of course, that will not accurately model what is happening near the mouth when it expands fast like this, a 2 dim time varying model is needed. Now we are getting into computational fluid dynamics.
I fell into the visual trap, but at least I had the concept!

Where do I look to earn how to build one? I am sure the WAF is high..
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Old 9th January 2013, 10:25 PM   #9
xrk971 is offline xrk971  United States
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Jrenkin,
There two threads that discuss this. The one that started it has lots of good info but is long.
Foam Core Board Speaker Enclosures?

Then there is one specific to the Cornu:
Ever think of building a Cornu Spiral horn? Now you can!
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Old 9th January 2013, 10:51 PM   #10
GM is offline GM  United States
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Quote:
Originally Posted by Don Hills View Post
Thanks for the insight, GM (can I call you G?)
I would classify those horns as having symmetrical mouths. If the dotted line is meant to indicate the "line of maximum SPL", it looks to me as if the effective mouth at HF is somewhat back in the horn.

Looking at xrk971's diagrams, the situation is complex. Each horn has a "short, straight" side and a "long, curved" side. At some point the "long, curved" side ceases to become a horn side and becomes a "local boundary condition". the question is, what's that point?

There's a further complication in that these horns are designed to be mounted on a wall, which adds another boundary condition, but I'm conveniently ignoring that for now...

I'll code up a model for the ripple tank simulator. I'll also see if this can be modeled in ABEC, I've been waiting for a reason to learn it.
You’re welcome!

I guess, for sure I’ve been called far worse!

Well, it’s the acoustical axial path-length exit angle, so as the horn’s 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 ‘line of maximum SPL’.

Correct, the frequencies increase as one moves back in time to the beginning of the wave-form [moving towards the throat], so all the HF response is back at the throat stub.

I assume the Cornu horn is a foreshortened [truncated] one since it’s designed to be wall mounted, so where the WLs are long compared to its perimeter, the individual horns sum as one and slowly become individual point sources, same as in a stacked speaker array. As such, each mouth abruptly ends at the four corners plus however much the pipe end correction of each acoustically lengthens/expands them.

I’ve never built one, but assume that it’s 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.

When mounted to a wall, it’s in 2pi space, so the horn loads down around an extra half octave due to doubling the effective mouth areas via the mirror effect.

GM
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