Gedlee, wouldn't perforated horns eliminate HOMs? - diyAudio
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Old 30th December 2009, 09:10 PM   #1
NV&H is offline NV&H  United States
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Default Gedlee, wouldn't perforated horns eliminate HOMs?

From what I've read, one of the major objectives of your work is to eliminate HOMs while controlling dispersion to limit early reflections.

Years ago I worked in a power station. Very, very loud environment. Telephones in the loud environments were all mounted in partially enclosed V-shaped housings lined with perforated metal behind which was sound absorption material. Open ended as they were, it was amazing how quiet it was in those things, in spite of the 130+db environment.

Similarly, waves bouncing around at tall angles inside a horn would get quickly absorbed, while the waves moving along the hard but perforated horn surface would not be so quickly absorbed, it seems to me. You may wind up with fewer dBs at the mouth, but whatever sound that made it that far would be HOM-free. And I don't see why the horn's dispersion angle would be affected either. Finding the optimum size and distribution of the sound absorbing perforations would need to be engineered, of course. I can easily see why the Altecs and JBLs not trying this back in the day, since they were interested primarily in getting maximum sound pressure, with HOM colorations being of little concern.

But since the objective of your work has been to fight inside-the-horn HOMs and early room reflections, I thought perhaps you considered this approach.

Thoughts?
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Old 31st December 2009, 03:52 PM   #2
Borat is offline Borat  United States
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Old 31st December 2009, 05:19 PM   #3
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Might work on the outside... Anything wrapping around the front radius
needs killed off anyway. On the inside, perforation probably touches off
way more tangential modes than it could ever suppress? Got no scientific
reason for saying so, just prattling at random bout something I don't know.
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Old 31st December 2009, 06:20 PM   #4
Defo is offline Defo  Norway
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Quoted for truth.
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Old 31st December 2009, 10:23 PM   #5
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HOM is an invention of a manufacturer to sell their product and make it "better.". HOM can not even be measured by the "inventor."

It is like an automobile manufacture that claims their car gives you "better" or the "best" gas mileage efficiency but it can only explained in an advertisement or "white paper" but can not be measured by anybody. Does it have a benefit? yes, to the manufacture that gets a way with it.

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Old 31st December 2009, 10:43 PM   #6
thadman is offline thadman  United States
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Quote:
Originally Posted by TrueSound View Post
HOM is an invention of a manufacturer to sell their product and make it "better.". HOM can not even be measured by the "inventor."

It is like an automobile manufacture that claims their car gives you "better" or the "best" gas mileage efficiency but it can only explained in an advertisement or "white paper" but can not be measured by anybody. Does it have a benefit? yes, to the manufacture that gets a way with it.

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Have you taken the time to actually read Dr. Geddes' research or do you simply enjoy spreading misinformation?
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Old 31st December 2009, 10:53 PM   #7
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Have you taken the time to actually read Dr. Geddes' research or do you simply enjoy spreading misinformation?
Oh, you must have missed the entire thread the manufacture/inventor had on this subject at this website where he could not measure his HOM.

Please explain how you and the manufacture measure the HOM.

The manufacturer's advertisements or white paper does not count because it does not measure HOM.

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Old 31st December 2009, 11:10 PM   #8
thadman is offline thadman  United States
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Originally Posted by TrueSound View Post
Oh, you must have missed the entire thread the manufacture/inventor had on this subject at this website where he could not measure his HOM.

Please explain how you and the manufacture measure the HOM.

The manufacturer's advertisements or white paper does not count because it does not measure HOM.

HAPPY NEW YEAR!!
You do not need to measure HOMs to know that they exist.

Visualize an oscillating piston located at the end of a cylindrical duct. Neglecting resistance at the boundaries (sides of the cylinder), the wave will propagate unimpeded. However, once the wave reaches the end of the cylinder (an impedance mismatch exists at this boundary), an abrupt transformation of the wavefront occurs. A secondary wavefront is created at this boundary, which propagates antiparallel to the original wavefront (ie a reflection). The secondary wavefront will propagate (anti-parallel to initial wavefront) unimpeded until it reaches the piston. An impedance mismatch between the fluid (air) and solid (piston) exists at this boundary, where the wave is once again partially reflected. This wavefront will propagate (parallel to the initial wavefront) unimpeded until, once again, it reaches the end of the cylinder, where it is partially reflected. This process will continue until all of the energy leaves the system.

I believe these secondary wavefronts are what Dr. Geddes' refers to as Higher Order Modes. They are a result of the reactance of the duct and interact with the primary wavefront.

Unless you can demonstrate that waveguides are purely resistive transformers, Higher Order Modes will exist.
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Last edited by thadman; 31st December 2009 at 11:17 PM.
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Old 31st December 2009, 11:28 PM   #9
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I see, you can't measure them either. Reminds me of religion. I also have never seen a peer reviewed controlled double blind test to correlate the HOM advertisement and audibility. Can you point me to one?

Quote:
Originally Posted by thadman View Post
You do not need to measure HOMs to know that they exist.

Visualize an oscillating piston located at the end of a cylindrical duct. Neglecting resistance at the boundaries (sides of the cylinder), the wave will propagate unimpeded. However, once the wave reaches the end of the cylinder (an impedance mismatch exists at this boundary), an abrupt transformation of the wavefront occurs. A secondary wavefront is created at this boundary, which propagates antiparallel to the original wavefront (ie a reflection). The secondary wavefront will propagate (anti-parallel to initial wavefront) unimpeded until it reaches the piston. An impedance mismatch between the fluid (air) and solid (piston) exists at this boundary, where the wave is once again partially reflected. This wavefront will propagate (parallel to the initial wavefront) unimpeded until, once again, it reaches the end of the cylinder, where it is partially reflected. This process will continue until all of the energy leaves the system.

I believe these secondary wavefronts are what Dr. Geddes' refers to as Higher Order Modes. They are a result of the reactance of the duct and interact with the primary wavefront.

Unless you can demonstrate that waveguides are purely resistive transformers, Higher Order Modes will exist.
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Old 1st January 2010, 12:24 AM   #10
gedlee is offline gedlee  United States
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Quote:
Originally Posted by TrueSound View Post
HOM is an invention of a manufacturer to sell their product and make it "better.". HOM can not even be measured by the "inventor."

It is like an automobile manufacture that claims their car gives you "better" or the "best" gas mileage efficiency but it can only explained in an advertisement or "white paper" but can not be measured by anybody. Does it have a benefit? yes, to the manufacture that gets a way with it.

HAPPY NEW YEAR!
Quote:
Originally Posted by TrueSound View Post
Oh, you must have missed the entire thread the manufacture/inventor had on this subject at this website where he could not measure his HOM.

Please explain how you and the manufacture measure the HOM.

The manufacturer's advertisements or white paper does not count because it does not measure HOM.
That my speakers sound better than "normal" horns is indisputable (see the reviews), there are a lot of things that I do to improve things. Do we know that5 HOMs are the whole story - no we don't and I have always said that. But HOM are not some "made up" concept they can be shown to exist theoretically and they can be measured on compression drivers, but to your point, no I have not actually taken the time to try and trace down an HOM from any other waveguide aberation. That in no way makes any of your statements correct however because "did not" which is true, is quite different than "could not" which is not true since that implies a trie and failed, which never happened.

So the misinformation here is clear - ignoring the vast amount of data that "something" makes my speakers sound better than everything else, and instead focusing - incorrectly - on a small aspect of their not having been measured directly. Your later point is irrelavent while the first point is the key.

There is, by the way, absolutely no evidence that HOMs do not exist and are not a problem. Makarski did in fact measure them for your information, but he concluded that they were a small effect, and they are, but then so id THD. The size of the effect and its perception are entirely different things.

Get your story straight next time.
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