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Old 31st May 2009, 10:08 PM   #1
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Default How is HOM measured?

I see several references in this forum where horns or "waveguides" are considered inferior or bad because of "hom" .

I asked one of the participants that refers to it all the time how it is measured and for some data showing it. They never replied so I will ask the group.

How do you measure HOM in your horns and waveguides?

Do you have the data on the horns and waveguides showing the HOM?

I checked the JBL website and found no reference to it, and when I do a search I keep coming up with "Gedlee" but see no data on how it is measured. I asked "Gedlee" here but he has not responded.

Are there any successful/reputable horn or waveguide manufactures that show how they measure HOM? What level is considered good or bad? Is there a correlation with the measured HOM and perception of sound quality (like IM distortion) with reproduction of sound?

Do any manufactures or hobbyists have the data and method of measuring and comparing their horn's HOM to others?
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Old 31st May 2009, 10:47 PM   #2
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Old 1st June 2009, 02:03 AM   #3
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Quote:
Originally posted by soongsc
soongsc - ?????????????

Have you a way to measure HOM?
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Old 1st June 2009, 02:16 AM   #4
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Quote:
Originally posted by TrueSound


soongsc - ?????????????

Have you a way to measure HOM?
I sent a smiley because no single method will explicitly indicate the High Order Modes, nor the amplitude.
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Old 1st June 2009, 02:21 AM   #5
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Quote:
Originally posted by soongsc

I sent a smiley because no single method will explicitly indicate the High Order Modes, nor the amplitude.
So where is the data you concluded that from? What series of methods are used? Do you measure HOM? Can you point me to the data?
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Old 1st June 2009, 08:01 AM   #6
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Quote:
Originally posted by TrueSound


So where is the data you concluded that from? What series of methods are used? Do you measure HOM? Can you point me to the data?
Basically because nobody has done it. Si it's not that simple. Where is my view that I openly expressed.
http://www.diyaudio.com/forums/showt...77#post1774677
I'm sure there are lots more issues involved. But I would like to see work from others before I go deeper into this issue.
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Old 1st June 2009, 08:18 AM   #7
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HOMs are hypothetical, with a basis in waveguide theory. That's not to say they're not real, merely that neither their existence nor their significance is established. The best evidence appears to be that installing open-cell reticulated foam in a horn or waveguide improves the perceived sonic quality, though how that operates is not well understood, either.

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Old 1st June 2009, 09:42 AM   #8
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From it's description, since the HOM is delayed from the original wave, I would think they would be relative easy to record by taking fast snapshots of an impulse of the horn at it's mouth. Like viewing a standing wave in a room but in a smaller space and at a higher frequency.

As far the foam goes, it seems it either slows the delay down (it can't speed it up) delaying it more or it lowers the amplitude of the delayed waves. I'm not sure how this can be done without lowering the amplitude of the initial wave or delaying the initial wave though.

So there is no data or comparisons among horns on this?
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Old 1st June 2009, 11:15 AM   #9
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To my understanding (according to an explanation I have read here on forum), the foam works because HOMs are traveling more in the radial direction, as opposed to the direct radiation which travel axially, therefore HOMs are absorbed more because they travel longer in the foam.

Intuitively, I would think that some sort of "anisotropic" foam (more permissive in the axial direction) should be better, if it can be done (it would be in principle similar to the multi-cell horns, just with a lot of cells)

As to the measurability, there is an ongoing discussion on another thread here: it does not seems straightforward to separate HOMs in a measurement from the rest.
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Old 1st June 2009, 11:24 AM   #10
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To the measurability, I meant this post:

http://www.diyaudio.com/forums/showt...43#post1841943

Granted, I have also not seen any concrete example and I would not know how to "separate the non-minumum phase part of the response" as suggested above.
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