How is HOM measured?

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TrueSound

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Joined 2009
#1
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?
 
#6
TrueSound said:


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?
Click to expand...
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/showthread.php?postid=1774677#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.
 
#7
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.

[Elusive little beasties.... 😉 ]
 
#8
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?
 
#9
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.
 
#12
I think i read every HOM related thread here and no exact measurement method was ever mentioned. That doesnt mean that there is none, but rather that noone ever cared to try it. Current state of things is "listen to the device and judge for yourself if you need it".

From gedlees perspective, there is no real use in designing one, since his OS waveguides are the mathematically optimal solution, so even if he measures them, it wont make a difference for his designs.
 
#13
What if you used a sine chirp of a high frequency only one cycle long? Maybe then you could see the reflected sines coming in just after the first?

Anyone know if SE can do something like this? I've never used the analog measurements section.

The phenomena is grounded in waveguide theory and wasn't it about a year and half ago that someone had posted a sim from hornresp or some horn FEM that showed HOMs being generated? Being able to predict them should shed light on how to measure them.
 
#16
I don't think so. Relefections at the mouth are more likely the cause.
Click to expand...

I have read the Geddes waveguide thread, the horn vs waveguide thread and in summary the Reflections is the HOM which makes that unwanted Horn sound.

The OS waveguide is designed to have minimum reflection issues and more importantly the foam insert is the key. I have read several cases where people have experimented with the foam idea and it has helped with the horn issue. Geddes even indicates that its the foam that is very important, hence the patent on it!
 
#17
I would agree that OS initial expansion is important, however, bear in mind that the solution is based on an initial plane wave. In my simulations that I have posted in the Geddess thread, it seems that once you start having a compression factor prior to the throat, there is no way the plane wave can be maintained throught the bandwidth of the design.
 
#18
Why is it so difficult to develop a concise method to measure? Because they get mixed with diffraction and manifest as common ripple? Is it like those atomic particle issues that can be predicted but are obscure to get caught?😎
 
#19
Based on what I've read, this is how HOMs work. And how the foam reduces them.

homs.jpg


Pardon the crude drawing, I knocked it out in 15 minutes.
 
#20
HOMs are probably part of the problem. But I don't think the whole horn honk thing. The mouth termination makes a big difference - I know - I've done tests. IIRC, Geddes talks about HOM mostly in the horn thoat. I'm sure both contribute. How each contributes should be dependant on the design of the horn.
 
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