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Why do HOMs Suck?
Why do HOMs Suck?
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Old 13th July 2019, 05:53 AM   #1
Patrick Bateman is offline Patrick Bateman  United States
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Default Why do HOMs Suck?

A dude on the FB forum was asking why horns suck, and I wanted to explain that it's mostly due to HOMs.

But when I went googling around, I couldn't find any succinct explanation of why HOMs suck.

So here goes:

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Sound travels 13.5 inches in a millisecond. Our hearing mechanism is particularly attuned to events that happen in the first few milliseconds that the sound is radiated.

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Anyone who's put a pair of speakers on a bookshelf can attest to this phenomenon. When you put a pair of speakers on a bookshelf, it isn't just that the response changes, the imaging changes also.

When you put the same speakers on a set of stands and place them a couple of feet into the room, the sound improves dramatically. It's smoother, less fatiguing, it images better. We've all heard this.

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In a conventional horn, the first millisecond or two is radiated in the "bell" of the horn.

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In a direct radiator loudspeaker, only a fraction of the first millisecond is interacting with the loudspeaker enclosure. For instance, in this Dynaudio speaker, after a fraction of a millisecond, the wavefront has radiated away from the enclosure.

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In 2019, we see a lot of speakers which occupy a "middle ground." The baffle isn't flat like a conventional direct radiator, but it's not as deep and narrow as a conventional horn.

On his website, Nelson Pass wrote the following:

Dick Olsher famously remarked that “The first watt is the most important watt.” This sentiment has also been expressed by others as “Who cares what an amplifier sounds like at 500 watts if it sounds like crap at one watt?” With this in mind, I created First Watt in 1998 as a "kitchen-table" effort, exploring unusual low power amplifiers with an emphasis on sound quality.

I think that there's something very similar that happens with loudspeakers, but instead of being conscious of the first watt, we have to be conscious of the first millisecond.

This begs the question: What is a higher order mode?

The answer is very simple: Sound travels at... the speed of sound. That's 13.5 inches per millisecond. If there's *anything* that impedes that wavefront, you have a higher order mode. The impedance could be a cabinet edge, it could be a sharp edge inside of a horn throat, a discontinuity between your tweeter and the baffle, it could be a bolt on your tweeter's faceplate that screws up the wavefront.

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There are a million things that can disturb the wavefront that's radiated from your loudspeaker. The reason that most speakers look like a bar of soap in 2019 is because manufacturers appreciate that higher order modes are a problem. Whatever it takes to get that wavefront from the loudspeaker to your ear, undisturbed, they'll do it. That might mean a spherical enclosure or a woofer that has a surround that's low profile or a tweeter baffle with recessed screws. It's all about eliminating the sharp edges and the dramatic angles that are a hallmark of old-school loudspeakers.

All of this behavior can be simulated and measured. You can create a loudspeaker in Hornresp or ABEC or Axidriver, and quickly see that even the SMALLEST aberration within and inch of a loudspeaker will lead to problems in the frequency response, the impulse response, and a noticeably negative effect on the sound quality of the loudspeaker.

Click the image to open in full size.

Click the image to open in full size.

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It's not an accident that modern speakers look like a bar of soap. No sharp edges, tons of gentle curves, very gradual angles. These smooth transitions preserve the impulse response of that first millisecond. With no sharp edges, the loudspeaker enclosure seems to disappear. The things that "clue us in" to the fact that we're listening to a loudspeaker are the sharp edges on the enclosure, or the sharp edges inside of a waveguide.

Click the image to open in full size.

Thirty years ago, horn loudspeakers looked like this. Full of sharp edges on the horn, the waveguide, and diffraction slots.

Click the image to open in full size.

And now studio monitors look like this. Gentle curves, smaller footprints, not a hard edge in sight.

Last edited by Patrick Bateman; 13th July 2019 at 06:00 AM.
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Old 13th July 2019, 07:19 AM   #2
norman bates is offline norman bates  United States
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Bar of soap.

I like that.

Nice summary.
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Old 13th July 2019, 09:22 AM   #3
andy19191 is offline andy19191  Europe
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At the risk of being a wet blanket what you are saying looks reasonably correct but your terminology is not. Assuming HOM means Higher Order Modes (why only higher order?) then diffraction from edges are not HOMs. They are effectively secondary sound sources. Diffracted sound from edges in a horn is likely to drive/energise the modes of the horn but they are different entities.
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Old 13th July 2019, 11:34 AM   #4
bentoronto is offline bentoronto  Canada
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Why do HOMs Suck?
Fussing about edge diffraction is just putting lipstick on a pig if the source is a crude cone mechanism based on the 90 yr old Rice-Kellogg model. With dome drivers a small step better. Just marketing.

If anything, a horn should in theory ease the transition of sound from the driver into the room because true horns act as transformers between the monumentally heavy cone assembly and the light-as-air air.

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Old 13th July 2019, 04:50 PM   #5
norman bates is offline norman bates  United States
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Lets get rid of homs and call them reflections ?

Hom = higher order modes


So, a reflection caused by taper rates mismatch from inside compression driver to the flare rate of the horn, a reflection caused by the mouth not rounding over nicely to the baffle, a reflection caused by a diffraction slot, a reflection caused by horn paths straight with occasional angle changes (instead of a gradually bending line), reflections caused by corners in a horn, etc.

And we have reflections (diffraction) of highs off of the speakers baffle edges.


I'm interested in the reflections of inside a phase plug compared to a dome tweet.......
My buddy (rip) djk swore by some community drivers because they had lower compression ratios.


Conical horns have received high praise but the horn mouth reflections are a problem, as is the less loading on the lower end of the horns response.

Domes may not have the magnet strength to load a horn up high.

I find it curious that Avantgarde uses some domes to load the horns instead of compression drivers.

People liked zaph's horn loaded dome project.
The orangutans use a horn loaded dome (stereophile) but i find the morel cat378 (similar has a dispersion plot too small for me).
cat378 morel - Google Search

These look nice from newell (did many tests years ago).
But a deep horn (probably tighter dispersion as you go higher).
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Old 13th July 2019, 08:24 PM   #6
Robh3606 is offline Robh3606  United States
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Quote:
But when I went googling around, I couldn't find any succinct explanation of why HOMs suck.
Well more importantly what exactly are HOM's.

I think Norman covers it.

Then you have 2 issues.

How do you measure them?

What are the thresholds for audibility?

I find it quite difficult to discuss their effects without a better understanding of their audibility and some kind of metric.

How can you compare 2 horns for HOM's without a metric.

If you are depending on looking at 2 horns visually and saying one has none and the other more sharp edges so therefore it must have more audible HOM's that's a pretty poor way to do an evaluation.

Rob
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Old 13th July 2019, 10:05 PM   #7
charlie2 is offline charlie2  England
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My opinion at this moment in time is the flare rate.

Could Homs be measured not by doing a frequency sweep but by a pulse test tone burst of all frequencies expected to be reproduced by the horn/waveguide.

Old horns do have problems as they were built to maximise bandwidth as much as possible in relation to the compression drivers built at the time, squeezing as much low frequency to the detriment of high frequency

Question for you Patrick

Do danley synergies suffer this fate

Do your meh’s Suffer this phenomenon

Are hom’s new in the technical term in hifi??? I was reading about higher order modes in literature compiled about 1970 but not speaker related as such
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Old 14th July 2019, 02:29 PM   #8
Soldermizer is offline Soldermizer  United States
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"Bar of soap.

I like that." -- for cleaner sound, perhaps? All this talk about HOMs and, alas for Dr. Geddes, curved surfaces as a remedy and not a royalty-earning foam plug in sight?


Do I detect a trend of handles to fictional movie killers in this thread?


As almost always, thanks "Patrick" for well-written posts. I continue to be awed by the amount of DIY time you can devote to this hobby, as well as writing posts here, in addition to what I assume is a very busy normal life style!
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Old 14th July 2019, 02:47 PM   #9
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by charlie2 View Post
Could Homs be measured not by doing a frequency sweep but by a pulse test tone burst of all frequencies expected to be reproduced by the horn/waveguide.
Yes. Geddes is a fan of the impulse response. I took to using this method too, and it's been useful.
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Old 14th July 2019, 10:40 PM   #10
charlie2 is offline charlie2  England
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Foam plugs

Foam plugs I tell you ten a penny cheap as chips as much as you want and where to get it............after all is said and done this a DIY speaker building forum..

Let’s not forget.......

Unless you are marketing speakers with a crap sound and need particulate foam to hide the inadequacies and charge stupid dollars n call it HOMs
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