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The HOMster!  (or How I Learned How to Fix a Horn)
The HOMster!  (or How I Learned How to Fix a Horn)
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Old 9th September 2009, 04:15 PM   #1
Patrick Bateman is offline Patrick Bateman  United States
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Default The HOMster! (or How I Learned How to Fix a Horn)

There's been a great deal of discussion about diffraction and higher order modes on diyaudio lately. In a nutshell, it appears that diffraction may be more offensive than harmonic distortion. At the very least, diffraction muddies the soundstage by creating unnatural soundstage cues.

I have personally experimented with a number of the treatments used by Dr Geddes to treat diffraction. I started a few years back by applying HOM-reducing foam to a set of tractrix horns, built a few oblate spheroidal waveguides, and purchased a set of Gedlee Summas.

Subjectively, I noticed that the better waveguides were less affected by the treatments that Geddes prescribes. For instance, the improvement of the foam was less audible with an OS waveguide than with a tractrix horn.

I was curious to investigate this further, so I took a pair of the craziest horns that I have, gave them a serious set of tweaks, then measured and listened.

I hope you find the results as enlightening as I did!
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Old 9th September 2009, 04:19 PM   #2
Patrick Bateman is offline Patrick Bateman  United States
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Before we dig into this, I should define what a "HOM" is. According to Geddes:

"Higher Order Mode, its a term that I coined to define waves that propagate in a waveguide that do not go down the axis, but travel by bouncing off of the walls. They are not predicted by the Horn Equation, so most people didn't even know that they existed (I was the first person to hypothesize there existance). The Waveguide Theory predicts them, and low and behold, it turns out that they are quite significant to audibility. Minimizing them yields a far better sound quality. But with "horns" its not possible to minimize them because you don't know what to do - the equations aren't rigorous enough to predict them so they are simply ignored."

http://www.diyaudio.com/forums/showp...&postcount=106
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Old 9th September 2009, 04:36 PM   #3
Patrick Bateman is offline Patrick Bateman  United States
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Based on the definition from the previous post, if someone wanted to create gobs and GOBS of homs, here are a few ways to do it:
  • Use a horn that has a diffraction throat. The diffraction slot improves the directivity of the horn, but at the expense of sound quality.
  • Use a horn that is poorly terminated. For example, a horn that is poorly mated to the baffle.
  • Even worse, use a horn that doesn't even have a baffle. That will create a ton of HOMs.
  • HOMs are created when the wave bounces off the walls. Therefore, a horn with a mouth that isn't equidistant from the throat will have a lot of HOMs. In other words, a horn that doesn't have a round mouth.
  • HOMs are cumulative, so if you really wanted to create a HOMster(tm), combine all of the previous things!

Ladies and gentlemen, I present the HOMster:

Click the image to open in full size.

This is a horn that has a diffraction slot, partial termination, and a rectangular mouth.

Let's tweak this thing and see if we can make it better.
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Old 9th September 2009, 04:41 PM   #4
Patrick Bateman is offline Patrick Bateman  United States
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After looking at the pic above, a lot of you are probably wondering what the hell that thing is. It's a horn for a car. I bought a set a few years back. As I alluded in an earlier post, I noticed that the treatments espoused by Geddes seemed to work better on horns than on waveguides. My theory is that horns have a ton of HOMs, therefore the improvement is more audible.

I am not aware of a way to measure HOMs. Having said that, my Summas image like crazy, and I believe the treatments which reduce HOMs also improve imaging. Though I can't measure HOMs, I *can* measure on and off axis response.

So here's the question:

Will the tweaks improve the on and off axis response?

If there is mesurable improvement, it should correlate with an improved soundstage, since soundstaging is improved dramatically when the off-axis response is consistent with the on-axis response.

Let's find out...

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Old 9th September 2009, 04:42 PM   #5
Pano is online now Pano  United States
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The HOMster!  (or How I Learned How to Fix a Horn)
A "Car Horn".

But where is the diffraction slot?
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Old 9th September 2009, 04:45 PM   #6
Patrick Bateman is offline Patrick Bateman  United States
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Quote:
Originally Posted by panomaniac View Post
A "Car Horn".

But where is the diffraction slot?
There's a vertical diffraction slot in the horn, about two inches forward of the throat. It appears to have a dramatic effect on the power response. The off-axis response is remarkably consistent considering the horn is dramatically asymmetrical.
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Old 9th September 2009, 04:57 PM   #7
Patrick Bateman is offline Patrick Bateman  United States
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Here's the frequency response of The HOMster, in my car, completely untreated. It was measured at 0, 15, 30 and 45 degrees with the microphone at ear level, and the HOMster mounted under the passenger side of the dash. In this mounting position the dash forms a baffle for the HOMster at the top, but it's basicall unbaffled on the bottom. IMHO the lack of a baffle will create tons of HOMs. Let's find out...

Click the image to open in full size.
In the 0 degree (purple) curve, we see a deep notch at 2khz. Spanning almost an octave and 8db deep, it will be difficult to EQ out. Even worse, it begins to move as you get off axis. By the time you're 30 degrees off-axis the notch is only 4db deep, less than a quarter of an octave wide, and higher in frequency.

You could look at this two ways. The good news is that the horn will sound better off axis than on-axis. In a car, this is A Very Good thing. OTOH, the speakers that image the best have a consistent response on axis and off. That doesn't mean that the response must be the same level, but the curves should be consistent.

Can we improve this?

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Old 9th September 2009, 05:03 PM   #8
Patrick Bateman is offline Patrick Bateman  United States
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Here's what the horn looks like mounted in the car. I took this pic from their website.

Click the image to open in full size.

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Old 9th September 2009, 05:10 PM   #9
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Are you also going to measure the directivity of the device?
If a device relies on diffraction to control its directivity, then maybe adding foam could affect its directivity.
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Old 9th September 2009, 05:19 PM   #10
Patrick Bateman is offline Patrick Bateman  United States
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The use of a roundover on a speaker or a horn should improve the off axis response. The reason is that the sound from the radiator diffracts off the edges, creating secondary sound sources delayed in time. Those secondary sources muddy the image. Reflections can be a significant fraction of the energy, so their affect on soundstaging is dramatic. Because the effect is delayed in time, it also reduces intelligibility.

One of the things that I love about my Summas is their ability to retrieve details in the mix that are inaudible over conventional speakers. For instance, I'll listen to a track and note that there's a background singer that I'd never noticed before, and it's also a champ at differentiating various instruments in the mix.

Can a simple roundover bring some of this goodness to The HOMster?

To find out, I took a piece of 2" PVC pipe, cut it in half, and duct taped it to the *bottom* of The HOMster. I didn't bother with the top, since it's flush with the dash. I didn't bother with the left or right at this stage either.

Here's the measured response with and without treatment:


Click the image to open in full size.

As above, the horn is measured at 0, 15, 30 and 45 degrees. Both measurements were performed at the same voltage level. In the graph I offset the second set of measurements by 10db. Ignore the SPL levels; I am too lazy to measure the voltage so I simply adjust the volume to generate a clean measurement.

So what do you think?

Here's what I notice:

  • The on-axis notch at 2khz is wider and shallower.
  • The peak at 2800hz is reduced by 3db
  • After treatment, the 0 degree curve and the 15 degree curve match a lot better. (zero is purple, 15 is red.)
  • The treated horn is more consistent on axis and off, particularly at 0 and 15 degrees.
  • Though the 45 degree curve (yellow) isn't as good as the 30 degree curve (orange), it's better with the roundover than without.

IMHO, these are great results for a dollar worth of PVC and a bit of duct tape. I'd love to see someone try this with a conventional loudspeaker.

But I'll take it to the next level anyways... In the next step we'll tweak it further.
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