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How to judge a Horn/Waveguide
How to judge a Horn/Waveguide
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Old 11th April 2019, 01:39 AM   #1
camplo is offline camplo
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Default How to judge a Horn/Waveguide

I have an idea, some formulas, but its not a whole picture. Can someone supply the formulas to judge a horns performance? I understand one part of the scope has to do with axial length and the other has to do with the height and width, and yet another part of whole has to do with the angle on the height or width.
just trying to get all the way there. Thanks in advance.
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Old 11th April 2019, 03:10 AM   #2
AllenB is offline AllenB  Australia
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How to judge a Horn/Waveguide
There are multiple, non-complementary aspects of a horns performance that might be of interest. If you specify what you want it becomes simpler to answer this question.
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Old 11th April 2019, 04:26 AM   #3
Robh3606 is offline Robh3606  United States
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Hello

I would say one of the most important aspects would be constant directivty horns and waveguides vs those that are not like an exponential horn.

With traditional horns the flat axial response is gained at the expense of the off axis response. The horn design uses increasing directivity to maintain a flat on axis response and the coverage angles decrease with frequency and follows the DI curve. This makes them beam at high frequencies, makes the in room response impossible to EQ without messing up the on axis response and makes placement more difficult.

With the CD horns the off axis response closely follows the on axis response over a defined angle coverage window. This makes them more EQ and placement friendly.
The issue with this type of horn is you need to EQ them for flat on axis response either using a passive network or through an active set-up with EQ/DSP.

This makes passive crossover design more challenging for these horns. I find them much better overall performers compared to older designs.

To judge differences you really need to compare them under real life conditions no set of formulas is going to tell you how they will sound and which you will prefer.

Heck I like CD you may hate them you never know!

Rob
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Old 11th April 2019, 06:07 AM   #4
cspieker is offline cspieker  United States
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Rob did a nice job explaining it, but I recently had a light bulb moment understanding horn behavior that might be helpful to you. It is an oversimplification, but it's helpful nonetheless.

Think of the horn/waveguide as a water nozzle spraying sound, BUT, the width of the spray varies with frequency.

Whatever the angle of the horn is at that wavelength will equal its spray angle.

Now on a constant directivity waveguide, the angle will be pretty similar from small wavelength (throat) area to high wavelength (mouth) area, giving all frequencies similar spray angle, but the response will drop down a lot as frequency goes higher. I believe simply because compression drivers tend to do that. If you want to help that, you tighten up the spray at those wavelengths by using a tighter spray angle for those frequencies (a tighter throat like Tractrix /exponential). It's louder on axis than a cd waveguide at high frequencies because you are simply focusing the spray more at those frequencies. Now in this way of understanding it, length of the horn is not important in and of itself, it is going to be whatever it has to be to get you to the diameter you need to equal the wavelength you want to control with your nozzle.

I realize that there are issues with this. For one, directivity and gain will fall off about an octave sooner than this suggests, but as a quick and dirty way of understanding what's going on with horns, i find it helpful.

Also very relevant is the understanding that you want a smooth exit from the mouth to avoid diffraction. Think of that as turbulence in the air as it passes over the edge of the mouth making noise similar to vehicle aerodynamics. Another imperfect analogy but easy to "get it" quickly.
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Old 11th April 2019, 12:50 PM   #5
camplo is offline camplo
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So when GM analyzed the "Goldwood GM-450PB" in my other thread he said
Quote:
just doing a quickie 'ballpark' assessment assuming wall angles are actually 90x40, then using 10^6/[angle*width], ~740 Hz x 2841 Hz with a mouth cutoff = ~246 Hz, axial length = ~372 Hz, so...
so 10^6/[angle*width] and 10^6/[angle*height] tell...I'm not sure exactly. I think its the point where the horn starts being directional? Or maybe stops being directional? I never get how to assumed the axial cutoff. The length is like 9 inches so maybe the rule is the length has to be ~half the size of the wavelength? I have no idea. Some where in my reading, I read that, a horn should be the size of the wavelength it intends to support axial length and perimeter of mouth opening...and thats only if I recall correctly what it said. So, if any knows the mathematical formulas to the judge a horns performance based on its dimensions, please share.
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Old 11th April 2019, 01:02 PM   #6
AllenB is offline AllenB  Australia
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How to judge a Horn/Waveguide
The length is not responsible for the directivity. It is the width, and assumes the wavefront shape is also correct.

The length will have an impact on the nature of axial reflections, if any.
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Old 11th April 2019, 01:10 PM   #7
Robh3606 is offline Robh3606  United States
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Here this will help you understand the 2344 spec sheet and a capture from the AES paper

2344

Rob
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File Type: jpg 2344 dimesions angle coverage.jpg (49.2 KB, 352 views)
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Old 11th April 2019, 01:33 PM   #8
cspieker is offline cspieker  United States
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Not all horn/waveguides follow the same rules for cutoff frequency, you have to look at the measurement I believe. A Tractrix horn stops being directional about an octave higher than its spec. So my 375 hz horn for example starts losing directivity at about 700hz and it is almost completely gone by 450 hz. But a more conical waveguide will lose loading earlier, correct me if im wrong.

Read this:

https://www.google.com/url?sa=t&sour...e7Li3VAlsWtewE
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Old 11th April 2019, 05:43 PM   #9
camplo is offline camplo
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The terms loading and directivity, is that the same thing? Do they coincide or can a horn load a particular frequency ,while not being so directional at that same frequency?
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Old 11th April 2019, 06:21 PM   #10
plasnu is offline plasnu  United States
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Quote:
Originally Posted by cspieker View Post
Not all horn/waveguides follow the same rules for cutoff frequency, you have to look at the measurement I believe. A Tractrix horn stops being directional about an octave higher than its spec. So my 375 hz horn for example starts losing directivity at about 700hz and it is almost completely gone by 450 hz. But a more conical waveguide will lose loading earlier, correct me if im wrong.

Read this:

https://www.google.com/url?sa=t&sour...e7Li3VAlsWtewE
I think so. IME, Waveguides and modern JBL shallow horns can go lower at the same mouth size.
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