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Horns and waveguides 101
Horns and waveguides 101
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Old 17th April 2019, 10:11 AM   #1
marco_gea is offline marco_gea  Italy
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Default Horns and waveguides 101

I saw a number of recent threads asking some pretty fundamental questions about horns and waveguides and how the various types differ.

The following is my attempt to provide some guidance.

There are fundamentally SIX types of horns that you can use, and they all have their pro’s and con’s.


1) VERY old school symmetrical round or square exponential/hypex horns. Examples of these would be the early WE ones and the ones still used in those Japanese Goto, Ale, etc. Installations. These horns (i) load the drivers well down to the horn’s cutoff, and (ii) equalize the compression driver’s natural high-frequency mass-induced roll-off at the high frequencies, resulting in pretty flat on axis, but they beam a lot. They also lead to some pretty serious reflections at the mouth, unless they are either used only above 2-3x cutoff, and/or are improved with some suitable additional round-over at the mouth. The axi-symmetric JMLC horns fall into this camp, and they are arguably the best at addressing the mouth reflection issue.

2) Old school exponential/hypex radial horns. Examples of these would be the old Altec “sectoral” horns, like the 511 and 811, or the Fostex radials (H420, H320, H220, H400, H300, etc.). These still achieve the same (i) and (ii) feats above, but they manage to also yield almost constant directivity on the Horizontal plane over much of their bandwidth (at the expense of vertical coverage, which is monotonically decreasing and even narrower overall). The more “classic” radials also have some issues with internal reflections and diffraction, because of the abrupt profile discontinuity at the throat. But, these latter issues can be mitigated by clever design improvements, as done in the Yuichi Arai horns, as well as in several other similar Japanese wooden horns like those by Yamamoto, Ta-On, GT Sound, etc. I PERSONALLY CONSIDER THESE TO BE THE BEST OVERALL COMPROMISE FOR HOME HI-FI, but of course I’m aware that it’s all a matter of picking one’s favourite trade-offs, and that others may disagree, and for very valid reasons.

3) Tractrix horns. These tend to be very similar to n. 1), but with reduced mouth reflections, and poorer driver loading towards the lower end of their bandwidth. The main problem I have with these is that the theory on which they are based is very shaky. In simple terms, the tractrix profile “looks” nice but there is no sound physical justification for it whatsoever. So I don’t consider it a very credible contender, especially given that the JMLC horns (see 1) tend to solve the same issues and offer improved loading.

4) Conical horns. These are essentially simple “funnels” with straight walls that simply constrain the driver’s radiation into a solid angle of choice. There are two BIG problems with them, though: (i) they do NOT load the drivers effectively at all, resulting in very poor low frequency extension even in the case of very large horns, and (ii) they present the worst possible abrupt discontinuities at the throat and at the mouth, resulting in awful reflections and diffraction. In other words, in my humble opinion, why bother?

5) Modern “constant directivity” horns. JBL and Altec were the first to develop these in the 70s/early 80s, with the aim of ensuring a more homogenous coverage of the venues served by PA systems in Pro sound applications. The main problem with these early efforts was that the profile was decomposed into two sections: a very short expo/hypex throat section, expanding in only one dimension, and feeding a second conical section through a diffraction slot. While this achieved the desired constant directivity, while also giving marginally better loading than a simple conical horn, there were serious drawbacks in terms of (i) diffraction, and (ii) still poor loading nonetheless. The JBL 2385 is a typical example of these horns. All the later JBL horns use in their home products such as the K2 S9800, S9900, etc. still essentially belong to this camp, but they backtracked a little on the diffraction slot, smoothening it to reduce the severity of the effect, and in so doing also giving up a bit of directivity control...

6) Even more modern “constant directivity” waveguides. These are the result of a radical re-thinking of the purpose of having horns in the first place. Basically, their rationale is that today power is cheap and we don’t really need efficiency and loading any more; all we should aim for is directivity control, AND the least possible amounts of diffraction. The best examples of these are Earl Geddes’ OS waveguides. They are essentially conical over most of their expansion, but with an ideal, mathematically-derived throat section that transitions from the planar wavefronts at the compression driver exit to the spherical wavefronts within the conical part of the waveguide. Loading is still very poor, but at least they are (very) good at something, I.e. diffraction minimization. To me, if you buy into the argument that directivity control is essentially all that matters, these are a much better proposition than the “constant directivity” designs at point 5) above.

So, there you have it. As I said many times already... pick your poison! ��

Marco

Last edited by marco_gea; 17th April 2019 at 10:14 AM.
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Old 17th April 2019, 10:55 AM   #2
Sub Sonic is offline Sub Sonic  Australia
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Just what I needed, thank you!!!
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Old 17th April 2019, 12:40 PM   #3
lowmass is offline lowmass  United States
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Marco, can you talk a bit more about "loading" ?

Im not sure I have enough understanding here. I have been doing some work on waveguides used with small ribbons. I understand its not exactly the same as a compression driver etc BUT I get quite usable lower freq sensativity increase with these and thought this was a result, in part anyway, from "loading". Or is it simply doing the "directivity" thing and thus the in window lower freq sensitivity increase.

At the moment these small ( 75mm long ) ribbons are being used down to 800hz in the waveguide at 95 db sens. ( they are used to about 1.2khz as direct radiators at 88 db sens) .There +- 1db from 800 to 10khz BUT then roll off to abot -7db at 20K

Last edited by lowmass; 17th April 2019 at 12:43 PM.
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Old 17th April 2019, 01:03 PM   #4
jzagaja is offline jzagaja  Poland
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Horns and waveguides 101
Marco what about patent 2,690,231? :-)
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Old Yesterday, 09:50 AM   #5
marco_gea is offline marco_gea  Italy
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Quote:
Originally Posted by jzagaja View Post
Marco what about patent 2,690,231? :-)
That is certainly an interesting early attempt to control directivity.
In practice, it would still fall under category 5), though.

Marco
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Old Yesterday, 07:06 PM   #6
jzagaja is offline jzagaja  Poland
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Horns and waveguides 101
Observations from my friends:

1) Waveguides are unable provide "detail" at HF - maraca instruments, 7-8kHz. E.g. BC DE360 and Marcel Batik's STH 12" wavguide.
2) Minphase horn (horns-diy.pl) is very good tweeter - but very big. It is something between horn and OS waveguide. Hope author is still here.
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