Some time ago I presented the first article about my development of the ALO William Neile horn type and the underlying construction method. Although, most of my previous worksheets use a super ellipse for each 3D layer / spline along the horn axis. To have an alternative option for William Neile horns I have implemented the the necessary math together with evenly distributed Neile parabolas used in this context to build up the horizontal construction wave front. All William Neile (WN) horns based on the super ellipse algorithm will get the extension “SE” in their name.
By varying the Lamé exponent of the super ellipse formula many different shapes from elliptical to almost rectangular are possible. At throat everything always starts with a Lamé exponent of 2 which indicates an ideal ellipse. Of course, if major and minor axis of the ellipse are equal there will results a perfect circle at throat. If major and minor axis differ an ellipse will result. Generally, the major axis is the horizontal plane because it is intended to radiate more broad compared to the vertical plane (minor axis). For higher Lamé exponents of the super ellipse formula the resulting shape will be an almost rectangular spline but a transition function is needed to provide a smooth transition from exponent 2 to higher values along the horn axis. A very similar procedure was already used for my spherical wave horn (SWH) and JMLC worksheets.
This article is about the first making of such a horn by DonVK who much preferred the native elliptical shape and asked for my assistance to optimize a horn for his setup. Finally, we ended by with two horn of different cut-off. This article describes the making of the first smaller version.
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The second part of this article series deals with 1in4 (1.4 inch) William Neile ALO horn. This is the next logical step on my agenda. Also because the new 1in4 William Neile ALO horn is supposed to replace my current TH4001 which is a good sounding horn so the expectations are set quite high. The category of the so-called fin horns seems to me to be quite popular and their good sound properties are usually reported. However, I am of the opinion that the known underlying assumption for the construction of fin horns, especially the fin shape and arrangement, does not lead to a coherent wavefront over all frequencies at the exit of the individual channels. I have developed an improved version of different curved fin arrangement which implements equal path lengths of each channel together with the right and exact opening angle for proper exponential acoustic loading, but this is another story. So to speak the TH4001 is a good performer but has some serious design issues. The William Neile ALO horns use what I call natural dispersion instead but with it’s own limits one has to accept. What I want to say is that it is not possible to achieve such wide radiation for a very low-loading horn similar to a fin horn. In my opinion, this is not necessary at all for use in normal listening rooms. But many are somehow still of the opinion that some kind of cinema horns with extremely wide dispersion should be set up at home but these were intended to reach hundreds of people in a large cinema hall with evenly distributed sound impressions at every single seat. A slightly narrower dispersion can definitely be an advantage, especially in smaller listening rooms and when only a few listening positions in the room have to be considered. The intrinsic problems of fin horns, which are related to the the individual channels (shape, length, arrangement), such as cavity resonances or excitations or the suboptimal addition to a coherent wave front, cannot occur with a natural dispersion horn. For example, anyone who doubts that e.g. 30 degrees of even radiation can be sufficient should mark this angle with two strings on the floor from each loudspeaker to the listening position and preferably with the loudspeakers angled slightly inwards and then look at the area of even radiation delimited by the strings on the floor. My 1in4 William Neile ALO horn will achieve about twice as wide dispersion in the horizontal plane.
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It is quite a long time ago since I wrote my last article about William Neile Horns. There was definitely considerable progress exploring and refining this new type of horns, but unfortunately the lack of spare time did not allow it to be documented. The attentive reader will certainly not have missed the fact that one of my fundamental goals is to achieve a good acoustic horn loading almost down to the desired cut-off, but the William Neil horns presented so far behave more or less like classic waveguides with regard to horn loading as there is no visible cut-off, instead of this a very slight roll off of the radiation impedance towards low frequencies happened. Although, some people might in fact prefer the loading properties of my first William Neile horns.
This article series will deal with acoustic loading optimized (ALO) William Neile horns which means that acoustic loading should be pushed to the most reasonable level down to the desired cut-of frequency but at the same time keeping the resonances / reflections of a classical exponential wave front surface area expansion to a minimum. I am aware that there is a controversial view about horn loading in the community. Some say that horn loading is almost unimportant as you can simply push the driver where you need the output. Directivity control should be the major design objective of a horn . I have a different view on this issue as generally without proper horn loading you need to push the driver more and more towards lower frequencies where it hurts most as the excursion doubles with each octave towards lower frequencies and when there is any need to push the driver even more – it might work technically – the excursion needs are even larger, so this is never the best solution. Horn loaded compression drivers are an ideal combination with low power single ended tube amplifiers using a passive crossover – well, usually there is almost no output power left to push anything. The speaker has to sound great with the first Watt of output power and even with much less. So with more low frequency loading from the horn you get more SPL in that region and less output power means less excursion for lower frequencies. My experience is that a compression driver used within the acoustic loading optimized frequency band of a horn (resistive loading) will give you much better micro dynamics with an open and effortless sound. What I intend with new ALO William Neile horns is to combine good acoustic loading characteristics and good directivity control especially for the horizontal plane. This is not an easy task but as we will see that it is possible.
The BEM simulations of the final ALO William Neile horn designs were indeed so promising that is is planned to have the first prototypes made since the inherit property of the Neile parabola obviously provides the capability to obtain very good directivity control especially with smooth transitions along the frequency pass band of the horn. This property combined with an exponential throat section and an appropriate mouth termination flare there is a valid prospect of an excellent horn design.
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