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.
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.
In two previous posts I presented my PETF algorithm and JMLC inspired horn calculator. Assuming that many of my readers are familiar with the native JMLC horn performance like loading or radiation polar it should be a common acceptable consensus that JMLC horns do not belong to the so called constant directivity (CD) category. Towards higher frequencies they tend to slightly beam which is because of the curved horn walls. This might not be an issue for some applications or some people might even like this behaviour but as general rule of thumb the lower the horn cut-off value the longer the horn profile will be and the smaller the initial opening angle both causing an increasing tendency to narrow the dispersion of higher frequencies. A more focused dispersion of higher frequencies might be an advantage in small environments if it is fairly constant or if the intention was to compensate the natural roll-off of most compression drivers but generally a design goal of wider dispersion is one of my personal preferences. More precisely, one of my main goals is to find a good compromise between good horn loading and good directivity control.
There can be found some BEM simulation examples for round JMLC horns in the web that clearly show the increasingly more narrowed dispersion towards higher frequencies especially for the lower loading versions with 350Hz or lower cut-off. On the opposite JMLC horns shine if the target design intention was mainly a nearly perfect horn loading down to the desired cut-off frequency or by looking at the smoothness of radiated wave fronts when the formalism inherited roll-back is present.
I already presented that the PETF algorithm produces a faster opening of a horn profile while straightening the horn walls. In this article I will investigate what horn properties we can expect by applying PETF to a given profile.