Distance between compression driver / horn and mid

Hello,

Im in the process of designing some 1.4 + 12 x 2 (horn at the top, with both mids below it) reflex mid-tops and due to the adding of a text logo and design in general there could be around 85mm of space from the bottom very of the horn to the top of the midrange chassis, im just wondering how much impact this would have on sound? The mids are 18sound 12nd710s and the horn is a faital lth-142 loaded with a faital hf146 and ill probably but crossing around the 1.4k mark

I know the ideal placement would be close as possible to get more of a point source, but im willing to make a little compromise to sound for the aesthetic side of things.

Im just wondering how much of a concern to sound around the given distance between them would have?

I am not quite sure what will happen when we'll get near edge of cone or horn mouth, so on following text I focus only on center to center distance, which can be assumed to be worst case scenario, also I assume both drivers have same phase response:

In an ideal world you would want two sources to be at least within 1/3 or even 1/4 of wavelength apart. Wavelength in this question would be the crossover frequency, assuming the acoustics crossover to be same as electronic, 1400Hz. That is the frequency where two sources will have most overlap going on. Moving away from that point in either direction, other source will get quieter and thus won't affect the summation that much.

Full 1400Hz wavelength is 24cm. Having center to center spacing of 8cm (1/3 wavelength) will be impossible with typical arragnment of say, 12" cone with 32cm driver diameter and let's say 20cm HF horn. If those two sources were within 1/3 wavelength away from each other, they would sum positively on full 180 degree vertical front field of that loudspeaker. Of course cone beaming and horn dispersion characteristics come to play here but that's not the point right now.

If these sources were put right next to each other, center to center spacing would be 26cm (32/2+20/2). This means that measured at 90 degree off axis vertical, these two sources would emit sound almost exactly wavelength apart and would show some kind of summation behaviour, depending on signal. I don't bother drawing or calculating triangles right now but I assume that somewhere around 40-70 degree off axis vertical they would be half a wavelength off, so that's where you would get phase cancellation at the crossover frequency.

This is somewhat academic with typcal PA loudspeakers that have something like 40-60 degree nominal total vertical dispersion anyway.

Actually I just bothered to inspect your example

12" chassis radius = 16cm
Space between 12" and horn = 8,5cm
Horn height = 24cm, center to lowest point = 12cm
12" to HF center to center distance = 16+8,5+12 = 36,5

With these measurements you will get cancellation at crossover frequency around 20-30 degrees off axis vertical. Time difference between HF and 12" will affect this and so do dispersion behaviour of both sources. it will also be different going down or up the vertical axis.

There might be few brain farts somewhere, I should be sleeping right now and not thinking about these things.

Teunos you are more scientific than me, would you agree with my post? Wrote it half sleep

What do you mean? "How much?" How would you rate it? All I'm saying there will be cancellation lobes on vertical axis not that far off from straight on axis. If you want to push those cancellations further away from on axis, you need to have those sources closer to each other. When listening on axis, you will most likely not notice much difference in having sources as close to each other as compared to 85mm gap between. Some tiny "image" difference could be heard I guess but you never know until you try.

Ok, so I use horns and totally separate comp boxes for treble. I keep them as close as possible in the stack but they could be closer if the units did not have separate walls, and feet and all the stacking configurations vary. You do want to be as close as possible, but the further apart they are, the further back I need to be before they sound normal. Close up, they separate, and you hear treble over here and mid over here. I would call it a pretty small problem, but that is what I find. Keep them as close as possible, but scientifically, the treble is almost always a little too far from the center of a medium-large format mid driver, especially if it is hornloaded, without doing like a coaxial arrangement of some sort.

Edited by b grade - 24 September 2016 at 1:37pm

@b grade

Thanks man,

well the design im making will mean that the 2 x 12 section, logo, and horn section are actually all seperate, seperate baffles just joined together by some smaller parts of ply on the top and bottom of the logo section (glued and screwed), they will look kinda seperated which imo will look 'cool', but anyways, regarding prototyping, the beauty of this design means that before joining the horn section, logo section and 2x12 section together, i could simply put the horn straight on top of the 2x12s and use parts of thin sheet to keep increasing the distance and see what i make of it, that way the sections could be used for the final model and the prototype, the awkward thing is i currently have no dsp or amps or anything yet.

Im glad you consider what you said a small problem, its up to me to do what i want but im just so inexperienced im just trying to hear some opinions, just by being logical it did occur to me that the closer to the speakers you are the more difference the distance makes, and as stated this will be a system used for smaller crowds


Edited by Cheif_Stringer_ - 24 September 2016 at 2:04pm