Originally Posted by JackNC
I had seen your thread over at DIYA but didn't put 2 and 2 together here. Why not a SEOS 18 in the middle instead of a KEF?
I'm going in the same direction as you with WMTMW but crossing over per the D'Appolito criterion, where CTC distance = 2/3 lambda, which works out to 950 Hz and 169 Hz. I don't understand the difference between expanding line array and D'App. I bought SoundEasy hoping it would predict in room polars for me but haven't gotten very far with it yet.
In my design T is a SEOS18 and the Ms are offset driver midhorns. So far at least, I have the T at ear height instead of midwall. I see from the geometry that if I move it to half wall like yours, its well within the MTM vertical beamwidth. I'm pondering the consequences of that. I would get more volume for the woofers, which would be nice but I wouldn't want to force the crossover to the woofers any lower. I might be able to get them to 2' and 6' with benefits in cancelling some modes....
Interesting design. How does SoundEasy predict polars? Can you specify drivers as coaxials or horns? How does it account for the SEOS horn, for example?
The KEF is a coaxial unit, i.e., no lobing between the mid and tweeter. The hope is that the M to W CTC can be reduced to less than 1/2 wavelength and lobing will mostly be eliminated. The response will be even in the vertical direction. With the SEOS-18, it will be a struggle to eliminate the vertical lobes because the width of the horn in the vertical direction is usually larger than 1/2 wavelength at the crossover frequency.
Regarding the expanding line array, here's Dave's explanation:
"It was important to me that I achieve a system with smoother directivity and without the usual response nulls off axis. At Mac I did a THX system with 3 tweeters in a short row, not a line array so much as just a short multi-element array. If the center tweeter was used for its full range and the outer tweeters were progressively rolled off I could get something that was lobe free. It was directivity without “pain”. At Snell I just extended that over a number of sections and a wider frequency range. I was using Peter Shuck’s early Xopt crossover/system modeling software and was finding ways to stack elements tight and roll off each section in particular ways that gave very well behaved directivity. I found I could achieve flat response on axis, less than a dB or so droop at +-15 degrees vertical and a totally uniform 6dB or so shelving at +- 30-40 degrees. This was exceedingly well behaved. In essence, with the smooth transition from the inner tweeter to adjacent mids, to surrounding woofer, you are making a system that has an effective length proportionate to radiated wavelength. Looking for a marketing hook, we called it an eXpanding Array, meaning that its effective length expanded as you went down in frequency (that seemed more positive than it contracting as you go up in frequency!)
So with JBL I worked with constant directivity wave guide devices, at McIntosh with medium and long line arrays and at Snell with highly optimized symmetrical arrays. Of the 3 approaches I think the symmetrical array is best. Response is very well behaved and the directivity increase is about right, not as extreme as it is with the long arrays."
You can read about it in the interview here (page 1 is interesting too):
With the coaxial unit, the most difficult part of the design, which is getting the tweeter very close to the mids, has been eliminated. With the corner array, I can get control in both the vertical and horizontal directions to a very low frequency. It breaks down a little at the KEF unit, which is just a single driver, but it is better than most other designs at eliminating the vertical reflections. Also, the KEF coaxial is a great unit. Quite smooth in response and low in distortion.
I did the horn thing. I have a SEOS-24, 18 and 12 and a large tractrix. I have the large Altec VOTTs as well. But I'm not too convinced with the horns, too much expense and size, and too much complexity for little to no benefit. With the SEOS horns, I never could get the balance in the highs quite right. If I make on-axis flat, it sounds a little bright, but if I provide droop, the balance sounds ok, but then some detail is missing. I think a rising directivity is appropriate in most small rooms. You don't want it to be too narrow, but constant seems too bright to me. Of course, YMMV, depending on your room size, amount of absorption and where it is located.
The corner array design I have right now was just an experiment, but it has been quite enjoyable. This is the way to go in my opinion: corner line arrays, or line arrays in general. The corner speakers gave me my room back, as in the floor area. They are much more social as well, whereas horns tend to be large and asocial. If you listen in stereo only then you want the side wall reflections. It adds some ambiguity and a pleasant sense of envelopment to the sound.