This is a very interesting approach to sound reproduction! I would like to learn more, in general, and also wrt this particular approach. Assuming rectangular listening spaces.
This is the first and only approach I know of which allows you to "be there" without removing most of the room effects with cumbersome/expensive/esthetically compromised solutions.
Thank you for the encouragement. I assume you have some background in the psychoacoustics of reflections in rooms, because usually the best reaction I can hope for is polite skepicism.
A 90 degree waveguide gives good off-axis soundstaging across a wide area, sometimes with the enjoyable listening area extending outside the speakers. Not every location within this large area will be equally good, and really good soundstaging is confined to up and down the centerline.
Which way should the speaker point to get the best amount of side wall reflections?
I use 45 degrees of toe-in, which (with a 90 degree waveguide) results in the near-side-wall being far enough off-axis that it is only weakly "illuminated" by the waveguide.
Can one trade off dispersion vs. pointing angle by the speaker?
Yes. With a narrower pattern you can use less toe-in, but imo you still want the axes criss-crossing in front of the listener. With a wider pattern (which I haven't tried) you might want to use more toe-in to avoid a significant early same-side-wall reflection.
Is this applicable to any speaker, or only those which have a back firing component like your designs?
The toe-in suggestion is at best hit-or-miss with speakers which do not have a fairly narrow and uniform radiation pattern. An add-on Late Onset Reflection Assist module has been beneficial with other speakers having more conventional radiation patterns, but I don't yet have a wide enough range of experience with it to make sweeping claims. My guess is that "it depends".
The LORA stand was designed with possible use for other stand-mount speakers in mind. It has a switchable notch filter which introduces some dippage in the 3-5 kHz region, to allow for the off-axis energy flare that many stand-mount two-way speakers have above their crossover region, due to the tweeter's wide pattern at the bottom end of its range.
(I do remember you mentioning a 15deg angle to the speaker, is that assuming an equilateral triangle between the listener and the speakers?)
Yes, but that's not carved in stone. Adapt angles and toe-in to your speakers and room.
- How low in frequency should the directionality (90deg) be maintained? In order to play with it I am building a waveguide which can go down to <150Hz
Pattern control down to 150 Hz is FANTASTIC!! Dude, you are HARD CORE!!
The lower you can get pattern control the better, all else being equal, BUT all else is NEVER equal (in particular enclosure size and/or complexity). The region from 700 Hz to 7 kHz is apparently the region which matters most (based on David Griesinger's writings). The little Alana misses that 700 Hz target by about an octave, but we still think it's a worthwhile improvement.
- You talked about the backward facing drivers bandwidth and directionality. I have same questions about how low it needs to go, and would adding time delay help/what should it be (how far should the speaker be from the rear wall, and can you compensate with time delay)
The directionality is to minimize any "leakage" arriving during that first-10-milliseconds-window when we're trying to minimize early reflections.
In this application the rear-facing driver starts rolling off in the same region as the front drivers' crossover (1.4 kHz), but the rear-facing driver's rolloff is quite gentle down to about 700 Hz, then accelerates rapidly. This gentle rolloff over an octave or so approximately offsets the pattern-widening of the midwoofer as we go down from the crossover region.
Minimum distance from the wall for the rear-firing driver is a foot or less. The up-and-back angle + the directivity gives us about 10 milliseconds of delay, relative to the direct sound.
You absolutely could use delay for the rear-firing drivers, and in that case you could avoid the complexity of aiming that energy up at an angle.
OR... you might even consider deliberately aiming the additional driver(s) at the theoretically ideal first-reflection zones, along the same-side-walls, delayed to arrive maybe 15-20 milliseconds behind the direct sound. Toole reports that the ideal arrival direction for reflections is about 30 degrees forward of directly to the left and right, or at about 10 o'clock and 2 o'clock. Apparently Ken Kantor felt the same, unfortunately his MGC-1 was a failure in the marketplace:
Anthony H. Cordesman wrote about the MGC-1 in October 1985 (Vol.8 No.6):
www.stereophile.com
What we're doing is a passive solution that you can just drop into a "normal" system, but there are arguments for going fully active and taking advantage of the capabilities DSP offers.
- I assume that one can experiment by just adding a drive unit to any speaker (which already has the right characteristics for the front facing part). Am I correct?
Imo there are multiple viable options. In a sense ye olde rear-firing tweeter is a variation on the theme... or maybe we're just doing a variation on the rear-firing tweeter theme. However OUR song-n-dance has MUCH better choreography!
- If adding room treatments, where should one put them? They are essentially low pass filters on reflected sound. How does your approach change their application?
I am NOT an acoustician, so consider this to just be opinion.
The front wall between the speakers becomes even MORE susceptible to illumination because of the aggressive toe-in. I'm not sure you'd necessarily want to treat the entire area between the speakers, but you might want to pay attention specifically to where the first reflections occur (as seen from the sweet spot).
The wall behind the listener is almost always an issue. If it's really close behind your head, that might be a place to put just enough thick absorption to catch the first reflection from each speaker.
I'm less inclined to treat the floor and ceiling bounces.
Personally I'm inclined to re-direct the first reflections as long as the room isn't inherently too lively to begin with. I prefer diffusion over absorption in general, especially along the side walls. In particular, we want to avoid absorption of the rear-firing energy's first few reflections on its way to the listening area.
Sorry about all the questions, and if you are not comfortable sharing any of the secret sauce, I understand!
No problem, the deepest darkest secrets remain hidden, and thanks for your interest!