1.) How far behind the engineers head should the first reflections from the side walls intersect? What's the safe zone?
Hard to say: depends on the Q (directionality) of the speaker, off-axis response, the angle, the frequency, etc. Probably a foot or so is about as close as you'd want to get, but two feet would be better. (I'm not aware of any actual research to back that up: just my own personal hunch.)
BTW, is that a Sony console you have there? Kind of looks like it, but then again, it doesn't...
2.) Do the baffles of the soffits count as the reflection surface when calculating the RFZ?
Ohh yes! The front baffles ARE the new wall of the room, as far as the speakers are concerned. That's why you have to build them massive and rigid.
I'm assuming that standard soffits are constructed in such a way that they will reflect the frequencies needed for an RFZ...
Exactly, which means "as low as the speaker goes, and then a bit more, just in case". Their main purpose in life is to force the low frequencies (below the baffle step point) back into half space, so they have to appear very solid, very massive, very reflective to those low-frequency sound waves. Of course, that also means that they appear even more solid and reflective to the highs! Thus, they absolutely must be taken into account for your RFZ ray tracing.
This applies to the left speaker reflecting off the right baffle and vice a verse
Exactly. But take a look at your speaker manual, and specifically at the polar pattern (dispersion pattern, or whatever it is called: the one that shows you the SPLs around the speaker for each frequency range). You need to take that into account when you check your reflection angles, ray tracing, etc. For example, at 60 degrees off axis, you won't find very much 20 kHz at all, but you might find a lot of 200 Hz, for example. And just to confuse you even more, don't forget that highs bounce like light rays, while lows bounce like expanding balloons...
So for lows you really should draw a cone that expands out from the point where the ray hits, and check how that cone is with respect to your ears...
(Hey, I DID warn you that things are a bit more complex than they look at face value!)
OK, put down the gun and suicide note, and don't sweat things too much: if your ray tracing gives you a couple of feet clearance all around your head, and the reflection point is a reasonable distance away, you should be fine. So go with the largest angle that gets things a few feet clear of your head, and that's about all you need to worry about.
I think there should be a sticky that demonstrates ray-tracing and RFZ design at a more detailed level. It would be a great help for us newbies!
That would be useful, wouldn't it? One day when I have time, I'll see if I can write up something... (Hmmm... checking my calendar, I should be able to fit that in some time in early 2058...
)
What I would REALLY like, is a free plug-in for SketchUp to do all the ray tracing for me!!!! Oh yeah! (Well, dreaming is free, at least).
- Stuart -