Saturday, October 28, 2006

Studio Stereo Localization Test Observation & Question For Curt

Hi Curt--
Putting aside the double boundary PZM fixture (which is interesting and I'll take up in another post), some of us guessed that a rig with an angle between that of our Parallel Boundary Rig and that of our Pseudo SASS rig (110 degrees) might be good to try because the parallel rig exaggerated the sides a tad too much in some people's opinion. (I happened to prefer the additional contrast). I was looking at your site and thought you might have had something similar in mind with this rig. Of course, the parallel rig has front-facing caps and the Pseudo SASS has flush-mounted caps. For those who want more center, I'm inclined to suggest front-facing caps on two boundaries tilted in on the leading edge a bit, maybe 10-15 degrees. Any thoughts about this suggestion? Rob D.


Curt Olson said...

I tried what you're suggesting here, Rob, and quickly backtracked to the parallel approach. In fact, I've even gone the other direction recently on my AT3032 rig, angling the two boundaries outward by about 10-12 degrees each to get a little ORTF-type effect with the slight increase in directionality at high frequencies.

My my experience has been that with any of these rigs -- whether forward-facing omnis on parallel (or nearly parallel) boundaries or Pseudo SASS -- the distance from the capsule to the leading edge of the boundary will be the primary dermining factor in the width of the stereo image. The angle of the boundaries comes next, followed by the distance between the mic capsules. These three factors each make their own unique contribution and can be adjusted to acheive the desired illusion.

(Also, keep in mind that overall size of the boundary will have an effect too. I've been surprised to discover that 1" x 4" dimension wood is big enough for me. Any larger than that, and I start hearing unpleasant mid-range bumps. Any smaller, and the desired baffling effect of the boundaries begins to taper off.)

I always shoot for reasonably accurate sound source vectoring across the whole sound stage, as rendered through my "standard" monitoring chain, which is Sony Hi-MD > Sony MDR-7506 headphones. I usually prefer a soundstage that is slightly wider than the real thing, but never by more than a few degrees.

Regarding the Psudeo SASS approach vs. the parallel boundary approach, my observation is that Psudeo SASS seems to yield an image that is rather similar to ORTF -- very pleasant to my ear, especially on headphones, but slightly "flat" on stereo loudspeakers. By comparison, the parallel (or nearly parallel) boundary approach, using forward-facing omnis seems to deliver a considerably richer sense of depth and space than either Pseudo SASS or ORTF. I usually can hear this most clearly on loudspeakers.

There are other subtle but significant differences between the two approaches that I can hear, but haven't been able to put into words yet. The closest way I can describe it is that ORTF/Psudeo SASS seems to be somewhat like "looking into" the sound stage through a bay window, while the parallel boundary approach is more like standing in the sound stage right in front of the bay window. It's a very weak analogy, but it works for me at the moment. I'll happily defer to any better way of putting it.

Curt Olson

us said...

Thanks Curt! Very instrucrive. It seems we should divide up the measurement from the leading edge into smaller steps to carefully evaluate.

You think something like these would be in the range?
2.5" recessed
2.0" recessed
1.5" rececsed
1.0 "recessed
.5" recessed

Finding the right spot for the smaller, 5.8mm diameter capsules we are using might required more precision than the AT-3032s you are using with 20mm(?) capsules.

"..the ORTF/Psudeo SASS seems to be somewhat like "looking into" the sound stage through a bay window, while the parallel boundary approach is more like standing in the sound stage right in front of the bay window."

I agree. I felt like the tighter, yet smoother front stage of the Pseudo SASS made it feel slightly more "oapaque" and flatter too. It seem like the Pseudo SASS dedicates more of the "heart" of the omni polar patterns across the middle and has less off-axis frequency roll-off at 180 degrees compared to our parallel boundary rig. There shouldn't be any off-axis drop in frequency response at 180 degrees for our ears, but the less bright edges of the parallel boundary rig perhaps creates greater contrast with the front. It's all an illusion as you say.

Testing the image against speaker monitoring as well is an important point. Everyone in the group has matching headphones, and we've beening tuning our perceptions with this common reference first.
Thanks again for your prompt and extremely time-saving response. Rob D.

Curt Olson said...

In almost all my parallel boundary experiments, the set-back from the leading edge usually ends up being around 2.25 - 3 inches. Intuition might suggest that this should vary more widely depending on the microphone diameter, but my experience so far seems to indicate otherwise. 2.25 - 3 inches or so is usually it, no matter what mics I'm working with. I'd be curious to see if your findings turn out to be the same or different.

On a related note, here's something to consider in the DIY arena: a forward-facing pair of WL 183s (or maybe EM158 capsules) mounted to a large PVC drain pipe coupler (about 5" OD and 3.75" high). Yesterday I resurrected a rig like this that I first built last year. Now that I have a better idea what I'm hearing, I'd say it sounds like a fairly close cousin to the parallel boundary rigs. Don't mount the mics directly across from each other, however. Move them backward until they are about 4 - 4.5" apart and angled outward about 70-90 degrees or so. A smal wooden "nose" at the front of the rig might help also. It's a cheap, rugged and easy way to go, requiring almost no fabrication, except for drilling a few holes. It can sound great too, if you're careful where you mount the mics. How's that for a student project?

Finally, I've been looking recently for some decent omnidirectional dynamic mics to experiment with. Last week I picked up a pair of AT804's, and so far I'm quite impressed. Nice sound quality, higher output than many other dynamics, and reasonably cheap (BSW has one of the best prices of the major vendors I checked). Rough preliminary field tests are quite encouraging with these mics mounted on opposite sides of a small fishing reel case I picked up at Gander Mountain for $14.95.

This obsession is getting nutty, Rob, but at least the costs are modest, and for me they're tax-deductible.

Curt Olson

us said...

OK-- We'll test a range of recess distances and then round off the corners of the square boundaries and then test the recess distances again. So the round PVC pipe is mimmicking the curvature of a head? We have a styrofoam wig holder we'll throw in too. Rob D.