Here's my approach to thinking about the problem. My apologies if you already have a thorough understanding of how this works, but hopefully it will help someone or at least spark a conversation. In any case it helps me to get it clear by thinking it though enough to describe it. This starts with the Stereo Zoom idea but goes much farther. A philosophy of mic setup?
I'll assume that the orchestra as viewed from the mic stand all fit well within, say, a 120º angle and that you are using a pair of cardioid mics for your stereo pair. You can choose various combinations of the angle and spacing between mics which all have an angle of pickup of 120º. If you just choose angles and spacings randomly then the angle of pickup changes, but if you are careful about which combinations you choose, the pickup angle remains constant and you only change other variables.. like how your stereo pair handles the reverberant pickup of the room in the remaining 240º.
For any particular
angle of pickup (my assumed 120º angle into which the orchestra fits as viewed from the mic stand) the relationship between the angle between the mics and their spacing forms a continuum - one end of that continuum is X/Y with a very wide angle between mics and no distance at all between them, using the directionality of the mic pattern alone to produce stereo. The other end of the continuum is no angle at all between mics (both facing straight ahead) and a good amount of spacing, which treats the mics as if they were a pair of spaced omnis, using the only time of arrival differences between them for producing stereo. As mentioned, you can choose any point between those two extremes, trading angle for distance and maintaining the 120º pickup angle. This is what
Michael Williams' Stereo Zoom technique is all about. If you want a 120º pickup angle (he calls it SRA- or stereo recording angle and writes 120º as +/-60º) with cardioid mics, you can refer to his cardioid chart and read spacing and angle combinations off the +/-60º curve of the graph. For instance you might choose to set your mics up in X/Y @ 140º, or 14cm @ 90º, or 32cm @ 45º, or 44cm @ 0º (mics pointing straight ahead). All of those combinations have a 120º pickup angle. ORTF, DIN, NOS and others are just commonly used points on various SRA curves.
Williams charts are set-up to let you choose between angles and spacings for a certain SRA using a particular pickup pattern. They also tell you a little bit about how even the distribution of instrument sources across the playback stage will be within that pickup and playback angle. But other than that they don't tell you anything about why you might choose one combination over another. One perhaps obvious, or at least fairly often discussed choice is a preference for either the sharper imaging of mic setups that rely primarily on level differences generated by mic pattern or the less sharp imaging but deeper and spacious sound of setups that primarily use the spacing between mics to record time of arrival differences. Rarely discussed and I'd argue more important, especially for less than ideal situations such as this one, is the effect on the reverberant pickup of everything outside of the SRA. I feel the sound of the room and the correct balance of that contribution with the direct sound is the most critical aspect when choosing a setup. For me, the real value of the Stereo Zoom concept is providing the ability to hold the pickup angle variables constant and change how the reflections and reverberation in the room are recorded.
As far as the Stereo Zoom is concerned, two cardioids spaced 44cm apart and pointing straight ahead with no angle between them and two omnis spaced 44cm act identically. They produce the same SRA and distribution of sound sources between the speakers. But we all know they'll sound very different. One reason for that is the cardioid pair is angled so that the mics reject all sound arriving from directly behind them as a pair, whereas the omni pair doesn't favor any particular direction.
What I'm suggesting is to 'read between the lines' of the Stereo Zoom info and using it to consider ways of setting up your mics to reduce the negative aspects of the recording environment. You could use your cardioid pair like spaced omnis and cut down significantly on the contribution of the sound bouncing off that back wall. The more angle between the mics, the less the stereo pair acting together will limit pickup of sound bouncing off the back wall.
I find the Stereo Zoom relationship between mic pickup pattern, distance and angle between the mics very interesting and relatively easy to get an feel for. Most recordists develop an instinctive understanding of the concept even if they may not have a clear intellectual idea of what is going on or if they've never heard of the Stereo Zoom stuff. The part that fascinates me is the huge contribution of everything outside of the Stereo Zoom SRA. The idea of choosing a setup to cut down on the sound bouncing off the back wall is part of that. Other important factors that I'm trying to understand more completely are the nature of the the reverberant and reflective contribution from 'that other 240º'. Questions like: how much of the reverberance from the rest of the room is recorded as mono vs stereo information? Were does that reverberance and room reflections appear in the stereo soundstage illusion on playback? How differently does the stereo pair capture reverberance and reflections from above and below as opposed to the side or back. If this sounds interesting you might take a look at a thread I posted last fall which gets into that:
Why Blumlein sounds more spacious than other coincident or near-so arrays The link there to
Stan Linkwitz's page on Mapping from recording to playback is pretty easy to understand and explains why 48% of the total reverberation of the room is captured as mono reverberation using an ORTF pair.
Everything I've mentioned so far considers only sound on the horizontal plane. Lnkwitz only looks at the horizontal plane as I recall, yet reverberant sound comes from all directions. When considering the entire 3-D pickup of a stereo pair in a real room, that 48% mono reverberation figure for ORTF would be much higher still. Michael Williams' 1991 AES paper
91st AES Convention in New York - Preprint 3155« Early Reflections and Reverberant Field Distribution in Dual Microphone Stereophonic Sound Recording Systems » does a good job of explaining what happens above and below the horizontal plane. This one was enlightening for me with 3-D analysis of both individual mics and of stereo pairs. In most cases, the farther the source of sound is above or below the horizontal plane of the mics, the more it is recorded as mono instead of stereo information, and that happens surprisingly rapidly.
This spring I ran across another excellent paper by Geoff Martin that he's made available on his website which goes further in depth:
General Response Characteristics of Microphone Configurations. If the Linkwitz page and the Williams papers make sense, then take a look at this one. It is quite technical and through. Even if reading the whole paper is not your thing, you may get something out of checking out the colored globe images that illustrate the 3-D pickup of stereo pairs and analyzes what I'm getting at.
