You could also mount both vertically, but not with one inverted - put them one in front of the other. One of the Blumlein specialists on GS Remote does this with his Sennheiser MKH 30 setup. [snip]
I wonder about the effects of non-coincident placement in that arrangement, and how much it matters. I've wondered about this for a while.
Thinking about it out loud -
● There will be a path-length difference to the two microphones which will vary with the angle of wavefront arrival. That path-length difference will be maximized for a wavefront arriving from directly in front or behind, and there will be no path-length difference for a wavefront arriving directly from either side. This is the inverse of the behavior of a typical near-spaced stereo pair arrangement.
● This is essentially the same relationship as will be found with a stereo shotgun that incorporates a Side mic behind the interference-tube Mid, except producing a somewhat greater path length difference (small diaphragms in a single housing are able to be positioned more closely that two separate LD microphones).
● The path-length differences will manifest symmetrically across the L/R plane.
● Any audible effects are likely to manifest at high frequencies where the wavelengths are short in comparison to the path-length difference.
● A typical vertically-coincident arrangement has the same path-length difference thing going on but it manifests across the vertical plane, not in the horizontal plane which is the primary plane of stereo interest.
● Turning the mount 90° and each microphone 45° for use as a Blumlein arrangement (with the two microphones side by side) rather than for Mid/Side, makes the path-length differences symmetrical for Blumlein, and inverts the wave-front arrival timing relationship- There will be no path-length difference for wavefront arrival from directly in front or behind, and maximum path-length difference for a wavefront arrival directly from either side. This is the behavior of a typical like a near-spaced stereo pair arrangement, only with a smaller than typical spacing between the two microphones.
So does it matter? I suspect we need to maintain effective coincidence through the upper midrange frequencies. Not sure how necessary above that range. Respected microphone manufactures do offer end-address stereo shotgun microphones with the Side capsule positioned behind the Mid capsule.
A few years ago I encouraged B9 Audio to offer a modular stereo microphone. Essentially a variant of their existing small diaphragm side-address Blumlein stereo fig-8 microphone, but with a removable top fig-8 capsule, which the user could replace with either a blanking plug for use as a mono fig-8, or an end-address capsule of whatever pattern they wished that B9 has available (omni, subcard, card, supercard, shotgun), to form an end-address Mid-Side arrangement. While the physical stereo arrangement would be that of a stereo shotgun, the modular nature of it would be unique in allowing choice of Mid pattern. We talked about it a bit here at TS but in the end he rejected the idea due to this non-coincidence inherent in that arrangement.
Love to do some listening comparisons of vertically coincident verses front/back coincident to hear the effect and empirically determine how much it really matters.