I own two Rode NT5 cardioid microphones. After reading Günther Theile's paper On the Naturalness of Two-Channel Stereo Sound (J. Audio Eng. Soc. 39 (10) Oct. 1991), I was wondering how I can achieve some of the "naturalness" of a sphere microphone with what I have, i.e. using my cardioids instead of omnis.
In this setup, two cardio mikes are horizontal and parallel, pointing forward to the source, with their capsules almost touching two opposite points on the equator of a sphere. The mikes stick outside the sphere much like the figure-of-eights in the Schoeps KFM360. Here is a couple of pictures of this setup:
The sphere I'm using is a white lamp made in polycarbonate, you may find such things in gardens or on exterior walls. It's essentially a plastic shell, few millimeters thick and hollow, 20cm diameter. Mike Rivers points out that I may get resonances in the plastic and in the air inside the cavity.
Soundhaspriority points out that a cardioid doesn't behave as a cardioid if put against a boundary. I could argue on that, but he made me think twice on what I was doing: although Crown and other manufacturers have marketed directional microphones against some boundary surface, it is true that the surface has always been chosen to be flat (with one notable exception I know of, i.e. Schoeps KFM360). Directional microphones are arguably designed to have a uniform response in frequency when in the far field, i.e. when hit by plane waves. In the near field, sound from a point source is made of spherical waves and not plane waves, so the well-known proximity effect can be put in relation with the curvature of the sound wavefront. Now, plane waves hitting a reflecting spherical surface will bounce with a curved wavefront, so I can expect some proximity effect. If I'll ever work out the math, I think I'll try to derive an "effective distance" of the sound source from the Laplacian of the sound wave.
Based on formulas in a paper by Steve Turley, I made some plots of sound intensity around the sphere (the plots are generated with SciLab 4.1):
Click here to download an animation from 80Hz to 8000Hz in 1/3 octave steps (1.47 MB).
The equipment: a Rode NT5 matched pair, a Rolls portable phantom power, an M-Audio MicroTrack 24/96 as a recorder. Apologies for my amateurial recording: I have very little experience in audio engineering.
The first take is from a rehearsal session before the concert. For the sake of comparison I put also the very same piece as performed an hour later in concert with a similar arrangement but without the sphere and with the mikes in a near coincident configuration 11.5cm apart at an angle (mic-to-mic) of 50°. According to Wittek's Image Assistant, this configuration has a recording angle of 180° with a fairly smooth dependency of the "phantom" (direction of the stereo image of the source) on the azimuth of the sound source.
If you download and listen to the samples, please post a comment on rec.audio.pro.
Another try would be with a furry disc (Jecklin disc): one could expect results comparable to a furry sphere, but it is not hollow and it is more convenient to carry around.