Festival! The Best of the 1995 Santa Fe Chamber Music Festival Microphones part 2
Crossed cardioids at 90 degrees, therefore, give an inherently narrow stage, but in practice the situation is even worse than this. Real (as opposed to "perfect") cardioid mikes have difficulty in maintaining their pattern at low frequencies, becoming more omnidirectional in character; bass frequencies, therefore, are effectively recorded in mono by 90 degrees cardioids. It has been often remarked that with such an arrangement, the low-frequency ambience seems to "pool" in the center of the soundstage. Their pattern also tends to narrow at high frequencies, giving rise to a treble "hole in the middle" and often unacceptable levels of coloration for centrally placed instruments.
Cardioid microphones used in ORTF configuration.
Opening out the angle between the cardioids' axes gives an increased amplitude difference between the two channels for extreme left- and right-hand sound sources: an included angle of 135 degrees, for instance, will give a difference of around 10dB, so the stereo stage will be significantly wider than that produced by a 90 degrees pair. Placing the cardioids back-to-back---ie, at a 180 degrees angle---will give the maximum channel separation, but also gives maximum sensitivity to far left and right. Thus reverberation will be emphasized at these positions, giving a "pulling" of the ambience to the sides. Central images will be 90 degrees off-axis to both mikes where the HF response of "real" cardioids doesn't hold up, giving a treble hole in the middle. Instruments that increase in treble as they are played louder will, if central when quiet, "splash" to the sides with 180 degrees cardioids.
There is another way of widening the stage from a crossed pair of directional mikes, however. This takes advantage of the Haas or Precedence Effect, best known from its effect on off-center listening to a pair of loudspeakers. In that situation, if the same signal is fed to the two loudspeakers, introducing a time delay in the feed to one of the speakers will shift the otherwise centrally placed image for a centrally placed listener toward the other speaker. If that delay is roughly 2ms or more, then the image will be displaced completely toward one side.
What, then, if we take our coincident cardioids and introduce a time delay for off-center sources by spacing them slightly? The additional time-delay information will reinforce the amplitude-only information and, if optimally arranged, widen the narrow amplitude-only stereo stage to an acceptable degree. The French broadcasting organization ORTF (Office de Radiodiffusion-Télévision Francaise) favors two cardioid microphones angled at 110 degrees and spaced about 7" apart---the average distance between a human's ears. This was how I chose to use the B&K cardioids. They were mounted on a stereo bar, and hung by their leads from the center of the ceiling 11' above the level of the stage and the same 8' back as the omnis (fig.1). This meant that the main microphone axes pointed to the outside edges of the instrumental group, while the conductor's podium on the hall's centerline was 45 degrees off-axis to both microphones.
Again, the array was tied back to the balcony rail with fishing line. And, as explained by Wes Phillips above, I pulled the cardioid mikes back by 12" for Appalachian Spring after the sound check in order to add a little more ambient bloom to the sound. I returned it to the original position for the Milhaud and Kohjiba works, however, as the instruments at the back of the stage---particularly the piano (a rather nice Hamburg Steinway)---otherwise sounded too "wet," or reverberant, in those works.
The cables used were a bit of a mixed bunch, partly because we had to set up our recorder and monitoring equipment in a room much farther away than we had originally planned. The omnis were hooked up with 125' of Beyerdynamic and 15' of AudioQuest Lapis balanced cable, while the cardioids fed a combination of 75' of Cardas Microtwin 300 and 50' of Canare balanced cable.---John Atkinson