Can anyone comment on the qualitative and quantitative differences between MK21 and MK22 with regards to far off-axis sources? Essentially across the "rear hemisphere" of the pattern, specifically across the region between 120° through 240° or so? I'm interested in the difference in subjective sound quality as well as the difference in sensitivity between the front and back averaged across that relatively wide angle, rather than as measured specifically at 180°.
Although this question is relatively straight forward, my reason for asking is in regards to a rather atypical application. I've long thought about using these types of patterns in place of omnis in the wide A-B portion of my arrays, which serve a dual roll of low frequency pickup and ambient/audience/reverberant capture. For the purpose of low frequency pickup the omnis are unsurpassed. Yet for the purpose of ambient/audience/room capture there is too much pickup of direct arriving sound from the front to allow for the most optimal balance with the other microphones dedicated to frontal direct-arrival pickup and imaging. This places the two roles at odds. A rear-facing wide A-B pair of cardioids sufficiently limits sensitivity to direct arrival from the front, yet lacks the low bass response (and sufficient ambient openness). And used in this way, a cardioid may adversely color pickup of the dominant front-arriving sound if its response across it's region of minimal sensitivity is not especially well behaved. Fortunately in general, with greater sensitivity to the rear comes increased smoothness across that region (more open patterns being generally better behaved across their least sensitive quadrant).
Its my speculation that a rear-facing A-B pair of subcardioids or open cardioids in place of omnis could sufficiently reduce sensitivity to the front while retaining sufficient low frequency sensitivity and openness. I'd love to experiment with a pair of A-B fig-8's coincident with the omnis to really get a handle on dialing in the front/back sensitivity balance afterward, but based on my experience I expect a subcardioid or open-cardioid type pattern would be just about right.