The „lungs“ of coincident stereo setups or: Coincident stereo setups in reverberant rooms.

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Product links are affiliate links. Some observations during my recording sessions motivated me to spend some time of further investigation. How stereo setups capture the „main characters“ is well understood and there are plenty of articles on that. Today a bit more focus on how they capture rear sound, which is important in reverberant rooms. I updated one of my calculators and designed a 360° Phase Monitor, that depicts, which sound - depending on its incidence angle - will be out of phase; so L and R will have opposite polarity, when played back through speakers. The graph show the level difference in decibels between L and R. If the graph moves towards the center, one signal is softer than the other, making the phase issue less noticeable. If the graph disappears, there's no phase problem.

For each figure there are two links. One leads to the setup in the polar patterns calculator, where you can adjust the settings by yourself and investigate the results. It features an abstract auraliser as well. The second link points to the setup in the Virtual Stage, where you can freely move around the mics and sound sources and listen to the result. Two sound source (noises) are put already in the rear of the stereo setup, as this article focuses on the capture of the ambience.

Fig. 1 shows the corresponding graph of a Blumlein setup, which - as expected - shows that sounds from the sides are out of phase. The dashed lines mark the recording angle. A Blumlein setup consists of 2 Mics with a pattern of a figure of 8, with an angle of 90°. Here is an example of a fig. of 8.
Deep link to this setup in the calculator
Deep link to this setup in the Virtual Stage

Fig. 2 – for XY120 with cardioids – shows no graph, resp. no phase problems.
Deep link to this setup in the calculator
Deep link to this setup in the Virtual Stage

Using super cardioids (XY120-SC) shows a very remarkable picture, like human lungs (Fig. 3). They indicate phase problems at ~ ±150°.
Deep link to this setup in the calculator
Deep link to this setup in the Virtual Stage

Today's „bad boy“ is MS with C as M. The animation shows, how with increasing level of the S-signal/decreasing level of the M-signal the recording angle is decreased (which results in a wider stereo image). FL and FR show the resulting virtual mics – in other words: the decoded MS.

Its rear capture is mostly out of phase. Only when the recording angle is set very wide (=narrow stereo image), the out-of-phase-area is limited to the region, where the response of the whole setup is very small anyway. For smaller recording angles the out-of-phase-area is considerably larger and - what is really the worst - almost completely depicted in the center of the stereo image. I depicted that in the „stereo potato“, where the inner border marks the left, the outer border the right speaker and the position of the graph between the borders reflects the position of the phantom source between the loudspeakers. You can see, that all sounds out of the recording angle tend to move to the center between the speakers, and the more they are in the center, the more out of phase they are. The conclusion is, that MS with C works best for very close miking in lesser to non-reverberant rooms. This might be surprising, as a cardioid is known for attentuaing rear sound. But that's obviously just a part of the game.
Deep link to this setup in the calculator
Deep link to this setup in the Virtual Stage

A better option for attenuating rear sound is Schoep's „Open Cardioid“ for the M in MS. Still very good attenuation, but already much better phase behaviour and the rear sound is more spread over the stereo image.
Deep link to this setup in the calculator
Deep link to this setup in the Virtual Stage

The XY-equvialent (approx. same recording angle, phase behaviour, power sum) would be XY132-SC.
Deep link to this setup in the calculator
Deep link to this setup in the Virtual Stage

If the aim is not to suppress the room reverberation, but instead to include it in the recording, an omnidirectional microphone is a good choice. However, most commercially available omnidirectional microphones have strongly frequency-dependent patterns. In an MS array, this leads to the stereo image becoming frequency-dependent. The stereo image becomes wider at high frequencies. This is critical for organ recordings, for example. Organs achieve their volume through higher-frequency registers. This means that organ recordings would sound wide in forte and narrow in piano when using MS with a common omni.
Deep link to this setup in the calculator

While many sound engineers prefer when omnis tend to become more directional in the high frequencies, this is not helpful in an MS-setup. One solution would be to mount a mic with a very good off axis response vertically. This requires the sound source to be very well in the same plane as the mic setups is directing. However, there are a few omni models on the market, that specialize on a very even omni pattern, with as little directionality as possible. DPA offers a nose cone for their 4006. Another option is their 4090. Note that MS with omnis has way less phase problems than the previous setups.
Deep link to this setup in the calculator.

Before doing the MS processing a frequency response balancing can help improving the stereo image: Flip the frequency response curve of each mic upside down and feed a curveEQ (with linear phase settig to on) to get remove bass/high roll offs. The image shows a MS with MK22. Upper graph: frequency dependent stereo image with unequalized mics, lower graph: equalized mics.

To get the equalizing curve, use this tool: https://www.stefan-kiessling.com/?Thema=proximitydiffuse. Choose your mic. Click on „Equalizer…“ in the „Resulting Frequency Response“ and follow the instructions.