Condensor Microphones diaphrams facing each other

Question

I once recorded grand piano with two condenser microphones (AKG c414) and due to the length of the arms of the microphone stands had the two diaphragms pointing past each other (in a cardioid shape). Someone much more experienced than me then told me that that is something you should never do, the diaphragms should always face away from each other. The recording I made sounded good, and I was not given any good reasons why my set up was not correct/preferred.

Any explanations as to why this is the case would be greatly appreciated.

Edit: the mics were placed at the top half of the strings close to the player. The mic on the performers right was facing the bass strings and the mic on the right was facing the treble strings, hence the diaphragms face each other slightly.

Answer 1

He was most likely worried about phasing issues, as I'm assuming you weren't doing a stereo split for a dual mic piano setup. Basically, when you have them in the center pointed out, one mic primarily picks up the lower registers and the other primarily picks up the higher registers so you don't have to worry much about phase when they are combined.

If they overlap, however, then if the frequency of one string hits one microphone at the top of the waveform and the other mic has the bottom of the waveform hit at the same time, then the string will effectively cancel itself out. This is a type of phase canceling and can also end up as a partial mismatch resulting in a bump or beat in the waveform due to the sometimes constructive, sometimes destructive interference between the phases.

You might or might not get a serious issue depending on the positioning and sensitivity of the mics, but it is generally something best avoided. You want to ensure that when dual micing a piano, you want minimal overlap in coverage between the mics unless both are positioned similar distances from all strings (or you are using filters to restrict the frequencies you use from each mic).

If you are using it for a stereo image, then you additionally encounter the stereo image problems that leftaroundabout covered in great detail, but my primary concern would be the phasing.

Answer 2

It's not entirely clear what setup you're talking about, but I'll discuss both options.

• The mics are basically at the same spot, just offset a bit to aim in directly opposite directions.
  
  This is essentially an XY pattern, with an extremely wide angle. The setup will thus work a lot like XY: quite reliable phase for mono compatibility, stereo field only from amplitude differences. You may not get ideal signals because center sources are strongly off-axis on both mics; especially with large-diaphragm mics this gives you quite a different frequency response. But at least with small-diaphragms, this technique can certainly be quite useful when you want XY with an extra portion of stereo width and room content. It's also useful when you want to separate two given sources, e.g. two singers¹.

• The mics are placed some distance apart, pointing at each other.
  
  Problem with this is that you get "contradicting" stereo information: sound coming from the left hits the blue mic first, but shortly later gets captured with higher intensity by the red mic. If you use this as a room technique, it'll definitely sound weird – the transients of an instrument on the left will seem to come from the right. For a single "wide" instrument like piano however, the stereo field you hear doesn't exactly represent the spatial placement of any particular sound source (like a single string) anyway, so it might not sound notably unusual at all.
  This technique can also be applied to "inflate" a source with not much of an actual spatial stereo width, but sound emission that strongly depends on direction. That is the main principle you exploit when miking an acoustic guitar with both mics pointed to the sound hole from different angles.

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¹_{This is often quoted as a canonical application of one figure-eight mic, but that's IMO a poor-man's choice – figure-eight has a lot of little-known applications where it excels, but simply picking up two sources is not something where it's really better than two cardioids, which unlike a fig-8 allow you to mix/process both singers individually. Only caveat is that the cardioids pick up more off-axis sound, but that's unlikely to be much of a problem for a studio duet recording (and live you'd more likely give each singer a supercardioid).}

Answer 3

If it sounds good, do it. But listen back in mono to check there aren't any phase issues.