MS Setup: microphone sensitivity MKH50 vs CM3

Question

I've been wanting to try out a cardiod in my MS setup (MKH30&50/60) to get a better image. But instead of buying a MKH(80)40 i'm trying out Line audio cm3's. They are nice mic's for a very low price and sound pretty good. The sensitivity is much lower (6mv/Pa) compared to the MKH30(25/Pa).

So I tried to match sensitivity by A-B ing the MKH50 and CM3. This showed a 6.21dB difference in favour of the MKH50. I did some test recordings with both mics ganged to the gain of the MKH30. The idea was that in post i could easily add the 6.21dB to the Mid channel (CM3) and get a good stereo image after MS matrixing.

However, back in the studio I ran into some difficulties. Some recordings were fine and others not (phase issues and extreme wide stereo image). So i thought, I must be able to calculate the difference. I searched the archive and found this post: What do sensitivity measurements of microphones really mean? Following this formula I reached almost the same result, a 6.19dB difference. So great, I was close in my test, but why is one recording good and another not?

Now I'm thinking something is wrong with my train of thought. Could it perhaps be that when I use more or less gain the sensitivity ratio (25:6) is influencing the output differently?
I didn't take note of the preamp gains so I don't know if this had any influence. I will continue my tests, but maybe somebody here has a more academic/technical approach?

Thanks in advance!

Arnoud

Answer 1

Well, in the end it's just a matter of gain (and personal taste) - so it doesn't really matter all that much since you can alter the gain (and even EQ) pre-matrix in the DAW - this will effectively alter your stereo width (and potential phasing issues) down the line. There';s no hard and fast rule with MS in this regard. The image can be whatever you want. Sometimes I master more narrow and focused - other times more wide, depending upon the source content or what I'm trying to reject.

Also you may need to alter the delay of one mic against the other pre-matrix - that could be causing some phase issues as well. This would be important if your M/S capsules are not perfectly aligned during recording.

With my 8040 + 30 combo, I find that about 8-9 dB reduction of the 30 feel good to me for a present XY feel (did about 5 dB on the 702, and in post dropped it another 3 or so). I didn't do any math though, I just went by taste and what sounded good to my ear.

What are using to run the matrix itself? I've done a few audible tests and I find weird phase imaging problems with the S1 Imager (the Width paramter may be a culprit too) - enough that I actually thought I needed to delay my side mic by 4 samples. After much fussing around, I switched out the Imager for the sub-plugin S1 MS Matrix and the LCR perception was much smoother (without needing the 4 sample delay - which now actually sounds wrong when it's there!), even when I adjust the pre-matrix gain. I'm going to be doing some additional tests to see how the S1 runs again doing it old-school in the DAW with panning and phase inversion - just to see of the results from both are truly identical (will perfectly phase cancel out). it has me very curious and right now I have trust issues with the S1.

Learned the hard way with this one too: mic position is everything. I have a recording or two where I was capturing street activity on Sunset Blvd in LA, oblivious to the pedestrians' behind me. There's a moment or two where they walk by (either a distinctive voice or heel clicks) but the 8040 doesn't really pick them up from moving behind me, and those sound a little sketch through the matrix. Again only because I didn't consider, unlike XY, how the rear axis can have a drastic affect upon the resulting image Also why I go by ear for my stereo width. Phase relationships are always a "moving target" in a sense with MS, with position and reflections potentially creating undesired results at times.