Q. Can I use two completely different mics in an M/S array?

Sound Advice : Miking

I have a vintage Neumann U87 mic, a GAP Pre73 preamp and also a Zoom H4N recorder (massive contrast, I know). If I want to, can I use these in a Mid/Side setup with the Neumann as the Mid mic and the H4N for the Side? Or should I use the U87 in a figure-of-eight polar pattern for the Side and another mic — my Shure SM7B or Rode M3 — as the Mid? How, then, would I combine the two into a Mid/Side audio file?

Via SOS web site

SOS Technical Editor Hugh Robjohns replies: Using such radically different mics to form an M/S array is never going to give great results; the stereo imaging will tend to be rather variable and frequency dependent because of their mismatched frequency responses.

However, for experimental purposes I’d suggest using the U87 in figure-of-eight mode for the Side mic and either the SM7 or M3 as the Mid mic. I suspect you’ll still find quite a lot of image instability, but it should work well enough to give you a feel for the versatility and practicalities of the M/S mic technique.

All you need to do to create a Mid/Side file is allocate the Mid mic to channel one and the Side mic to channel two, and record as a normal stereo file.

Regarding rigging the mics, the normal convention is that the in-phase side (front) of the figure-of-eight mic — the U87, say — faces to the left of the sound stage (as viewed from behind the mic array, looking towards the sound sources).

Although our reader may never achieve great results using vastly different mics in an M/S setup, it may still be worth him experimenting with his U87 in figure-of-eight mode as a Side mic and his Rode M3 as a Mid mic.

You should also set the preamp channel gains to provide a similar sensitivity for both mics, to optimise the signal-to-noise ratio and make the decoder work properly. The easiest way to do that is to decide how wide you want the sound stage to be — decide where the notional edges of the stereo pickup area are — and then get an assistant to stand at the nominal left-hand edge and make a fairly constant noise, such as humming. Assuming the Mid mic output is already providing an appropriate level (if not, adjust that first), set the gain of the Side mic’s preamp to give an approximately equal output level to that provided by the Mid mic’s preamp.

Question of the month

Published in SOS March 2012