I had a talk with the very helpful David Copp, who does the mic
repairs at Sennheiser USA. HDavid said that MKH mics can drift out of
alignment (meaning sensitivity change) due to the condition of the
capsule, and due to aging of components. He aligns each mic to its
best performance, but doesn't measure absolute SPL sensitivity. He
said there is no way to predict the sensitivity of a mic that's been
in the field. Some are just fine after many years, but others have
drifted off. His way to determine the condition of a mic capsule is to
compare it by ear to another known good one.
Omni mics could be calibrated in the field with a sound level meter
calibrator, but this would require removing the windscreens. There
currently isn't any field method for calibrating directional mics,
like MS arrays or dishes. One can measure the SPL with a meter (omni
mic), but the ambient field may be directional (think creek), which
will give different results depending on how the directional mics are
pointed.
I'm continuing to brainstorm about manufacturing a field calibrating
system that would provide a convenient way of balancing multi-mic
arrays, and give soundscape recording more scientific credibility.
-Dan Dugan
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