Noise specifications are generally among the least reliable specifications that you will find for microphones, and "A"-weighted noise specifications are the worst of the bunch. It's not just that the "A" weighting curve is way wrong for such low sound pressure levels (though it is); it's also the fact that "A" weighted noise measurements are nearly always rms--they're averaged across a relatively long "time window" which smooths out all impulse- or shot-like noise. Since impulse noise is much more audibly disturbing than steady, smooth hiss, as a consumer you don't want it filtered all out of the noise measurement process.
In addition, the different manufacturers measure noise so differently that if you gave any two of them the same microphone to measure, they might come out with 6 dB differences in the value they would report. This is not just conjecture on my part--a group of cooperating manufacturers (part of a standards committee of the AES) actually went through this as an exercise a few years ago, and the results were written up in an AES paper that I helped to translate and edit for publication. Most of the manufacturers agreed pretty well on such items as sensitivity, frequency response and polar response, but when it came to noise levels there was much greater divergence. This difference was never explained and should not be necessary, but it exists.
In general, if a manufacturer accurately specifies a microphone's equivalent noise as a CCIR-weighted, quasi-peak measurement, that should correlate fairly closely to what we would hear in actual use. However, such measurements tend to be 8 to 12 dB higher (i.e. seemingly worse) than "A"-weighted rms figures--so when only one specification is published, it is generally the more favorable-seeming "A"-weighted rms value.
Finally there's the whole question of just what a manufacturer means by their specifications in the first place. Do they mean "if your microphone doesn't live up to this level of performance, we'll repair or replace it at no charge"? Not very likely. The first-class manufacturers would pay attention if someone made a credible claim like that, and would fix the microphone if they agreed that it needed fixing, but they never offer specifications as any kind of legal guarantee to the public at large.
More often, specifications from responsible manufacturers mean, "These are values which we define for our microphones, which they generally live up to--but any one sample could deviate by a few dB"--while with less responsible manufacturers the specifications don't come from any actual measurements at all, and are simply chosen by the marketing department to suggest an impression of the level of performance that they would like you to believe they are offering.
--best regards
Topic: NT4 on quiet material? (Read 5697 times)