volki wrote on Mon, 24 January 2011 14:44 |
(...) Thus, the speakers on their own only generate THD, their two signals combine acoustically in front of the mic, and you can be sure that the DFD you measure is actually caused by the mic itself.
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This test does not address the question of how to isolate capsule distortion/artefacts from amp distortion or the interaction of the two in generating unwanted distortion/artefacts.
A previous post mentioned to simply hook up the capsule to a mic amp with (high headroom) op-amp design.
But that is not real-world scenario either:
take the fet47 which uses the same capsule as the U47 tube. So this would be theoretically a good capsule distortion testing device, as the fet47's amp has a higher headroom than the U47 tube by at least 10dB, depending on setting.
Yet, the fet47 is known for an initial fast transient response with a lag time response to the full signal (a priced quality for kick drum application.) That means, the anomalies or characteristics of a mic amp cannot be easily taken out of the equation, even when, on paper, its static specs look flawless and suitable to isolate the capsule's flaws.
I just think the issue of isolating whether capsule or amp distort first when hit with massive SPLs may be more of a red herring upon deeper investigation, and may just leave us with this:
Testing a capsule's contribution to a mic's distortion may only be possible by substituting either capsule or mic amp with like components (after establishing a distortion base line through testing of one or several healthy specimen.) Then, one would listen for a decrease in audible distortion or other artefacts. If no decrease is audible after capsule or mic amp substitution, one can safely assume the relative level of distortion onset for that model, and, if that level is objectionable, that the mic as a whole may not be suitable for the type of recording envisioned.