I recently saw pictures of Neumann MM3 measurement mics. Each was accompanied by a response graph referenced to a mic calibrated by the Physikalisch-Technische Bundesanstalt Braunschweig (PTB), a national measurement/calibration institute in Germany.
How did the PTB arrive at the original calibration of their reference mic, to serve as standard for all submissions? And what mic did they use for that? And how did they know that their mic truly was linear enough to qualify it as reference for manufacturers of reference mics, like Neumann's MM series?