During the last two decades, international research and cooperation have advanced the methods of primary calibration of microphones and developed improved International Electrotechnical Commission (IEC) standards for primary calibrations by the reciprocity technique in pressure fields and in the free field. However, for high-quality primary calibration, a laboratory must carefully investigate its procedures and apparatus to determine their contributions to the various uncertainty components that establish the overall uncertainty. Issues in secondary calibration (for example, comparison) and characterization of measurement microphones and other acoustical instruments also require careful, systematic effort, and increasingly are receiving international attention. Topics of current interest at NIST include acoustic center positions and source--receiver standing waves during primary free-field calibration, the influence of anechoic chamber imperfections on primary and secondary free-field calibrations, and the effects of instabilities and nonlinearities in sound sources and microphones. Recently improved instruments including dynamic signal analyzers capable of FFT and other signal processing, as well as relatively new concepts in their application to acoustical measurements, are necessary to this work.