Precision electroacoustical measurements at the NIST (formerly the NBS): Selected examples

In addition to primary calibrations determining the pressure and free-field sensitivity levels of laboratory standard microphones by reciprocity techniques, a broad variety of electroacoustical measurements are performed at the NIST. These include the calibration and characterization of measuring microphone systems, audiometers and audiometric earphones, and other instruments at very low sound-pressure levels, as well as the investigation of the linearity of these instruments over relatively large dynamic ranges. Techniques and apparatus used for these purposes include free-field measurements in the NIST anechoic chambers and sound-pressure measurements in acoustic couplers. Separation of signals from noise is achieved by means including analog filters, synchronous detectors (lock-in amplifiers), and dynamic signal analyzers. These incorporate a variety of excitation source types (e.g., sine-wave, chirp, bandlimited-impulse, periodic noise, and random noise signals) and signal-processing techniques (e.g., FFT, extraction of the fundamental component of a sine-wave signal from a digitized time record). Specific examples from the NIST research and measurement services are discussed.