Randall P. Wagner
Natl. Inst. of Standards and Technol., Bldg. 233, Rm. A147, Gaithersburg, MD 20899
Natl. Inst. of Standards and Technol., Gaithersburg, MD 20899
Acoustic center corrections for microphones are necessary for accurate free-field calibration by the reciprocity technique. Such corrections for several pairs of the relatively new type LS2aP microphone [IEC Std. 1094-1: 1992] were obtained by utilizing the theoretical inverse relationship between the sound pressure amplitude at the receiving microphone and the distance between the acoustic centers of the source and receiving microphones. For a nominally 10-V sinusoidal rms excitation of the source microphone, source-to receiver voltage ratios were measured with a dynamic signal analyzer at 500-Hz intervals in the extended frequency range (2 to 50) kHz. This procedure allowed all the data for a microphone pair to be gathered within several hours as a function of microphone diaphragm separation at 10-mm intervals from 101 to 311 mm. At each frequency, these ratios were corrected for atmospheric effects, including attenuation of sound, and then fit to a straight line (ratio versus diaphragm separation. Acoustic center corrections were calculated from the derived values of the fit parameters. These corrections agree with appropriately scaled values [IEC Publication 486-1974] for ``1-in.'' microphones with recessed diaphragms. Physical phenomena that cause small deviations from the linear fits will be discussed along with uncertainty estimates, and effects of spatially truncating data.