Determination of acoustic center correction values for type LS2aP microphones at normal incidence

Abstract

Frequency-dependent acoustic center correction values are required to obtain accurate microphone calibrations in the free-field by the reciprocity technique. These values were determined for IEC type LS2aP microphones at normal incidence by utilizing the theoretical inverse relationship between the sound pressure amplitude at the acoustic center of a receiver and the distance between acoustic centers of source and receiver. A dynamic signal analyzer was used to measure the gain factor between the amplified output voltage of the receiver and the source input voltage at 500-Hz intervals in the extended frequency range 2–50 kHz. This procedure allowed all the data for a microphone pair to be gathered within several hours for microphone diaphragm separations from 101–311 mm at 10-mm intervals. At each frequency, the reciprocal of this gain factor as a function of microphone diaphragm separation was fit to a straight line after correction for atmospheric effects, including attenuation of sound caused by atmospheric absorption. Mean acoustic center correction values (from four microphones combined in six pairs) were calculated using the parameters obtained from the fit and found to be in good agreement with published values calculated by the boundary element method, as well as with values predicted by scaling published values of the acoustic center correction for microphones with a geometrical configuration similar to that of IEC type LS1P microphones.

A New Method for the Determination of the Acoustic Center of Acoustic Transducers

“…One can consider that in this last result most of the disturbances are eliminated. Finally, the results obtained experimentally here are in good agreement with several others presented in Figure 8, namely the standardized ones [1] (full line), and those obtained experimentally using the reciprocity method [3] (stars)a nd [4] (crosses). …”

“…The signal to noise ratio is increased because here the acoustic source is ao ne inch microphone flushmounted in aquasi-infinite plane bafflewhich provides in the lower frequencyr ange an acoustic leveln early 15 dB higher than the acoustic levelobtained in the current technique (a half inch emitting microphone set at the end of a semi-infinite half inch cylindrical rod [3,4]). …”

“…Several methods have been used for determining the acoustic center of at ransducer: one of them relies on phase measurements [2], butmost of them are based upon the measurement of the deviations of the amplitude of the sound pressure from the inverse distance law [3,4]. These methods are summarized and discussed in reference [5].…”

The acoustic center of laboratory standard microphones

“…However, any real anechoic room is less than perfect, and small reflections from the walls and interference between the microphones will contaminate the transfer function making the estimate of the acoustic center less accurate. The nature of these unwanted effects has been analyzed using statistical methods, 7 and using time-selective procedures. 8 Although time-selective methods can eliminate the unwanted contaminations, application of such methods may affect the accuracy of the estimated acoustic center.…”

A note on the concept of acoustic center

“…18 And finally Wagner and Nedzelnitsky have determined the acoustic center of microphones of type LS2 using measured values of the magnitude of the transfer function between two microphones. 19 A few examples of methods of determining the acoustic center from phase measurements have been found in the literature. Ando determined the acoustic center of a ''pipe horn'' loudspeaker from measurements of the phase shift between two positions.…”