Analysing the effects of phase sensitivity in low frequency primary microphone calibrations

Abstract Phase sensitivity data, in pressure reciprocity calibrations, has largely been overlooked due to the focus historically mainly only been on the modulus data. This study presents the low frequency variations that were observed between the modulus and phase sensitivities of LS2P microphones using multiple plane-wave couplers. It is shown that low frequency phase sensitivity data is more sensitive and is influenced more by the coupler and microphone geometries. Using plane-wave couplers with a longer cavity length, i.e. larger volumes, produces the best results. Differences has been observed between modulus and phase sensitivity data in the frequency range 1–100 Hz, which brings attention to the relevance of also evaluating phase sensitivity data more closely in the context of primary pressure reciprocity calibrations and not only the modulus data. It is observed that complex sensitivity data, more specifically phase sensitivity data has more relevance than just that of determining the resonant frequency of Laboratory Standard condenser microphones. Measurement uncertainties relating to low frequency reproducibility are also presented and it is shown that the uncertainties related to the phase sensitivity low frequency reproducibility are more prevalent than for the same modulus data. Evaluation of low frequency phase sensitivity data has resulted in a 66% smaller phase uncertainty of measurement at 1 Hz. It is concluded that smaller low frequency uncertainties can be obtained by using longer plane-wave couplers and that phase sensitivity data can be utilised as an analytical tool for improving low frequency measurement accuracy.

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