Influence of ground-shield configuration in reciprocity calibration of laboratory standard microphones

The open-circuit sensitivity of condenser microphones, and in particular Laboratory Standard (LS) and Working Standard (WS) microphones, depend on the mechanical ground-shield configuration of the preamplifier. Therefore, the mechanical ground-shield configurations of preamplifiers for LS and WS microphones are defined in the IEC 61094 standard series. The sensitivity, dynamic range and acoustic impedance of the microphone loaded by a preamplifier also depend on the electrical configuration of the shield and the preamplifier’s input impedance. Minimizing the load on the microphone is generally preferable. This is achieved with “driven” shield where the shield is driven with the microphone output signal. Recently, it has been shown that adaptors used for free-field reciprocity calibration of WS3 microphones with the driven shield configuration may present a too high load to the preamplifier type traditionally used. In this paper, it is showed that a simple modification of the preamplifier can eliminate this problem, even for adaptors that present a rather extreme load to the preamplifier. The ground-shield configuration and the electrical configuration of the shield is discussed, both in the case of ideal and non-ideal electrical conditions, and in the context of reciprocity calibration of microphones and the 61094 standard series.