Wagner Randall P, Guthrie William F
National Institute of Standards and Technology, Gaithersburg, MD 20899 USA.
J Res Natl Inst Stand Technol. 2015 Aug 17;120:164-72. doi: 10.6028/jres.120.012. eCollection 2015.
The devices calibrated most frequently by the acoustical measurement services at the National Institute of Standards and Technology (NIST) over the 50-year period from 1963 to 2012 were one-inch condenser microphones of three specific standard types: LS1Pn, LS1Po, and WS1P. Due to its long history of providing calibrations of such microphones to customers, NIST is in a unique position to analyze data concerning the long-term stability of these devices. This long history has enabled NIST to acquire and aggregate a substantial amount of repeat calibration data for a large number of microphones that belong to various other standards and calibration laboratories. In addition to determining microphone sensitivities at the time of calibration, it is important to have confidence that the microphones do not typically undergo significant drift as compared to the calibration uncertainty during the periods between calibrations. For each of the three microphone types, an average drift rate and approximate 95 % confidence interval were computed by two different statistical methods, and the results from the two methods were found to differ insignificantly in each case. These results apply to typical microphones of these types that are used in a suitable environment and handled with care. The average drift rate for Type LS1Pn microphones was -0.004 dB/year to 0.003 dB/year. The average drift rate for Type LS1Po microphones was -0.016 dB/year to 0.008 dB/year. The average drift rate for Type WS1P microphones was -0.004 dB/year to 0.018 dB/year. For each of these microphone types, the average drift rate is not significantly different from zero. This result is consistent with the performance expected of condenser microphones designed for use as transfer standards. In addition, the values that bound the confidence intervals are well within the limits specified for long-term stability in international standards. Even though these results show very good long-term stability historically for these microphone types, it is expected that periodic calibrations will always be done to track the calibration history of individual microphones and check for anomalies indicative of shifts in sensitivity.
在1963年至2012年的50年期间，美国国家标准与技术研究院（NIST）的声学测量服务部门校准频率最高的设备是三种特定标准类型的一英寸电容式麦克风：LS1Pn、LS1Po和WS1P。由于NIST长期以来一直为客户提供此类麦克风的校准服务，因此它处于一个独特的位置，可以分析有关这些设备长期稳定性的数据。这段悠久的历史使NIST能够获取并汇总大量属于其他各种标准和校准实验室的麦克风的重复校准数据。除了在校准时确定麦克风的灵敏度外，重要的是要确信，与校准期间的校准不确定度相比，麦克风在两次校准之间的时间段内通常不会发生显著漂移。对于这三种麦克风类型中的每一种，通过两种不同的统计方法计算了平均漂移率和近似95%的置信区间，结果发现两种方法的结果在每种情况下差异均不显著。这些结果适用于在合适环境中小心使用的这些类型的典型麦克风。LS1Pn型麦克风的平均漂移率为每年-0.004 dB至0.003 dB。LS1Po型麦克风的平均漂移率为每年-0.016 dB至0.008 dB。WS1P型麦克风的平均漂移率为每年-0.004 dB至0.018 dB。对于这些麦克风类型中的每一种，平均漂移率与零没有显著差异。这一结果与设计用作传递标准的电容式麦克风预期的性能一致。此外，界定置信区间的值完全在国际标准规定的长期稳定性极限范围内。尽管这些结果表明这些麦克风类型在历史上具有非常好的长期稳定性，但预计仍将定期进行校准，以跟踪各个麦克风的校准历史，并检查是否存在表明灵敏度变化的异常情况。
