Wagner Randall P, Nedzelnitsky Victor
National Institute of Standards and Technology, Gaithersburg, MD 20899-8220.
J Res Natl Inst Stand Technol. 2007 Apr 1;112(2):107-14. doi: 10.6028/jres.112.008. Print 2007 Mar-Apr.
The electrical measurements required during the primary calibrations of laboratory standard microphones by the reciprocity method can be influenced by power line interference. Because of this influence, the protocols of international inter-laboratory key comparisons of microphone calibrations usually have not included measurements at power line frequencies. Such interference has been observed in microphone output voltage measurements made with a microphone pressure reciprocity calibration system under development at NIST. This system was configured for a particular type of standard microphone in such a way that measurements of relatively small signal levels, which are more susceptible to the effect of power line interference, were required. This effect was investigated by acquiring microphone output voltage measurement data with the power line frequency adjusted to move the frequency of the interference relative to the center frequency of the measurement system passband. These data showed that the effect of power line interference for this system configuration can be more than one percent at test frequencies harmonically related to the power line frequency. These data also showed that adjusting the power line frequency to separate the interference and test frequencies by as little as 1.0 Hz can reduce the effect of the interference by at least an order of magnitude. Adjustment of the power line frequency could enable accurate measurements at test frequencies that otherwise might be avoided.
采用互易法对实验室标准传声器进行初次校准期间所需的电气测量可能会受到电源线干扰的影响。由于这种影响，传声器校准的国际实验室间关键比对协议通常不包括在电源频率下的测量。在美国国家标准与技术研究院（NIST）正在开发的传声器压力互易校准系统进行的传声器输出电压测量中观察到了这种干扰。该系统针对特定类型的标准传声器进行了配置，使得需要测量相对较小的信号电平，而这些电平更容易受到电源线干扰的影响。通过获取传声器输出电压测量数据来研究这种影响，其中调整电源线频率以使干扰频率相对于测量系统通带的中心频率移动。这些数据表明，对于该系统配置，在与电源线频率呈谐波关系的测试频率下，电源线干扰的影响可能超过1%。这些数据还表明，将电源线频率调整到使干扰频率和测试频率相隔仅1.0 Hz，就可以将干扰的影响降低至少一个数量级。调整电源线频率能够在否则可能会避免的测试频率下进行准确测量。
