SIM AUV.A-K1.prev Microphone Intercomparison

Transcription

1 Participants: Report on SIM AUV.A-K1.prev Microphone Intercomparison by Knud Rasmussen, DPLA CENAM, División de Vibraciones y Acústica km 4,5 Carretera a los Cués C.P Querétaro, México DPLA, Danish Primary Laboratory of Acoustics, DTU Branch Oersted Institute, Acoustic Technology Oersted Plads, Bldg. 352, DTU 28 Kgs. Lyngby Denmark INMETRO, Laboratório Del Electroacústica Av. N. S. das Graças 5 - Xerém, Rio de Janeiro Brazil INTI, Centro de Investigación en Acústica Av. Gral. Paz e/albarellos y Av, de los Constituyentes C.C. 157-(165) San Martin, Buenos Aires, Argentina Abstract The results of an inter-comparison on calibration of type LS1P microphones among the above-mentioned National Metrology Institutes are presented. Two microphones were circulated from one laboratory to the next, DPLA acting as the pilot laboratory. Due to an abrupt change in the sensitivities of the microphones before the calibrations at the last laboratory (CENAM) the results are compared in two groups using the calibration results at DPLA before and after the circulation as references. A consecutive comparison between CENAM and DPLA confirmed the results of the first comparison. The maximum RMS deviation from the mean value for the two LS1P microphones measured by DPLA, INMETRO and INTI is,38 while the maximum deviation from the mean value measured by DPLA and CENAM is,32. Thus the largest value,,38 may be considered as the key comparison reference value. Page 1 of 11

2 Intercomparison of pressure calibration of LS1 microphones by the reciprocity technique among DPLA, CENAM, INMETRO and INTI. 1 Introduction An intercomparison of pressure calibration of two type LS1 microphones has been performed among the National Metrology Institutes of Argentina (INTI), Brazil (INMETRO), Denmark (DPLA) and Mexico (CENAM) during 1997 to 1999, DPLA acting as the pilot laboratory. In the period 1997 to 1998 a similar intercomparison covering both type LS1 and LS2 microphones was conducted among 14 National Metrology Institutes under EUROMET, where DPLA, NPL and PTB acted as pilot laboratories, each serving 4 other laboratories. The present intercomparison would then serve as an unofficial link between EUROMET and SIM represented by the abovementioned three institutes, CENAM, INMETRO and INTI. 2 Organization of the Intercomparison Due to the lack of financial support and large traveling distances between the involved laboratories it was decided to circulate the microphones from one laboratory to the next rather than performing a star circulation. A time schedule was set up ensuring that the intercomparison could be finished within SIM in 5 months. Unfortunately this time schedule could not be followed and the microphones circulated within SIM for a period of 12 months. Furthermore both microphones showed significant changes in their sensitivities when returned to DPLA. The changes apparently took place before the calibrations at CENAM and consequently a repetition of the calibrations was suggested with a tight time schedule. However, it turned out that only CENAM was able to participate in the repetition and thus the present report on the final results are divided in two parts. The first part covers the initial calibrations at DPLA and the results from INMETRO and INTI and the second part covers the calibrations at CENAM, the following calibrations at DPLA and the results from both DPLA and CENAM from the repetition of the calibrations. The microphones were initially calibrated at DPLA in April and September 1997 and transported to CENAM in September A calibration took place at INMETRO in May 1998 INTI in July 1998 CENAM in October -November 1998 DPLA December 1998 The repetition of the calibrations took place at DPLA in March-April 2 CENAM May 2 DPLA June 2 Page 2 of 11

3 3 Calibration methods The calibrations were performed as reciprocity calibrations using closed couplers at all laboratories in the frequency range 63 Hz - 8 in accordance with IEC INMETRO and INTI used a 2-cm 3 Large Volume coupler up to 1 and a 3 cm 3 Plane Wave coupler from 1 and upwards. CENAM used two Plane Wave couplers of 3 cm 3 and 6 cm 3 volume, while DPLA used four Plane Wave couplers of approx and 3.5 cm 3 volume. Both INMETRO and INTI used the manually operated B&K Type 4143 Reciprocity Calibration Apparatus for the measurements while CENAM used the new automatic (computer operated ) apparatus B&K Type Both types of equipment uses a capacitor in series with the transmitter microphone to determine the transmitter current. The equipment at DPLA is a non-commercial computer operated laboratory construction built in It uses a decade resistor as measurement impedance for determining the transmitter current. However, for the measurements performed in 2 a series capacitor was used. The standard uncertainties (k=2) reported by the participants are listed in table 1. Table 1 - Standard uncertainties (k=2) in as declared by the laboratories DPLA,4,3,3,3,3,3,3,3,3,3,3,4,5,6 CENAM,4,4,4,3,3,3,3,3,3,3,5,5,6,1 INMETRO,5,5,5,5,5,5,5,7,7,7,1,1,1,11 INTI,5,5,5,5,5,7,1,1 4 Calibration results The microphones to be calibrated in the first circulation were: B&K Type 416 serial no and B&K Type 416 serial no For the repetition measurements the second microphone were not available but substituted by B&K Type 416 serial no All calibration results are referred to the reference conditions of 23 C, 11,325 kpa and 5 %RH but the actual calibrations were performed at the prevailing environmental conditions except at CENAM, where a pressurized vessel were used to obtain a static pressure close to the reference conditions. As the declared uncertainties in the most important part of the frequency range are comparable the reference value of the microphone sensitivities is taken as the arithmetic mean value of the reported results. Page 3 of 11

4 4.1 Pre- and post calibrations at DPLA As reported above the microphones showed a significant change in their sensitivities when returned to the pilot laboratory, DPLA. Figure 1 shows the changes observed for the two microphones. The sensitivities of both microphones have increased, apparently caused by a decrease of the mechanical tension in the diaphragm. This is an unusual behavior and figures 2 and 3 show the measured sensitivities at four frequencies over a period of five years for both microphones. It seems that both microphones has stabilized on the new sensitivities. Due to these changes it has not been possible to compare the results of all four participants directly. However, the results indicate that the changes took place after the calibrations at INTI and before the calibrations at CENAM. Consequently the reporting has been divided in two parts as explained above. Sensitivity change of microphones no no Figure 1 Difference of the measured sensitivity at DPLA of the two microphones B&K Type 416 after the circulation among the partners relative to the measured sensitivity before the circulation. Page 4 of 11

5 re 1V/Pa Jan-95 Jan-96 Jan-97 Jan-98 Jan-99 Jan- Jan Hz 255 Hz 51 Hz 1 Figure 2 Measured sensitivity of microphone B&K Type 416 no at DPLA over a period of 5 year. Note the change during 1998 when the circulation took place re 1V/Pa Jan-95 Jan-96 Jan-97 Jan-98 Jan-99 Jan- Jan Hz 255 Hz 51 Hz 1 Figure 3 Measured sensitivity of microphone B&K Type 416 no at DPLA over a period of 5 year. Note the change during 1998 when the circulation took place. Page 5 of 11

6 4.2 DPLA-INMETRO-INTI comparison The results of the initial calibrations at DPLA and those at INMETRO and INTI before the significant changes took place in the sensitivities are shown in Tables 2 and 3 together with the mean value of the three results. Figures 4 and 5 show the deviation from the mean value for the three laboratories. Table 2 - Reported sensitivities in re 1V/Pa for Type 416 no DPLA -26,96-26,99-27,1-27,1-29,97-26,94-26,88-26,79-26,66-26,46-26,17-25,88-25,88-27,14 INMETRO -27, -27,2-27,3-27,4-26,98-26,96-26,9-26,82-26,7-26,51-26,22-25,94-25,93-27,17 INTI -26,99-27, -27,1-27,1-26,98-26,81-26,2-27,15 Average -26,98-27, -27,2-27,2-26,98-26,95-26,89-26,81-26,68-26,48-26,2-26,91-25,9-27,15 Table 3 - Reported sensitivities in re 1V/Pa for Type 416 no DPLA -27,8-27,11-27,13-27,14-27,11-27,9-27,5-26,99-26,9-26,76-26,59-26,47-26,64-27,86 INMETRO -27,1-27,13-27,14-27,15-27,8-27,8-27,2-26,97-26,89-26,77-26,6-26,5-26,69-27,92 INTI -27,1-27,11-27,13-27,13-27,11-26,97-26,6-27,91 Average -27,9-27,12-27,13-27,14-27,1-27,8-27,3-26,98-26,89-26,77-26,6-26,48-26,67-27,9 Deviation from mean value B&K DPLA INMETRO INTI Figure 4 Deviation of measurement by each participant from the mean value of all for microphone B&K Type 416 no Page 6 of 11

7 Deviation from mean value B&K DPLA INMETRO INTI Figure 5 Deviation of measurement by each participant from the mean value of all for microphone B&K Type 416 no The mutual differences between the results are all within the uncertainties declared by the laboratories and the maximum deviation from the mean values is.38 (at 8 for s/n 9738) and the corresponding maximum difference is,6. The measurement capabilities of the participants are demonstrated on figure 6. The figure shows the standard uncertainties (k = 2) declared by the participants, see table 1 (heavy lines) and a RMS-deviation calculated from the measurement results as ( Xi Xref ) 2 RMSdeviation = i i where i is the number of observations, X i and X ref the measurement result and the corresponding reference value. The RMS-deviations are shown as the values for each microphone (see tables 2 and 3), i representing the participants (thin lines) as well as the values for each participant, i representing the microphones (dotted lines). Page 7 of 11

8 .12 Declared uncertainty and RMS-deviation from mean value DPLA INMETRO INTI no no Figure 6 Uncertainties declared by the participants, see table 1 (heavy lines) and the RMS-deviations from the mean value of the measured values, see tables 2 and 3. The RMS-deviations are shown for each microphone (thin lines) and for each participant (dotted line) 4.3 CENAM-DPLA comparison The results of the calibrations at CENAM and DPLA after the changes in the sensitivity took place are shown in Tables 4-6 together with results of the repetition of the calibrations in the spring of 2. For microphone no the average value is calculated from all 5 measurement series. Figures 7 and 8 show the deviations from the average value for the two laboratories. In the major frequency range the deviations are less than,1 and the maximum deviation is,32 found at 8 for no Table 4 - Reported sensitivities in re 1 V/Pa for Type 416 no October - December 1998 CENAM DPLA April - June 2 DPLA CENAM DPLA Average Page 8 of 11

9 Table 5 - Reported sensitivities in re 1 V/Pa for Type 416 no CENAM DPLA Average Table 6 - Reported sensitivities in re 1 V/Pa for Type 416 no DPLA CENAM DPLA Average Deviation from mean value B&K CENAM DPLA DPLA CENAM DPLA Figure 7 Deviation of measurements from the over-all mean value for microphone B&K Type 416 no / - measurements in 1998 / - measurements in 2 Page 9 of 11

10 Deviation from mean value B&K B&K CENAM DPLA DPLA CENAM DPLA Figure 8 Deviation of measurements from mean value of each microphone B&K type 416 no and / no / no The measurement capability of the two participants are demonstrated in figure 9. The figure shows the standard uncertainties (k = 2) declared by the participants, see table 1 and the maximum observed deviation from the mean value for any of the participants and microphones. The observed deviations are all well within the declared uncertainties..12 Declared uncertainty and max. deviation from mean value DPLA CENAM Max. deviation Figure 9 Declared uncertainties by the participants (see table 1) and the maximum deviation from the mean value for both participants and for all microphones Page 1 of 11

11 5 Conclusion The present inter-comparison of pressure calibration of type LS1 microphones concerned four National Metrology Institutes: Centro Nacional de Metrologia (CENAM - Mexico), Danish Primary Laboratory of Acoustics (DPLA - Denmark), Instituto Nacional de Metrologia, Normalização e Qualidade Industrial (INMETRO - Brazil), Instituto Nacional de Technologia Industrial (INTI - Argentina). The calibrations were performed as a standard calibration for customers, ie using the normal measurement equipment and method at each laboratory. The reported results are referred to the reference environmental conditions of 23 C, kpa and 5% RH. The maximum RMS deviation of the reported sensitivities from the mean value was,38 and the corresponding maximum deviation,6 between DPLA, INMETRO and INTI. The reported results from DPLA and CENAM showed a maximum deviation of,32 from the mean value. For all participants the maximum deviations were found at the highest measurement frequency, 8. The demonstrated deviations from the mean value are all within the declared uncertainty interval by the participants. Page 11 of 11