Reciprocity calibration of Laboratory. Standard Microphones. There is simply no substitute for knowing what you re doing

Transcription

1 Reciprocity calibration of Laboratory Standard Microphones Maria Enge, Philipp Begoff, Frank Schulz There is simply no substitute for knowing what you re doing Jeff Case

2 Topics 1. Motivation 2. Introduction in the Microphone Calibration 3. Realization of the System Welcome at SPEKTRA 4. Challenges during implementation 5. Measurement uncertainty 6. Comparison PTB METAS DFM SPEKTRA 7. Conclusions and Outlook Implementation of primary method for pressure calibration 2

3 Motivation Why did SPEKTRA start working on primary acoustics? Increasing significance of acoustic quality in industrial product development Increasing need of calibration possibilities to ensure quality of measurements Creation of a test arrangement for the reciprocity calibration as a transparent system Several inquiries from customers, looking for an alternative to B&K Not satisfied with the support and service Insufficient transparency of signal processing in Black Box MP.exe 3

4 Introduction in the Microphone Calibration Microphone Calibration: Determination of the Transfer Coefficient (Relation from Output Voltage to Sound Pressure Level) Secondary Calibration Comparison Calibration Comparison of the Microphone with a higher Reference Microphone Different Methods available Primary Calibration Absolute Calibration Direct Determination of the Transfer Coefficient Only Reciprocity Method available 4

5 Introduction in the Microphone Calibration Microphone Calibration: Determination of the Transfer Coefficient (Relation from Output Voltage to Sound Pressure Level) Method Actuator response Method Pistonphone Calibration Substitution Method Comparison Method Reciprocity Method Measurement Uncertainty 0,1 to 0,5 db 0,07 to 0,3 db 0,06 to 0,14 db 0,06 to 0,14 db 0,03 to 0,05 db Origin: Brüel & Kjær, Microphone Handbook Volume 1, Technical Documentation,

6 Introduction in the Microphone Calibration Continuous evidence chain to the reference value of the national standard Traceability via calibration hierarchy Highest Level NMI Subordinated Calibration centers in relation to the national standard 6

7 Introduction in the Microphone Calibration Based on the assumption of reciprocity of microphones Basics: reciprocal Sound Converter U q i=0 = p i q=0 Microphone U p i=0 = q i p=0 Transfer Coefficient of the Microphone M = U i=0 p = q p=0 i Microphone usage as Sound Receiver and Transmitter 7

8 Introduction in the Microphone Calibration Based on the assumption of reciprocity of microphones Acoustical Connection in between two Microphones by a cylindrical excavation (Coupler) Sound-Field Microphone 1 Microphone 2 Electrical Transfer Impedance: Z e,12 = U 0,2 i 1 = M 1 M 2 Z a,12 8

9 Introduction in the Microphone Calibration Based on the assumption of reciprocity of microphones First Setup Second Setup Third Setup Microphone 2 Microphone 3 Microphone 3 Coupler Coupler Coupler Microphone 1 Microphone 1 Microphone 2 Implementation of primary method for pressure calibration 9

10 Realization of the System Block diagram: Raw date available from CS18 System (Voltage of Transmitter and Receiver microphone) Raw data are available in text or excel format 10

11 Realization of the System SRS 35 Vibration Control System with Microphone ANA and 200 V Microphone Supply Get raw date from CS18 System (Voltage of Transmitter and Receiver microphone) Raw data are available in text or excel File 11

12 Realization of the System Sapphire couplers with different length 3.06 mm 3.75 mm and 6.10 mm Pressure chamber: Microphone 2 Microphone 1 Coupler Coupler holder B&K 4180 G.R.A.S.40AU 12

13 Realization of the System Calculation of the microphone sensitivity acquisition of the measured values calculation of the acoustical transfer impedance correction of the thermal conduction, radial wave-motion, ambient pressure and temperature calculation of the electrical transfer impedance calculation of the pressure sensitivity fitting of the microphone parameter (front cavity, equivalent volume, resonance frequency, loss factor) 13

14 Realization of the System Calculations with MATLAB (Implementation of all calculations): Calculation of the acoustic and electrical transfer impedance Calculation of all correction factors (thermal, pressure, radial wave, ) Calculation of microphone sensitivity according to IEC :2009 Fitting of the microphone parameters The Software is not a black box! It is customized and all calculations are open 14

15 delta [db] Realization of the system Comparison MATLAB Script METAS SPEKTRA 5,0E-05 4,0E-05 3,0E-05 2,0E-05 MpETAS - MpSPEKTRA ,0E-05 0,0E+00-1,0E-05-2,0E-05 MP_Cpl30601 [db] MP_Cpl37501 [db] MP_Cpl47004 [db] MP_Cpl61001 [db] -3,0E-05-4,0E-05-5,0E-05 31,62 316, frequency [Hz] Excellent Conformity 15

16 Challenges during implementation Correction of ambient pressure and temperature Correction Values are given by B&K Based on evaluation of 20 type 4180 and 25 type 4160 Microphones No values for other microphone type (e.g. GRAS) in the standard available (only preliminary studies by NIM Canada 2005) Fitting of Microphone parameters (resonance, loss factor, equivalent volume) Theoretical background: Usage of 3 couplers with different length 9 measurements lead to 3 sensitivities per microphone Numerical Variation of Resonance frequency, loss factor and equivalent volume Goal: minimize the deviation of the sensitivities per microphone Found parameters differ from calibration to calibration (annual calibration or key comparison) Found parameters sometimes physical not logic 16

17 uncertainty contribution/db Measurement uncertainty 0,06 voltage measurement impedance of the microphone preamplifier coupler length coupler diameter leakage coupler microphone front cavity 0,04 equivalent volume resonance frequency loss factor temperature coefficient pressure coefficient effective membrane diameter temperature 0,02 air pressure humidity heat conduction correction radial wave-motion correction temperature/pressure correction rounding errors 0,00 31, frequency/hz repeatability residual factors 17

18 Measurement uncertainty Expanded Measurement Uncertainty 0,081 db for 31,5 Hz Residuale Residual Einflüsse factors Wiederholpräzision Repeatability Rundungsfehler Rounding errors Temperatur-/Luftdruckkorrektur Temperature/pressure correction Radialwellenkorrektur Radial wave-motion correction Wärmeleitungskorrektur Heat conduction correction Feuchtigkeit Humidity Luftdruck Air pressure Temperature Effektiver Effective Membrandurchmesser membrane diameter Luftdruckkoeffizient Pressure coefficient Temperaturkoeffizient Temperature coefficient Verlustfaktor Loss factor Resonanzfrequenz Resonance frequency Äquivalenzvolumen Equivalent volume Vorraumvolumen Front cavity Leckage Leakage Kuppler coupler microphone Mikrofon Kupplerdurchmesser Coupler diameter Kupplerlänge Coupler length Impedanz des Einspeiseadapters Impedance of the microphone preamplifier Spannungsmessung Voltage measurement 0,000 0,010 0,020 0,030 0,040 0,050 0,060 /db 18

19 Measurement uncertainty Expanded Measurement Uncertainty 0,171 db for 20 khz Residuale Einflüsse factors Wiederholpräzision Repeatability Rundungsfehler Rounding errors Temperatur-/Luftdruckkorrektur Temperature/pressure correction Radialwellenkorrektur Radial wave-motion correction Wärmeleitungskorrektur Heat conduction correction Feuchtigkeit Humidity Luftdruck Air pressure Temperature Effektiver Effective Membrandurchmesser membrane diameter Luftdruckkoeffizient Pressure coefficient Temperaturkoeffizient Temperature coefficient Verlustfaktor Loss factor Resonance Resonanzfrequenz frequency Äquivalenzvolumen Equivalent volume Vorraumvolumen Front cavity Leckage Leakage coupler Kuppler microphone Mikrofon Kupplerdurchmesser Coupler diameter Kupplerlänge Coupler length Impedanz des Einspeiseadapters Impedance of the microphone preamplifier Spannungsmessung Voltage measurement 0,000 0,010 0,020 0,030 0,040 0,050 0,060 /db 19

20 microphone sensitivity/db (re 1V/Pa) Measurement uncertainty -37,80-37,90-38,00-38,10 PTB -38,20 METAS SPEKTRA -38,30-38,40-38,50 31, frequency/hz 20

21 Sensitivity [db re 1V/Pa] Measurement uncertainty Comparison measurements between PTB, METAS, DFM and SPEKTRA ,8-37, ,1 PTB (3x 4180) METAS (3x 4180) -38,2 DFM (1x 4180, 2x 40AU-1) SPEKTRA (3x 4180) -38,3 DFM calibrated 2 x 40AU-1 and 1 x B&K 4180 Excellent Conformity at all particularly in between DFM and METAS -38,4-38,5 31, frequency [Hz] 21

22 Comparison calibration (LS2P) SPEKTRA PTB METAS Motivation to start investigations on the G.R.A.S. 40AU-1 B&K 4180 the only accepted reference standard microphone (LS2P) Non- availability of the B&K 4180 since more than 1 year Extreme price increase of B&K 4180 Getting a 2 nd source supplier for a very important part G.R.A.S. developed a complete new LS2P (before 40AU now 40AU-1) - New material - New production process - New sealing - New polishing 22

23 Comparison calibration (LS2P) SPEKTRA PTB METAS Comparison B&K 4180 and G.R.A.S. 40AU-1 B&K 4180 G.R.A.S. 40AU-1 Vent hole Side Bottom Seal Metal Metal Teflon Coated Membrane exchangeable No Yes 23

24 Sensitivity, db (re 1 V/Pa) En Comparison calibration (LS2P) SPEKTRA PTB METAS Microphone cartridge LS2P Brüel & Kjaer 4180 Primary reciprocity calibration according to IEC :2009 Very low deviation between calibration results Equivalence score E n lower than < 1 (~ 0,1 0,6) Estimated measurement uncertainty are valid (PTB, SPEKTRA, METAS) -37,70-37,90-38,10 SPEKTRA METAS PTB Brüel & Kjaer 4180 Ser. Nr.: Difference at 1 khz: 0,02 db... 0,03 db (Inside exp. uncertainty) 2,0 1,8 1,6 1,4 1,2 1,0 Equivalence score En between SPEKTRA and PTB (should be < 1) En score (SPEKTRA - PTB) Limitation (En = 1) -38,30 0,8 0,6-38,50 0,4 0,2-38, , Frequenz/Hz Frequenz/Hz 24

25 Comparison calibration (LS2P) SPEKTRA PTB METAS DFM Microphone cartridge LS2P G.R.A.S. 40AU-1 Primary reciprocity calibration according to IEC :2009 Sensitivity, db re 1V/Pa -38,20-38,30-38,40-38,50-38,60-38,70-38,80-38,90-39,00-39,10-39,20 GRAS 40AU Ser. Nr.: DFM Denmark (Danish National Fundamental Metrology Institute) METAS Switzerland SPEKTRA Germany Difference at 1 khz: 0, ,10 db Frequency, Hz Results: Very low deviation of the relative frequency response Small systematic deviation in between calibration results 25

26 Comparison calibration (LS2P) SPEKTRA PTB METAS DFM Microphone cartridge LS2P G.R.A.S. 40AU-1 Primary reciprocity calibration according to IEC :2009 Next Steps: Dec Long time stability measurements at DFM side Jan. / Feb various measurements at PTB side Long time stability measurements at SPEKTRA side Evaluation of reciprocity conditions by receiver transmitter exchange 26

27 Conclusions and Outlook General Discussion Technical barrier to realize the standard is quite high Standard seems to be written to calibrate B&K microphones Standard should be revised (IEC , IEC TS ) Outlook SPEKTRAs goal is to set up a manufacturer independent calibration system Contribution to the quality assurance of acoustic measurements 27

28 3rd DMS - Sep