Calibration of the UAF Chaparral Microphone Calibrator

1. Calibration of the UAF Chaparral Microphone Calibrator Charles R Wilson & John V Olson Infrasound Group, University of Alaska Fairbanks Presented at the Infrasound Technology Workshop De Bilt, The Netherlands October 2002 This presentation does not necessarily reflect the policies or views of the United States Government.

2. Overview of the Calibration Effort • Ten I53US Chaparral microphones were calibrated, establishing sensitivity and bandwidth, using the Los Alamos National Lab infrasound chamber • The calibrated microphones were then used to establish the sensitivity of the UAF field calibration units • Using previous measurements we then completed the calibrations of I55US (Windless Bight, Ant.) from previous measurements using the field units • Next, we plan to establish the use of the step compression and step rarefaction capability of the UAF calibrator to establish sensor characteristics Infrasound Technology Workshop

3. UAF Field Calibrator • Nominal pressures: 10 Pa, 100 Pa • Frequencies: 0.02, 0.1, 1 Hz • Phase synchronization • Step compression and rarefaction • Volumes: 1L, 10L Calibrator and power supply Infrasound Technology Workshop

4. Chaparral Model 5 Microphone High gain: ~0.2 V/Pa Low gain: ~0.04 V/Pa Bandwidth: ~0.02 – 50 Hz Infrasound Technology Workshop

5. Application of Calibrator in the Field Dr. Charles Wilson installing the field calibration unit at one of the GI/DOE array microphone sites Infrasound Technology Workshop

6. To Calibrate a “Calibrator” • Ten individual I53US microphones were calibrated using the Los Alamos National Laboratory Infrasound chamber using a 32 ubar, peak-to-peak pressure variation (and the generous assistance of R. Whitaker, and T. Sandoval) • Calibrated microphones were then used to establish the sensitivity of the field calibrator. • The UAF calibrator produces a 13.2 ubar peak-to-peak pressure variation. This cross comparison assumes the microphones are linear over these pressures. Infrasound Technology Workshop

7. Chaparral Microphone Calibration at Los Alamos National Labs Tom Sandoval and Charles Wilson prepare to calibrate a Chaparral microphone in the Los Alamos chamber Dan Osborne with the UAF portable calibrator at Los Alamos Infrasound Technology Workshop

8. Calibration of SN1451 Sensor Frequency Sensitivity dB (Hz) (V/ub) 4.124 0.0102 0 1.083 0.0102 0 0.480 0.0102 0 0.0783 0.0100 -0.172 0.0392 0.0098 -.347 0.0197 0.0091 -0.991 Infrasound Technology Workshop

9. Calibration Equation The sensitivity of a microphone, in volts/Pascal, can be determined from the equation S = A*W/CL*PA where S = sensitivity, volts/Pascal A = peak-to-peak amplitude of microphone output (adu) W = digitizer constant (volts/adu) CL = Chamber constant characterizing the Los Alamos chamber PA = atmospheric pressure (Pascals) In the first stage of our calibration this equation was used to establish the sensitivity of the microphones using the Los Alamos chamber. The same equation can then be used with the UAF calibrator to establish the chamber constant for the calibrator. Infrasound Technology Workshop

10. Calculation of the UAF Calibrator Chamber Constant The chamber constant for the UAF calibrator can be found using the same equation: CGI = A*W/PA*SL where A = peak-to-peak amplitude in digital units (adu) W = digitizer weight, volts/adu PA = atmospheric pressure in Pascals SL = Los Alamos microphone sensitivity (V/Pa) CGI is a dimensionless number that specifies the effectiveness of the UAF calibration unit. Infrasound Technology Workshop

11. Calibrator Unit #2 Sensitivity Sensor Frequency LANL CalConst (Hz) (V/ub) 1446 1.02 0.0104 1.362E-5 1448 0.999 0.0104 1.352E-5 1449 1.01 0.0103 1.341E-5 1450 0.986 0.0101 1.335E-5 1451 1.03 0.0102 1.278E-5 1452 1.01 0.0105 1.079E-5 1453 1.08 0.0103 1.360E-5 1454 1.01 0.0099 1.491E-5 1455 0.984 0.0101 1.335E-5 When used at the 10ub setting, unit #2 produces 1.325E-5*PA microbars. e.g. at 1 atmosphere (PA=1.013E6 Pa) the calibrator produces 13.42 ub (peak-to-peak). Average for calibration unit #2: (1.325 ± 0.101) E-5 Infrasound Technology Workshop

12. Calibrator Unit #3 Sensitivity Sensor Frequency LANL CalConst (Hz) (V/ub) 1446 1.57 0.0104 1.257E-5 1447 1.45 0.0098 1.424E-5 1448 1.56 0.0103 1.264E-5 1449 1.57 0.0103 1.329E-5 1450 1.59 0.0101 1.351E-5 1451 1.58 0.0102 1.304E-5 1452 1.53 0.0105 1.143E-5 1453 1.69 0.0103 1.348E-5 1454 1.03 0.0099 1.410E-5 1455 1.49 0.0101 1.296E-5 When used at the 10ub setting, unit #2 produces 1.313E-5*PA microbars. e.g. at 1 atmosphere (PA=1.013E6 Pa) the calibrator produces 13.30 ub (peak-to-peak). Average for calibration unit #3: (1.313 ± 0.081) E-5 Infrasound Technology Workshop

13. MB2000 Sensor Calibration We adapted the UAF calibrator to an MB2000 provided by Dr. Garces. As can be seen the lower tube of the calibrator is fitted with a pressure connection and flexible hose that can be attached to the input of the MB2000. Using the newly calibrated UAF calibrator it was possible to calibrate the MB2000 Infrasound Technology Workshop

14. MB2000 Calibration Results • The overall sensitivity near 1 Hz was found to be as follows: Frequency Sensitivity (Hz) (V/Pa) 1.0500 1.937e-2 0.9997 1.953e-2 Infrasound Technology Workshop

15. UAF Calibrator Step Input S/N 1451 step response • The UAF calibrator produces a step compression followed by a step rarefaction in the fore volume. This test is performed automatically during each calibration run. • A typical Chaparral microphone response is shown at the left. The two responses are identical • Some sensors (microphone + electronics) show overshoots rather than simple exponential decay. Infrasound Technology Workshop

16. Typical Microphone Step Responses/n 1451 These plots show the response to a compression and rarefaction. The plot on the right, using a log scale for amplitude, shows that the response is approximately exponential for the first few seconds. Infrasound Technology Workshop

17. Sn1449 Step Response This plot shows three successive responses to the UAF calibrator step input and indicates the consistency of the response over several successive compressions and rarefactions. The divergence of the curves for times greater than 20 seconds is due to the presence of low-frequency ambient noise. Infrasound Technology Workshop

18. MB2000 Response to Step Compression Infrasound Technology Workshop

19. Estimation of the Frequency Response from the Step Response Assuming the microphone and associated electronics are a linear, time-invariant system, the system function, h(t), and its transform, the system frequency response, can be found using the convolution theorem. Time domain Frequency domain y(t) = h(t)*x(t) Y(f) = H(f)X(f) Using the convolution theorem, H(f) = Y(f)/X(f) Since the data are discrete, we use the Z-transform. We fit y(t) with a polynomial and then develop its Z-transform. The input is the forevolume pressure. Infrasound Technology Workshop

20. Sensor Frequency Response Estimated from the Step Function s/n 1451 • The blue line at the left is the frequency response estimated from the step response. • The red points are the Los-Alamos calibrations. Infrasound Technology Workshop

21. Recommendation • A step compression can be used to estimate the sensitivity and bandwidth of the sensor and thereby monitor the sensor stability with time. • Combined with a sinusoidal input at a frequency in the center of the passband, a complete diagnosis can be made of the sensor characteristics. • We propose a simple field calibrator, built into each sensor system, that would produce a calibrated step compression/rarefaction pair. • Such a calibrator would be simple, inexpensive and could be commanded from the IDC for regular system checks. Infrasound Technology Workshop