DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 1 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Infrasound Sensor Dynamic Calibrator ALCOVERRO Benoît DASE/TMG

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 2 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Dynamic Calibration l Dynamic calibration requires an infrasonic source with: –precise pressure signal applied to the sensor (some percent). –no phase distortion introduced by the calibrator (or measured error). –[0.001 Hz Hz] linear frequency range needed to verify the CTBT band [0.02 Hz - 4 Hz] (flat in amplitude and smooth phase). –Low sensitivity to noisy signals. l Solution to generate a pressure: –Variation of a sealed volume at constant temperature. –If V/V0 small, at first order, P = P0. V P = P0. V – P is static pressure dependant. Volume variation V Sealed volume V0 Static pressure P0 Pressure generated P0+ P

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 3 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 l The sensor is connected to a sealed cavity where the pressure is generated. l The use of small volumes (< 50 l) & light piston (< 100 g ) could increase the linearity up to 100 Hz (linear amplitude & smooth phase responses up to 10 Hz). l Low energy required to generate the pressure ( < 5 W for 10 Pa generated by Electro-mechanical driver) l Small size portable solution. Basic Acoustical Principle Basic Acoustical Principle V Small sealed volume (30 l) External pressure P0 P0+ P Displacement of piston Sensor to be calibrated Connection pipe P0+ P

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 4 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 P0+ P Sensor to be calibrated Actual solution Actual solution l Cavities without leaks : low frequencies generation. l Sensor inlet inside the front cavity & small volumes : extend the high frequencies response. (resonant frequency could be around 100 Hz : flat amplitude & phase 30 Hz)

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 5 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 6 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Leaks Verification l Verification by the mean of a Static pressure calibrator. l Measurement over 1 hour must be stable.( P < 0.2 hPa for 5 hPa over pressure) l Could be made before calibration

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 7 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Accurate pressure adjustment l Pressure generated is temperature & static pressure dependant: –the displacement of the piston must be adjusted before each calibration. l The solution: use a calibrated microphone: –measure the pressure at high frequency (> 10 Hz) –attach to an absolute pressure reference l The frequency used for adjustment must be in the overlap band between microphone & calibrator: –< 30 Hz for calibrator –> 10 Hz for current capacitance microphones

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 8 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Microphone Calibration l The microphone used is flat & currently calibrated from 20 Hz to 2 kHz within (+/- 2.3 %). l A 1kHz calibrator allows the measurement chain calibration at 10 Pa (+/- 1%) l 16 bits digitizers used in control device allows 1 LSB error ( < 0.01 %) This is the link with absolute reference Pressure (Quality procedures)

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 9 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Complete system l Precise pressure generation requires adjustments before calibration: l The calibrator accuracy depends on the microphone calibration: –currently < 3 % but < 1% possible by specific calibration SIGNAL GENERATOR 16 bits Power Amplifier Pressure Measurement 16 bits CALCULATOR Sync GPS PC Logical commands Pref Calibrated Microphone Sensor to be calibrated FEEDBACK LOOP & COMMAND CALIBRATOR MLS + / - Vref Sinus + / - Vref Response of calibrated sensor l Gain l Narrow band filter l mean value calculation

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 10 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Calibration Bench

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 11 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Calibration Features l Signals generated: –Sine Waves at finite frequencies [ ] Hz –Positives Pulses from [ ] ms width –Maximal length sequences with order [ ] & [ ] Hz sample rate for Transfer functions measurements. l Amplitude from [1 - 50] Pa (with on board power amplifier & 12 V DC supply) l Stimulus could be synchronized to an external trigger (1PPS for ex.) l Self calibration at 20 Hz is used to reach less than 3 % in absolute amplitude error (if no specific microphone calibration used) but could be < 1 %. l High frequency calibrator response could be measured by microphone & M.L.S. to precise H.F. measurements. l Frequency response : flat in Amplitude & Time over [DC - 30 Hz]. l Temperature stability: < 3 °C for < 1% error in amplitude.

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 12 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 l For calibration, pseudo-random sequences are used. l The pseudo-random sequences characteristics: –Binary sequences between +/- maxi. –Random like distribution but identical sequences (depending on order) –Spectral density = 1 over bandwidth –maximal energy applied to the sensor –Ref VANDERKOOY JAES Vol 42, N° 4 April 1994 l Transfer function measurements: Full range Calibration using Pseudo-random sequences SENSOR TO BE TESTED DIGITISER (one channel) Pseudo-random sequence (order n) Generated by calibrator Correlation Pseudo-random sequence (order n) locally generated Sensor response (Amplitude & Phase) Possibility to be distant

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 13 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Self Calibration & Sine Wave

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 14 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 M.L.S.

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 15 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Ex: Sensor sensitivity response ê 1 measurement ê pts MLS ê S.R. = 25 Hz ê duration = 2614 s ê 10 Pa excitation ê Sensor Nominal sensitivity: 20 mV/Pa

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 16 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Ex: Sensor response + tolerances % / static calibration 20 mV/Pa nominal sensitivity mV/Pa in static calibration mV/Pa in dynamic calibration

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 17 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Ex: MB2000 DC output response ê The calibrator is flat from up to 10 Hz (1.005 mV/Pa in static calibration)

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO 18 INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001 Conclusion l Effective broadband infrasound calibrator l Internationally patented l On board MLS allows amplitude & phase measurements with 1 channel digitizer l Self calibration requires no adjustments