1. Characterization and Suppression of Wind Noise Using a Large-Scale Infrasound Sensor Array Carrick L. Talmadge Doug Shields Kenneth E. Gilbert The University of Mississippi, Oxford, MS USA

2. Infrasound Detection source propagation receiver Source characterization (wave form, signal level, directionality, etc.);. Effects of vertical variation in wind-velocity and temperature profiles; including diurnal effects Effects of turbulent scattering Terrain effects Must consider effects of: wind-generated sound atmospheric turbulence (intrinsic pressure fluctuations) pressure probe body on pressure measurements Robust system design needs data-driven models of all three components!

3. Advantages of Sensor Arrays Arrays provide directionality (source location); noise reduction using more sophisticated DSP algorithms; the ability to separately characterize the signal and the noise.

4. Old Array Technology N Sensors D A/D Signal Processing Total Cable Required N 2 D/4 Example: N = 40 D = 5m L = 200m Total Cable = 2000m

5. New Array Technology N Sensors D A/D P P P P P P Signal Processing Total Cable Required ND Example: N = 40 D = 5m L = 200m Total Cable = 200m

6. Smart Sensor Arrays Components of smart sensor element: Ruggedized sensor and sensor body Signal conditioning with programmable gain and anti-alias filter Analog-to-digital stage with programmable sampling rate Microcontroller-based networking using RS-485 standard (~ 2Mbps data rate). Remotely controlled self-calibration and diagnostic testing of individual sensors. Approximate delivery dates for bimorph sensors: 8 prototypes January 1, 2002 100 manufactured units March 1, 2002.

7. Second-Generation Array RS-485 networked sensor elements DSP-basedcollector units with RS-485 to wireless modem wireless link central processor sub arrays

8. Bimorph Capsules Resonant Frequency - 3 kHz Sensitivity - 1 to 4 mV/Pa Temperature Compensation –Reverse bimorphs –Insulated enclosures, small openings Charge Generating –Must operate into a high impendence

9. Potted Capsules & Housing

10. Acoustic Sensor Development Gabrielson Piezo-Resistive Transducer Features: 0.005 - 1000 Hz High Sensitivity (20 mV/Pa) Low Power Usage Low Cost (Less than $250 per unit) 9V Batteries Add anti-alias filter and 24-bit sigma- delta A/D converter. Replace acrylic back volume with insulated brass back volume instrumented for internal temperature. Move first-stage amplifier onto sensor board (small- outline IC).

11. Oxford Airport Array Experiment Chris Clark, Ken Gilbert, Doug Shields, Carrick Talmadge, Ron Wagstaff Chad Williams, Jay Williams, Zak Williams Experiment took place on April 6, 2001 40 minutes of data were collected over an ~ 1 hr interval 2 microphone arrays, sonic anemometer Levels calibrated using a B&K sound meter (not to scale)

12. Experimental Setup south B&K array (6x) anemometer bimorph array (11x)

13. Single Sensor (B&K) Results Sensor # Successive sensor outputs have been shifted by –10Pa. Run #1 (50-Hz tone) initial 10 seconds of recording

14. Amplitude and Phase Shown is the amplitude and phase for the 50-Hz tone from the sensor 1 read-out. The propeller-driven plane which lands around 150 seconds had no noticeable influence on the 50- Hz tone… pops Prop plane landing (CPA) Run 1: 50-Hz tone

15. Frequency [Hz] Pressure [Pa] Time [sec] Signal Amplitude Frequency [Hz] Run 1: 50-Hz Tone Time SeriesSpectrum (Fourier Transform) RMS Averaged Spectrum

16. Time [sec] Level [dB SPL] Count Level [dB SPL] 40-Hz bin 50-Hz bin 60-Hz bin 50-Hz bin Run 1: 50-Hz Tone median RMS

17. Run 1: 50-Hz tone Note that pops in playback create erroneous spectra using RMS spectrogram. Median spectrogram is more robust and will be used here throughout. Calibration of B&K sensors is excellent between 10-250 Hz (error is on the order of 1 dB). (RMS) median

18. Run 6: 25-50 Hz Tones, 5-Hz Steps wind-generated sound

19. Yocona River Array Experiment 1 16 15 30 6.46 m 0.8 mile Sound Source Observation Station 350 meters Yocona River Hwy 7 N Prevailing Wind Sewage Disposal Plant

20. Prevailing Wind 02468 1 16 15 30 6.46 m N 3

22. Wind Noise Correlation Strength vs. Distance Correlation Parallel to Wind Direction Correlation Transverse to Wind Direction Separation [m] Maximum Correlation

23. Conclusions Large-scale microphone arrays are a versatile, transportable alternative to pipe arrays. In general, they provide superior wind-noise reduction due to the more sophisticated DSP algorithms which are possible. The three components of wind noise (intrinsic pressure fluctuations; wind-generated sound; probe-body induced pressure fluctuations) can be decoupled and separately characterized.