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

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!

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.

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

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

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, manufactured units March 1, 2002.

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

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

Potted Capsules & Housing

Acoustic Sensor Development Gabrielson Piezo-Resistive Transducer Features: 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).

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, 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)

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

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

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

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

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

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 Hz (error is on the order of 1 dB). (RMS) median

Run 6: Hz Tones, 5-Hz Steps wind-generated sound

Yocona River Array Experiment m 0.8 mile Sound Source Observation Station 350 meters Yocona River Hwy 7 N Prevailing Wind Sewage Disposal Plant

Prevailing Wind m N 3

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

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.