NCPAAtmospheric

Acoustics

T3.1-P28 Status of Digital Infrasound Sensors Developed by the NCPACarrick Talmadge, The University of Mississippi

National Center for Physical Acoustics, Oxford, MS

higher frequencies strongly attenuated, phase becomes incoherent

very flat amplitude/phase response below 500 Hz–ideal for long-distance sensing

NCPA Analog Sensors

• Electronics front end that attenuates low-frequency noise (3-pole system).

• Seismic sensitivity 1 m/s2 less than 0.04 Pa equivalent signal (comparable to B&K ½��mikes) in seismically decoupled version.highly ruggedized

• 0.01 Hz- 500 Hz operating range, sensitivity = 150 mV/Pa

• 750 mW power consumption, ±15 V differential

• Can drive very long cable lengths (at least 500-m)

NCPA Digital Sensors left: near field �blast sensor� ~ 55,000 Pa maximum transducible signal (0.045 mV/Pa)

right: far-field porous hose ~100 Pa maximum transducible signal (25 mV/Pa

• Similar performance features to the analog sensor (robustness, seismic decoupling, etc)

•  4 W power consumption

•  Storage capacity 8 GB (expandable to 64 GB)

•  Programmable sampling rates 31.25-1000 sps

•  On sensor GPS

•  Built-in 802.11/b WIFI (can be disabled)

•  Optional USB and Ethernet ports

Porous hose (digital) installation High-frequency sensor under 45” (115 cm) diameter foam dome.

First generation digital sensors ready for deployment.

US Array-style sensor.

Measured vs Modeled Response

0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 55

7

10

20

30

Frequency [Hz]

Sens

itivi

ty [m

V/Pa

]

Median of measured responsesModeled responses

The low-frequency portion of the sensor transfer function is well understood. (Shown is the transfer function for a lower-gain sensor).

Relative Calibration of NCPA Sensors

0 10 20 30 40 50 60 70-10-8-6-4-20246810

Sensor Number [arb]

Rel

ativ

e Se

nsiti

vity

[Per

cent

]

Show are the test results for 68 digital sensors. The standard deviation of the relative sensitivity is 3.5%.

Electronic Noise Floor

0.1 0.2 0.5 1 2 5 1010-1010-910-810-710-610-510-410-310-210-1100101102

Frequency [Hz]

"Ele

ctro

nic

Noi

se F

loor

" [Pa

2 /Hz] Bowman Model

Chaparral 50 (published noise floor) MB2000 (published noise floor) NCPA Sensor (true noise floor)

Thermal Sensitivity

These results are for sensors that were chilled to 4°C, then placed in a calibration chamber. Their sensitivities were then tracked as a function of time. The �actual� temperature change over the measurement period was approximately 8°C.

Two-Tone Linearity Measurements

Comparison of two-tone measurements (two speakers) between different sensor types.

Seismic Sensitivity

Comparison of noise floors for sealed sensors in a sealed tank. Above 5 Hz, the MB2000 is dominated by seismic noise.

Challenges of Atmospheric Infrasound • Noise associated with atmospheric turbulence (�wind noise�), especially at frequencies below 0.1 Hz. Conventionally this is solved by adding large �wind-noise filters� to sensors. The cost of the filters typically far exceeds the cost of the infrasound sensor itself.

• Environmental exposure is a hazard to current, rather delicate microphones, so vaults are constructed to stablize temperature and protect the instrument from environmental exposure.

Applications of Infrasound Sensors • monitoring potential atmospheric nuclear tests (CTBT applications)

• natural hazard detection of volcanos, tornados, tsunamis

• monitoring natural phenomena such as hurricanes and bolides

This 4-element design reduces effects of temperature gradients across sensors, an important noise source for measurements outside of vaults.

An 8-element configuration can be used that cancels temperature gradients and signals associated with seismic motion.

Piezoceramic Sensors •  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

Commercial Sensors (Hyperion Technology Group)

Commercial Versions of NCPA Analog Sensor (left) and Digital Sensor (Right).

Dual Channel Sensor (Pressure + Acceleration)

The Hyperion Model 5300 analog sensor has dual output channels for pressure and acceleration. This sensor is based on an NCPA design. The maximum transducible acceleration for configuration used for these measurements is ±25 m/s2 and the maximum transducible pressure is ±100 Pa.

High Amplitude (Blast Wave) Measurements

Two short-range shock wave measurements using a “blast-wave” sensor. The Hyperion digital sensor can also produce dual

channel output. Unlike the NCPA sensor, it directly tranduces two sensor plates (one oriented upwards, and one downwards). In principle the gains for the pressure and acceleration channels can be decoupled with this design.

Future Development: Reciprocal Calibration

Frequency Response of the NCPA Sensors

10-5 10-4 10-3 10-2 10-1 100 101 1020.512

51020

50100200

Frequency [Hz]

Sens

itivity

[mV/

Pa]

Current Generation Reference Sensor Original Design

Shown here are nominal responses for analog sensors. Digital sensors have a sensitivity 1/6 of this, because the digitizer reference voltage is ±2.5 V.