Underwater Acoustics Research Group

CAV Workshop3 May 2010

POC:Chris BarberApplied Research LaboratoryPenn State UniversityPO Box 30State College, PA 16804-0030(814) 865-3837cbarber@psu.edu

Underwater Acoustics at Penn State

• Ocean Acoustics– Propagation– Acoustical Oceanography– Marine Bioacoustics– Underwater Noise Sources

• Transducers and Instrumentation– Piezoelectric Transducers– Vector (Intensity) Sensors– Sonar Arrays

• Signal Processing– Sonar signal processing– Beamforming and array processing– Noise source localization

ARL Acoustics Division

• Division in the Research and Academic Programs Office– 16 Staff Members– 18 Students (was 20 – 2 PhDs graduated this fall)

• Majority of staff:– Teach at both undergraduate and graduate level in various

departments of PSU– Advise/guide graduate students– Conduct research as PI’s for sponsor direct funded programs

• Major Sponsor: ONR

• Inter-laboratory and Inter-university collaboration

• Outside collaboration

ARL Acoustics Division

International Collaboration

• Australia – Visiting Scientist – AUV G&C Studies

• Canada – Visiting Scientists – Geophysical Inversion– LF Seafloor Backscatter

• Germany – Visiting Scientist – Signal Processing

• Denmark – Visiting Scientist – Harbor Defense Studies

• NURC – Exchange Scientists – Shallow Water Propagation– AUV Studies– At-sea Experiments – Seafloor Scattering

• Korea – At-sea Experiments – Shallow Water Propagation

• France – Naval Academy Grads – Seafloor Studies

UNDERWATER ACOUSTIC MEAUSREMENTS

Measurements at Sea

Broadband Seafloor Clutter Initiative 500 m

note 5m rise of bathymetry on NE trending track 

Regular spikes on temp data are artifacts from modem

AUV altitude

CWH

CTD data from AUV indicates warmer, fresher water venting from NE flank of MV (fault).

PI: Charles W. HollandSea trials with NATO Undersea Research Centre, Defence Research & Development Canada, Naval Research Lab

Harbor Defense

PI: Kyle M. Becker

Sponsor: Office of Naval Research

Students:1 Ph.D. (Graduated)1 MS (Aug 2010)

Collaboration: NUWC/Newport

ARL/UTBAE SystemsPSU Mech Engr Dept

Assessment (Measurements)

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J-15-3 40-600 Hz 175 dBHLF-1D 196 dB @ 260 Hz(~190 dB @ 100 Hz)

Acoustic Field Predictions

Transverse Acoustic Variability Experiment (TAVEX)

Experiment location is within the Northern East China Sea

Towed CTD area is approximately 33X10km

300 and 500Hz acoustic sources and 96 hydrophone HLA (NRL)

300kHz ADCP southwest of tow area (KORDI)

IEODO station (KORDI) 11

PI: David L. Bradley

Sponsor: Office of Naval Research

Students: USNA Trident Scholar (MS)Acoustics Graduate Student (MS)

Collaboration: Republic of Korea, Naval Research Laboratory, Applied Physics Laboratory / University of Washington

N

HLA

500 Hz Src

300 Hz Src

12

wav 1

Sound Speed Profile Track with Acoustic Track

13

Typical Water Sound Speed Profile with Internal Wave Packet

Susan ParksPresidential Early Career Award for Scientists and Engineers (PECASE) recipient – 13 January 2010

16 / 20

Range (m) Range (m)

Leve

l (dB

) –5

dB In

crem

ents

Leve

l (dB

) –5

dB In

crem

ents

Low Frequency (32Hz)

Mid-Frequencies (6300 Hz)

Estimation of Ship Radiated Noise in the Near Field

• Problem: Simple source models coupled with simplified propagation assumptions inadequate to capture sound field variability for real sources in shallow water

PI: Chris Barber

Sponsor: Office of Naval Research

Students: Engineering Honors Undergradute

Collaboration: Naval Research Laboratory Naval Surface Warfare CenterALION

Background –Radiated Noise Characterization

• Propagation models treat a ship as a spatially-compact simple harmonic source

λ >> a ka << 1

• Far field acoustic pressure assumed to have range varying component given by

• Leads to familiar expression for spherically spreading sound pressure level (SPL)

• For source with directivity

kcω

=

( ) 0 jkrjk cSp r er

ωρ −=

( ) ( ) ( )2

2

( )10log 20logsref

p rSPL r L rp r

⎛ ⎞⎜ ⎟= = −⎜ ⎟⎝ ⎠

( ) ( ) ( ), , ,j t j tp r e p r H eω ωθ φ θ φ=

Near-Field Radiated Noise

• Far-field approximation– source-to-receiver distance much greater than the spatial extent of

the source, r >>a– Receiver distance large in terms of wavelength, r >> λ

• Near-field scenario– Simple source model breaks

down as source-receiver separation is reduced

– Ship appears as distributedsource to close receivers

– Ranges small in terms of wavelength

– Environment typically cannot be ignored

Typical Acoustic Environment and Source-Receiver Geometry

Far Field Radiated Noiseby Numerical Methods

• Approaches to obtain estimates of radiation from realistic hull structures have been presented in the literature, including – Recent developments in efficient finite element methods– High frequency SEA methods– Superposition / SVD computational approach (Koopmann)

• Limitations – limited low frequency regime over which computation is practical– application limited to global hull modes of radiation– Fully detailed computational model of a realistic ship structure remains

impractical– Simplified models do not capture structural details critical to near-

field radiation

Far Field Radiated Noise Estimates by Transfer Function

• From Helmhotz integral theorem

• In matrix form assuming forcing function F

– Theoretically possible to compute transfer function given sufficient number of velocity points (sensor density)

– In practice, transfer function estimated by spatial averaging of available sensor data

• Far field radiated noise estimated from

( ) 0i s nS

Gp p j u G dSn

ωρ∂⎛ ⎞= − −⎜ ⎟∂⎝ ⎠∫r

[ ]{ } [ ][ ] 1{ } { }far np u F−= Φ = Φ Ψ

( )( )( )

2

2

,10 log

ii

P fTF

A fθ

θ=

∑

( ) ( ) ( )SPL f A f TF fθ θ= +

Sound Field Variability - Source

Near Field

– multi-source interference– multi-modal interference– directivity

Far Field– Simple source representation

(monopole x directivity)

• Source Characteristics– Spatial distribution of localized

(simple) sources

– Source directivity

– Radiation from complex (non-simple) distributed sources

• Sound Field Characterization

Near-Field Primitive Model Inverse Source Computation (U)

Near Field Estimate from Near field Measurements

Radiated Noise Estimation from NAH Measurements

• Measure pressure over a nearby conformal surface to recover normal velocity of the source surface– NRL Approach (Valdivia and Williams)

• Measure pressure on conformal surface, solve for density function on source surface

• Apply Euler’s equation to recover velocity on source surface• Numerical computation implemented via Indirect Boundary Element

Method (IBEM) or Equivalent Source Method (ESM) approximation

• Given knowledge of normal velocity of the source surface and source density function, compute received acoustic field due to ship radiation at tactically significant ranges – Apply inverse of computational algorithms used to recover normal

velocity

Radiated Noise Estimation from NAH Measurements - Analysis

• Source and path analysis for machinery sources based on NAH and interior hull vibration data– Local (plate theory) radiation– Excitation of global hull modes

• Contribution of local radiation vs global hull modes to received acoustic field for machinery sources– Use of hull-mounted sensors to identify global hull mode radiation

in-situ– Transfer function uncertainty

• Limits of simple-source (monopole x directivity factor) assumption– Agreement with near-field direct measurements– Convergence to measured far-field measurements