ORE 654Applications of Ocean Acoustics

Lecture 6bOcean Noise

Bruce HoweOcean and Resources Engineering

School of Ocean and Earth Science and TechnologyUniversity of Hawai’i at Manoa

Fall Semester 2011

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Noise• Noise spectra• NL = 10 log(<Inoise>/Iref)

dB re 1 μPa2/Hz

• Classic“Urick” curves

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Ship noise

• Radiated sound

04/21/23 ORE 654 L1 4Gerald D’Spain

Low frequency noise - wind

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Station ALOHA, 20 months

Bioacoustic sound

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D’Spain

Offshore a military training base

Measuring wind and rain

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Wind

Rain

Rain

Nystuen

Large whales – using SOSUS

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Andrew, Howe, and Mercer

Annual average

All – 6 yrs

Bowhead and beluga whale and walrus sounds

• Spectrogram of under ice

• northernBering Sea in May 2007

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159th Meeting of the Acoustical Society of America Spring 2010:

Baltimore

Comparison Wenz vs APL vs Ross Ross: Mechanics of Underwater Noise (1976)Ross: Mechanics of Underwater Noise (1976)

& Acoustics Bulletin (1993) & Acoustics Bulletin (1993)

Simplified noise spectra

• Low frequency flow turbulence/seismics/wave-wave

• Shipping• Wind/waves

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Simplified noise spectra - terms• Turbulence/microseisms• Shipping• Wind/waves• Thermal

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NLS1 = 17 - 30(log f)

NLS2 = 40 + 20(D - 0.5) + 26(log f) - 60(log(f + 0.03)

NLS3 = 50 + 7.5 w1/2 + 20(log f) - 40(log(f + 0.4)

NLS4 = -15 + 20(log f)

NLSAll = 10 log(10NLS1 /10 +10NLS2 /10 +10NLS3 /10 +10NLS4 /10 )NL = NLS + 10 log BW

Noise• Noise spectra• NL = 10 log(<Inoise>/Iref)

dB re 1 μPa2/Hz

• Bandwidth Δf• BW = 10 log Δf

• Total noise level NLtotal = NL + BW

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SNR BudgetAcoustic Signal-to-Noise Ratio (SNR) Calculation

Enter values in yellow boxes

Valueunits ValueUnits

Acoustic source level 262W 195.0dB re 1 microPascal at 1 mRange - spherical spreading 10000m -80.0dBAbsorption for center frequency 10000Hz -10.8dB

Scattering loss (3 dB/bounce) 0bounces 0.0dBTotal received signal 104.2dB

Noise level 10000Hz 52.0dB/HzNoise in rms bandwidth 1000Hz 30.0dB Total Noise 82.0dB

Received SNR 22.2dB

Coherent processing time 0.1sProcessing gain 20.0dB

SNR - 1 receiver 42.2dB

SNR - n receivers 1 42.2dB

Travel time precision 0.024351ms

Sound speed precision 0.005479m/sTemperature precision 0.001370KRange precision 0.036526mN digits 100digitsTime per digit 1msCycles per digit 10cycles

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SNR =SL−TL−αR−(NL + BW)+ PG

Rms precision of peak locations

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β f = 2π ( f − f0 )2 ϒ( f )⎢⎣ ⎥⎦2df

0

∞

∫⎡

⎣⎢

⎤

⎦⎥

1/2

rms bandwidth

β t = 2π (t − t0 )2 γ (t)⎢⎣ ⎥⎦2dt

0

∞

∫⎡

⎣⎢

⎤

⎦⎥

1/2

rms time duration

βφ = 2π (x − x0 )2 In (x)dx0

∞

∫⎡

⎣⎢

⎤

⎦⎥

1/2

rms antenna length, x in λ

SNRa ≡2E

Namplitude SNR

σ t =1

β f; σ ν =

1

β t; σ φ =

1

βφhalf peak widths - time, Doppler, angle

σ t ' ==σ tSNRa

; σ ν ' ==σ νSNRa

; σ φ ' ==σ φSNRa

peak precisions

σ t ' =1

f0SNRaif high SNR, can use phase/carrier frequency