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
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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
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