ORE 654Applications of Ocean Acoustics

Lecture 3bDoppler shift and example sound levels

Bruce HoweOcean and Resources Engineering

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

Fall Semester 2011

04/21/23 1ORE 654 L3b

Doppler shift or effect

• Apparent change in signal frequency after propagation caused by the relative motion of a source and receiver

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Doppler frequency shift

• Consider repetitive pulse every T seconds

• If distance D constant, receive time is D/c and apparent frequency remains f0 = 1/T

• Distance decrease between s and r because of relative speed vr, arrival time changes

• Length of time between pulses will change/decrease, and apparent frequency changes too

• As distance decreases (vr +), frequency increases, wavelength decreases

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t1 =D(t1)c

D(t) =D(t1)−vrt

t2 =T +D(t2 )c

=T+D(t1)−vrT

c

t2 −t1 =T −vrTc

=T 1−vrc

⎛⎝⎜

⎞⎠⎟

f =1

T 1−vrc

⎛⎝⎜

⎞⎠⎟

=f0

1−vrc

⎛⎝⎜

⎞⎠⎟

≈ f0 1+vrc

⎛⎝⎜

⎞⎠⎟

δ f = f − f0 = f0vrc

δ f =2 f0vrc; two-way echo

Doppler and ASW• Acoustic source

passing a fixed receiver• δf goes to zero at x=0• δf/f can be large (0.7%)

for relative speed of 10 kts (18.5 km/hr) – 5 m/s

• Characteristic signature - pick out of clutter, track over time

• δf/f >> radar

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vr =xv

x2 + H 2

vr =xv

x2 + H 2

δ ff0

=vc

x

x2 + H 2=vccosθ

NPAL / ATOC Kauai source

• 260 W• M-sequence coded signals• 75 Hz, 35 Hz bandwidth• 28 ms peak• 27.28 s period• 2 hour transmissions, 1 per day

DIVES 25 and 56 - examples

30

79

Red segments = ARS recordings

10

.8 m

s

}1

4.7

ms

}1

3.0

ms

}

}

1/75 Hz = 13.3 msExample time series

Example PSD

zoom PSD

Kauai example

Coherent processing of M-sequence coded signals

Arrival times(72.7195)

(72.282)

(72.8654)(72.9153)

(73.4143)

Peaks in each block shift due to changing s/r range

•Measured travel time changes•~3.7 ms per 27.28 s block

•Match glider kinematics•0.204 m/s, 136 m horizontally, 33 m vertically, in 12 minutes

Relative travel time – 27.28 s

Relative travel time – 0.3 s

Relative travel time – 0.4 s

Coherent gain

Even with glider motion, coherent processing was possible, with 9.4 dB of gain – Doppler consistent

Theoretical gain is 14 dB: peaks still not properly aligned – more to do

Time – 12 minutes

Relative travel time – 0.4 s

Relative travel time – 0.16 sPk = 1.42

Mean = 0.026

SNR = 34.9

Pk = 32.15

Mean = 0.20

SNR = 44.2

1 Block 26 Blocks

~ 5 cm/s differences

Doppler - other

• Doppler velocimeters– Scatters – plankton, bubbles (ideally passive tracers)– Basic limits on maximum unambiguous range Rm

and velocity Vm

– Coherent and incoherent systems

• Moving (ocean) surface induces Doppler shift in scattered signal

• Measure velocity of platform relative to fixed seafloor – “Doppler velocity log”

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Sound levels

• Remember 1 W = 170.8 dB re 1 μPa at 1 m, water

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p1m2 =

ρAc4π

Pac; Pac =acoustic power W

SL(dB) =10 log10 (Pac) +10 log10 (ρAc4π

) + 20 log10106 μPa1Pa

⎛

⎝⎜⎞

⎠⎟

SL(dB) =10 log10 (Pac) + [(50.8 +120) =170.8]; re 1 μPa at 1 m

Sound levels

• In air? 1 W =? dB

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p1m2 =

ρAc4π

Pac; Pac =acoustic power W, air

SL(dB) =10 log10 (Pac) +10 log10 (ρAc4π

) + 20 log10106 μPa1Pa

120

⎛

⎝⎜⎞

⎠⎟

SL(dB) =10 log10 (Pac) + [(15.3+120 −26) =109.3]; re 20 μPa at 1 m

Sound levels

• Remember 1 W = 170.8 dB re 1 μPa at 1 m, water• Remember that air ref is 20 μPa =>20 log(20) = 26

dB (re 1 μPa) • Remember that impedance difference => 20

log(1000 * 1500 / 1.3 * 330) = 35.8 dB (re ρc water)• So net difference between air and water is 61.5 dB• and 1 W air = 109.3 dB re 20 μPa at 1 m, air

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Sound levelsSource Broadband Source Level

(underwater dB at 1 m)

Lightning Strike on Water Surface ~260

Seafloor Volcanic Eruption ~255

Sperm Whale Clicks 163 - 236

Beluga Whale Echolocation Click 206-225(peak-to-peak)

White-beaked Dolphin Echolocation Clicks 194-219(peak-to-peak)

Spinner Dolphin Pulse Bursts 108-115

Bottlenose Dolphin Whistles 125-173

Blue Whale Moans 155 - 188

Humpback Whale Song 144 - 174

Humpback Whale Fluke and Flipper Slap 183 - 192

Snapping Shrimp 183 - 189(peak-to-peak)04/21/23 ORE 654 L3b 14

Sound levelsSource Broadband Source Level

(underwater dB at 1 m)

Tug and Barge (18 km/hour) 171

Supply Ship (Kigoriak) 181

Large Tanker 186

Icebreaking 193

Airgun array (32 guns) 259 (peak)

AN/SQS-53C(U. S. Navy tactical mid-frequency sonar, center frequencies 2.6 and 3.3 kHz)

235

SURTASS-LFA (100-500 Hz) 215 underwater dB for a single projector, 18 in vertical array

Heard Island Feasibility Test (HIFT)(Center frequency 57 Hz)

206 underwater dB for a single projector, 5 in vertical array

Acoustic Thermometry of Ocean Climate (ATOC)/North Pacific Acoustic Laboratory (NPAL) (Center frequency 75 Hz)

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Sounds in air – DOSIT web site

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