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ORE 654 Applications of Ocean Acoustics Lecture 3b Doppler shift and example sound levels. Bruce Howe Ocean and Resources Engineering School of Ocean and Earth Science and Technology University of Hawai’i at Manoa Fall Semester 2011. Doppler shift or effect. - PowerPoint PPT Presentation
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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
04/21/23 ORE 654 L3b 2
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
04/21/23 ORE 654 L3b 3
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
04/21/23 ORE 654 L3b 4
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”
04/21/23 ORE 654 L3b 10
Sound levels
• Remember 1 W = 170.8 dB re 1 μPa at 1 m, water
04/21/23 ORE 654 L3b 11
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
04/21/23 ORE 654 L3b 12
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
04/21/23 ORE 654 L3b 13
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)
19504/21/23 ORE 654 L3b 15
Sounds in air – DOSIT web site
04/21/23 ORE 654 L3b 16