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ORE 654 Applications of Ocean Acoustics Lecture 3b Doppler shift and example sound levels

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.

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ORE 654 Applications of Ocean Acoustics Lecture 3b Doppler shift and example sound levels

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  1. ORE 654Applications of Ocean AcousticsLecture 3bDoppler 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 ORE 654 L3b

  2. Doppler shift or effect • Apparent change in signal frequency after propagation caused by the relative motion of a source and receiver ORE 654 L3b

  3. 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 ORE 654 L3b

  4. 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 ORE 654 L3b

  5. 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 Red segments = ARS recordings 79 30 DIVES 25 and 56 - examples

  6. } Example time series 1/75 Hz = 13.3 ms 10.8 ms 14.7 ms 13.0 ms } } } zoom PSD Kauai example Example PSD

  7. (72.7195) Arrival times (72.8654) (72.9153) (73.4143) (72.282) Coherent processing of M-sequence coded signals Relative travel time – 0.4 s Relative travel time – 27.28 s • 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 – 0.3 s

  8. Pk = 1.42 Pk = 32.15 SNR = 44.2 SNR = 34.9 Mean = 0.026 Mean = 0.20 Coherent gain Relative travel time – 0.4 s 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 26 Blocks 1 Block Relative travel time – 0.16 s Time – 12 minutes

  9. ~ 5 cm/s differences

  10. 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” ORE 654 L3b

  11. Sound levels • Remember 1 W = 170.8 dB re 1 μPa at 1 m, water ORE 654 L3b

  12. Sound levels • In air? 1 W =? dB ORE 654 L3b

  13. 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 ORE 654 L3b

  14. Sound levels ORE 654 L3b

  15. Sound levels ORE 654 L3b

  16. Sounds in air – DOSIT web site ORE 654 L3b

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