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Sounds in the sea

Sounds in the sea. Snapping shrimp. Major source of biological noise in shallow temperate and tropical waters 20 dB above the noise level typical of sea state 6 Little diurnal and seasonal variations Broad frequency extent Extremely difficult to filter this noise

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Sounds in the sea

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  1. Sounds in the sea

  2. Snapping shrimp • Major source of biological noise in shallow temperate and tropical waters • 20 dB above the noise level typical of sea state 6 • Little diurnal and seasonal variations • Broad frequency extent • Extremely difficult to filter this noise • Can severely limit the use of underwater acoustics • Interfere with the transmission and reception of sounds by other animals

  3. Shrimp dominated ambient noise

  4. A single snap Intensity 10 -20 dB higher than dolphin echolocation click

  5. How sound is produced • Not claw hitting stationary mate • Cavitation • Water moves above a critical speed and experiences a drop in pressure • Allows tiny air bubbles in the fluid to swell • Fluid slows and the pressure again rises, the bubbles implode • Generates a shock wave and an accompanying sound • Tooth-shaped piece on the moving part of the claw plunges through a hole in the stationary part, shooting out a jet of water fast enough to cause cavitation

  6. Snapping shrimp

  7. Shrimpoluminescence High temperature and pressure in bubble as it collapses Too brief to be seen with the naked eye

  8. Rain • Major role in heat and water budgets • Accurate measures over ocean almost non-existent • Noise distinct from wind

  9. Heavy rainfall Quiet

  10. How rain sound produced? • Impact of drop on sea surface • Formation of bubble underwater • Most often loudest source • Bubble not in equilibrium so it radiates sound while reaching equilibrium • Changes in drop size change shape of splash and bubble and thus, sound production

  11. Small drop High resonance (ringing) frequency

  12. Large drop Low resonance (ringing) frequency

  13. Acoustic rain gauge

  14. Humpback whale chorusing

  15. Humpback whale song • “Most complex display in animal kingdom” • Singers lone, stationary males • Winter mating grounds • Structured • Phrases organized into themes in sequences • All males sing same song in one area • Song evolves over season • Function? • Sexual advertisement • Physical male-male competition • Territory defense • Production mechanism? • Have larynx but no vocal chords • Do not exhale to produce sound

  16. Humpback whale chorusing levels • In Hawaii from ~Jan-April • Song levels recorded on 1 hydrophone over 4 months • Chorus of many whales not in synchrony Dominant source of noise Au et al 2000

  17. Diel variability in chorusing level Peak whale abundance Few whales Levels below 110 dB

  18. Reasons for diel variability? • Whales singing louder at night • More singers at night • Moving closer to hydrophone at night (nearshore) • Cannot be separated with one hydrophone

  19. Ships/propellers on-axis source level spectra of cargo ship at 8 & 16 kts measured directly below ship B – propeller Blade rate F – diesel engine Firing rate G – ship’s service Generator rate

  20. Application – Manatee collisions • Hearing peak 16-18 kHz • Dominant vessel <1 kHz Gerstein and Gerstein 2004

  21. Manatee management • Slow vessel down • Lowers intensity of sound and frequency • Large vessel • 3 mph detectable 2 to 3 seconds (12 - 18 feet) away from the propellers (hull of the boat extends 24 feet ahead of the propellers) • 24 mph detectable 16 seconds (650 feet) before propellers • Small boat • 3-4 mph detectable 6 to 24 feet from the propellers • 24 mph detectable 600 feet from the propellers.

  22. Speed effects on vessel noise

  23. Ship speed and source level

  24. Vessel shadowing Effect strongest close to the surface

  25. Vessel shadowing

  26. Measuring sounds in the seaSampling rules • Convert analog (voltage) signal to digital • Nyquist frequency rule • Sampling frequency must be at least twice that of the highest frequency component of the signal • The signal can be fully recovered from the sampled signal

  27. fs = 8 fa fs =1.5 fa

  28. fs=1/t Aliasing fa=7/8 fs

  29. Digitization • Digital signals made up of bits • Each bit is a 0 or 1 • At most, digitizer can represent 2n values where n is the bit rate • Dynamic range • Dynamic range (dB) = 20 log (2n) ≈ 6 n • 12 bit A-D converter • 4096 values • 72 dB dynamic range

  30. Hardware • Pre-filter • Remove constant noise • Cut off above Nyquist frequency (Anti-aliasing) • Pre-amplifier • Improve analog signal/noise ratio • Improve dynamic range

  31. Finite Fourier Transform (FFT) • Represent signal in time or frequency domain • All signals can be described as the sum of a series of sin and cos waves of varying frequencies and amplitudes

  32. FFT examples

  33. Duration and bandwidth Each signal is 100 kHz

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