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Underwater Sound

Underwater Sound. A Maury Project Briefing CAPT Joe Brenner. Underwater Sound. Outline. Objectives: Identify major milestones in Ocean Acoustics Describe the process of ocean sound propagation Discuss sound attenuation/refraction in the ocean Listen to applications of ocean sound.

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Underwater Sound

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  1. Underwater Sound A Maury Project Briefing CAPT Joe Brenner

  2. Underwater Sound Outline Objectives: • Identify major milestones in Ocean Acoustics • Describe the process of ocean sound propagation • Discuss sound attenuation/refraction in the ocean • Listen to applications of ocean sound

  3. Underwater Sound The Pioneers? • 1490 - Leonardo da Vinci (sound travel) • 1687 - Sir Isaac Newton (theory) • 1826 - Colladon and Sturm (speed) • 1914 - Reginal A. Fessenden (transducer) • Sub comms; iceberg location; fathometer • 1919 - H. Lichte (refraction) • 1943 - Ewing and Worzel (Deep Sound Channel)

  4. Underwater Sound

  5. Concepts of Sound • Three (3) elements required for this to work • Source • Medium • Detector (Receiver) • The source VIBRATES causing a series of compressions and rarefactions in a medium • Most concepts already discussed will apply

  6. Underwater Sound Sound Properties • Sound is mechanical energy – transported as a compressional wave • Frequency – rate of vibration (pitch). A tone is a sound of constant frequency with a long duration. • Acoustic pressure is measured with a hydrophone • Sound Pressure is measured in pascals (Pa) • Speed (c) ~ 1500 m/s in water (340 m/s in air) a function of (T, S, P) (Image Source : NOAA)

  7. Underwater Sound Sound is Attenuated by: • Spreading • Scattering • Absorption • Reflection

  8. Transmission Losses • Spreading • Spherical (omni-directional point source) • Cylindrical (horizontal radiation only)

  9. Transmission Losses (cont.) • Attenuation • Scattering and Reverberation • Volume: Marine life, bubbles, etc. • Surface: Ocean surface, wind speed • Bottom: • Not a problem in deep water • Significant problem in shallow water • Absorption • Process of converting acoustic energy into heat • Increases with higher frequency

  10. Underwater Sound Sound is Refracted by: • Variations in the medium’s speed of sound with depth – Sound Velocity Profile • Explained by Snell’s Law (HALT Rule) • As sound propagates outward, it is refracted into a myriad of directions

  11. How do we detect a submarine? • Detect the reflected SIGNAL • Detect the signal over the background NOISE • SONAR (Sound Navigation Ranging) • SONAR equations • Look at losses compared to signal • Probability of detection

  12. Water Flow Water Flow Screw Cavitation Blade Tip Cavitation Sheet Cavitation Screw Speed , Pressure behind screw blades , Water Boils, Bubbles form, The subsequent collapsing of the bubbles cause the noise. What effect does increased depth have on cavitation?

  13. Self Noise • Machinery Noise • Pumps, reduction gears, power plant, etc. • Flow Noise • Relative motion between the object and the water • High speed causes more noise (more friction) • Hull fouling - Animal life on hull (not smooth) • Want LAMINAR flow • Cavitation • Local pressure behind allows steam to form (low pressure area) • Bubbles collapse, VERY NOISY

  14. Ambient Noise • Hydrodynamic • Caused by the movement of water. • Includes tides, current, storms, wind, rain, etc. • Seismic • Movement of the earth (earthquakes) • Biological • Produced by marine life • Passive and active • Ocean Traffic • At long ranges only low frequencies are present.

  15. Signal to Noise Ratio (SNR) Same as with RADAR. The ratio to the received echo from the target to the noise produced by everything else. Detection Threshold (DT) The level, of received signal, required for an experienced operator to detect a target signal 50% of the time. S - N > DT

  16. Passive Sonar Equation SL - TL - NL + DI > DT SL: Source level:- sound level of target’s noise source. TL: Transmission Losses: (reflection, absorption, etc.) NL: Noise Level: (Ambient noise) DI: Directivity Index DT: Detection Threshold

  17. DT Sonar Equipment DI TL NL SL SL-TL-NL+DI=DT SR Jonesy!

  18. DT Sonar Equipment SR Jonesy! DI 2TL TS SL NL SL - 2TL + TS - NL + DI > DT https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=Hunt+for+red+october+one+ping+clip

  19. Factors that affect Sound in H2O • Temperature • Pressure • Salinity It will bend towards areas of slower speed. SOUND IS LAZY!!

  20. Variable Effects of: Salinity Pressure Temperature Salinity Temperature Pressure Depth Depth Depth Speed of Sound in Water SOUND IS LAZY!!

  21. Expendable Bathythermograph Canister Loading Breech Canister Loading Breech Launcher Recorder Cable (4-wire shielded) Stantion LAUNCHER Optional Equipment Alternating Current PowerCable (3-wire) Terminal Board Depth/Temperature Chart RECORDER Wire Spool Thermistor PROBE (XBT)

  22. Typical Deep Ocean Sound Velocity Profile Speed of Sound (meters/sec) 1480 1500 1520 Surface Layer Seasonal Thermocline Permanent Thermocline 1000 Depth of Water (meters) Deep Isothermal Layer 2000 3000 SOUND IS LAZY!!

  23. Possible Propagation Paths

  24. SNR Calculated by STDA for OPAREA Regional Acoustic Assessment (Pd) SNR transform to Pd (W.A. Albersheim) MSRC Intro / Review Challenges Range Dependence METOC Support Getting an Accurate Picture 26 June July 7 25 June

  25. Questions? CAPT J. R. Brenner Military Professor Oceanography Department United States Naval Academy Brenner@usna.edu

  26. Links • Ocean Sounds: http://www.dosits.org/audio/agsummary/ • ASW Training in Hawaiian Opareas: http://govsupport.us/navynepahawaii/training.aspx • Low Frequency Active: http://www.surtass-lfa-eis.com/KeyFacts/index.htm • ASW CONOPs: http://www.navy.mil/navydata/policy/asw/asw-conops.pdf • Article on Sonar and marine mammals: http://www.whoi.edu/oceanus/viewArticle.do?id=37146

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