1. Principles of Underwater Sound Naval Weapons Systems

2. Learning Objectives Physical properties associated with sound travel in waterPhysical properties associated with sound travel in water Why sound energy is employed for surveillance and detectionWhy sound energy is employed for surveillance and detection Sound propagation lossesSound propagation losses Self-noise and ambient noise, SNRSelf-noise and ambient noise, SNR Comprehend concept of FOMComprehend concept of FOM

3. Learning Objectives Effects of temperature, pressure, and salinityEffects of temperature, pressure, and salinity Know basic thermal and sound-velocity structure of the oceanKnow basic thermal and sound-velocity structure of the ocean Comprehend use of Snell’s LawComprehend use of Snell’s Law Comprehend the three basic sound-speed gradientsComprehend the three basic sound-speed gradients Basic properties of ocean currentsBasic properties of ocean currents

4. Why do we use SOUND? Range of PenetrationRange of Penetration Identify ObjectsIdentify Objects Speed of PropagationSpeed of Propagation

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

6. Transmission Losses SpreadingSpreading –Spherical (omni-directional point source) –Cylindrical (horizontal radiation only)

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

8. Questions? WEDNESDAY:Review FRIDAY: EXAM 1

9. Principles of Underwater Sound cont’d Naval Weapons Systems

10. 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

11. Screw 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? Water Flow Blade Tip Cavitation Sheet Cavitation

12. 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.

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

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

18. Active Sonar Equations **Ambient Noise Limited:** Reverberation Noise Limited: (Reverb > ambient noise) SL - 2TL + TS - NL + DI > DT SL - 2TL + TS - RL > DT TS: Target Strength, A measure of the reflectivity of the target to an active sonar signal.

19. SL 2TL NL DI DT Sonar Equipment SL - 2TL + TS - NL + DI > DT TS SR Hall!!!!!!!

20. Figure of Merit (FOM) FOM = the maximum allowable one-way transmission loss in passive sonar, and the maximum two-way trans- mission loss in active for a detection probability of 50%. PFOM = SL - NL + DI - DT AFOM = SL + TS - NL + DI - DT

21. Factors that affect Sound in H 2 O TemperatureTemperature PressurePressure SalinitySalinity SOUND IS LAZY!! It will bend towards areas of slower speed.

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

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

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

25. Ray Propagation Theory The path sound travels can be depicted as a RAY or VECTOR RAYS will change direction when passing through two mediums of different density. REFRACTION! Sound will bend TOWARDS the region of SLOWER sound speed. Sound is lazy! Snell’s Law

26. ISOVELOCITY Range Maximum Echo Range Depth Transducer Temperature

27. Negative Gradient Depth Water Warm Shadow Zone Water Cool Sound Bends Down When Water Grows Cooler With Depth Depth Direction of Increasing Temperature and Velocity Negative Gradient Thermal Structure T C

28. Positive Gradient Water Cool Shadow Zone Water Warm When Temperature Increases with Depth, Sound Bends Sharply Up Depth Direction of Increasing Temperature and Velocity Positive Gradient Thermal Structure TC

29. Layer Depth Temperature Cool Shadow Zone Isothermal Sound Beam Splits When Temperature Is Uniform At Surface and Cool At Bottom Depth Direction of Increasing Temperature and Velocity Isothermal Gradient Thermal Structure T C Depth

30. Sound Channel Water Cool Shadow Zone Water Warm Depth Direction of Increasing Temperature and Velocity Negative Gradient Over Positive T C Depth

31. Convergence Zone (CZ)  3-4 deg C T

32. Bottom Bounce >25 Deg.

33. Possible Propagation Paths SoundChannel Negative Gradient Surface Direct Convergence Zone Bottom Bounce Surface Direct Isovelocity

34. Questions?