Principles of Underwater Sound

1. Principles of Underwater Sound Naval Weapons Systems

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

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

4. Why do we use SOUND? • Range of Penetration • Identify Objects • Speed of Propagation

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

7. Transmission Losses (cont.) • Attenuation • Absorption • Process of converting acoustic energy into heat • Increases with higher frequency • 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

8. Questions? WEDNESDAY: Review FRIDAY: EXAM 1

9. 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

10. 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?

11. 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.

13. 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

14. 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

15. 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

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

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

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

19. 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

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

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

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. 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

24. Temperature Range Transducer Depth Maximum Echo Range ISOVELOCITY

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

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

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

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

29. 3-4 deg T C Convergence Zone (CZ)

30. Bottom Bounce >25 Deg.

31. Surface Direct Isovelocity Surface Direct Negative Gradient Sound Channel Convergence Zone Bottom Bounce Possible Propagation Paths

32. Questions?