Principles of Principles of Underwater Sound Underwater Sound Naval Weapons Systems Naval Weapons Systems
Nov 30, 2015
Principles of Principles of Underwater Underwater
SoundSoundNaval Weapons SystemsNaval Weapons Systems
Learning Objectives
• Physical properties associated with sound Physical properties associated with sound travel in watertravel in water
• Why sound energy is employed for Why sound energy is employed for surveillance and detectionsurveillance 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
Learning Objectives
• Effects of temperature, pressure, and Effects of temperature, pressure, and salinitysalinity
• Know basic thermal and sound-velocity Know basic thermal and sound-velocity structure of the oceanstructure of the ocean
• Comprehend use of Snell’s LawComprehend use of Snell’s Law• Comprehend the three basic sound-speed Comprehend the three basic sound-speed
gradientsgradients• Basic properties of ocean currentsBasic properties of ocean currents
Why do we use SOUND?
• Range of PenetrationRange of Penetration
• Identify ObjectsIdentify Objects
• Speed of PropagationSpeed of Propagation
Concepts of SoundConcepts of Sound
• Three (3) elements required for this to workThree (3) elements required for this to work– SourceSource– MediumMedium– Detector (Receiver)Detector (Receiver)
• The source VIBRATES causing a series of The source VIBRATES causing a series of compressions compressions and and rarefactions rarefactions in a mediumin a medium
• Most concepts already discussed will applyMost concepts already discussed will apply
Transmission LossesTransmission Losses
• SpreadingSpreading– Spherical (omni-directional point source)Spherical (omni-directional point source)– Cylindrical (horizontal radiation only)Cylindrical (horizontal radiation only)
Transmission Losses (cont.)• AttenuationAttenuation
– AbsorptionAbsorption• Process of converting acoustic energy into heatProcess of converting acoustic energy into heat
• Increases with higher frequencyIncreases with higher frequency
– Scattering and ReverberationScattering and Reverberation• Volume: Marine life, bubbles, etc.Volume: Marine life, bubbles, etc.
• Surface: Ocean surface, wind speedSurface: Ocean surface, wind speed
• Bottom:Bottom:– Not a problem in deep waterNot a problem in deep water
– Significant problem in shallow waterSignificant problem in shallow water
Questions?
WEDNESDAY: Review
FRIDAY: EXAM 1
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
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 Water Flow
Blade Tip Cavitation
Sheet Cavitation
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.
How do we detect a submarine?
• Detect the reflected SIGNALDetect the reflected SIGNAL
• Detect the signal over the background Detect the signal over the background NOISENOISE
• SONAR (Sound Navigation Ranging)SONAR (Sound Navigation Ranging)
• SONAR equationsSONAR equations– Look at losses compared to signalLook at losses compared to signal– Probability of detectionProbability of detection
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
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
SL
TLNL
DI
DT
Sonar Equipment
SL-TL-NL+DI=DT
SR Maul!!!!!
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 thetarget to an active sonar signal.
SL
2TL
NL
DI
DT
Sonar Equipment
SL - 2TL + TS - NL + DI > DT
TS
SR Hall!!!!!!!
Figure of Merit (FOM)
FOM = the maximum allowable one-way transmissionloss in passive sonar, and the maximum two-way trans-mission loss in active for a detection probability of 50%.
PFOM = SL - NL + DI - DTAFOM = SL + TS - NL + DI - DT
Factors that affect Sound in H2O
• TemperatureTemperature
• PressurePressure
• SalinitySalinity
SOUND IS LAZY!!
It will bend towardsareas of slower speed.
Speed of Sound in WaterD
epth
Dep
th
Dep
th
Salinity Pressure Temperature
Salinity Pressure Temperature
Variable Effects of:
SOUND IS LAZY!!
Typical Deep Ocean Sound Velocity Profile
Dep
th o
f W
ater
(m
eter
s)
Speed of Sound (meters/sec)
1500 15201480
1000
2000
3000
Surface LayerSeasonal Thermocline
Permanent Thermocline
Deep Isothermal Layer
SOUND IS LAZY!!
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
ISOVELOCITY
Range
Maximum Echo Range
Depth
Transducer
Temperature
Negative Gradient
Depth
Water Warm
Shadow Zone
WaterCool
Sound Bends Down When Water Grows Cooler With Depth
Depth
Direction of IncreasingTemperature and Velocity
Negative Gradient Thermal Structure
T
C
Positive Gradient
Water Cool
Shadow Zone
Water Warm
When Temperature Increases withDepth, Sound Bends Sharply Up
Depth
Direction of IncreasingTemperature and Velocity
Positive Gradient Thermal Structure
T C
Layer Depth
TemperatureCool
Shadow Zone
Isothermal
Sound Beam Splits When Temperature IsUniform At Surface and Cool At Bottom
Depth
Direction of IncreasingTemperature and Velocity
Isothermal Gradient Thermal Structure
T C
Depth
Sound Channel
Water Cool
Shadow Zone
Water WarmDepth
Direction of IncreasingTemperature and Velocity
Negative Gradient Over Positive
T
C
Depth
Convergence Zone (CZ)
3-4 deg
C
T
Bottom Bounce
>25 Deg.
Possible Propagation Paths
Sound Channel
Negative Gradient
Surface Direct
ConvergenceZone
Bottom Bounce
Surface Direct Isovelocity
Questions?