Transcript Principles of Underwater Sound

Principles of
Underwater Sound
Naval Weapons Systems
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
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
Why do we use SOUND?
• Range of Penetration
• Identify Objects
• Speed of Propagation
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
Transmission Losses
• Spreading
– Spherical (omni-directional point source)
– Cylindrical (horizontal radiation only)
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
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
Blade Tip
Cavitation
Sheet
Cavitation
Water Flow
Water Flow
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?
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 SIGNAL
• Detect the signal over the background
NOISE
• SONAR (Sound Navigation Ranging)
• SONAR equations
– Look at losses compared to signal
– Probability 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
DT
Sonar
Equipment
SR Maul!!!!!
DI
TL
NL
SL
SL-TL-NL+DI=DT
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.
DT
Sonar
Equipment
SR Hall!!!!!!!
DI
2TL
NL
SL
SL - 2TL + TS - NL + DI > DT
TS
Figure of Merit (FOM)
FOM = the maximum allowable one-way transmission
loss in passive sonar, and the maximum two-way transmission loss in active for a detection probability of 50%.
PFOM = SL - NL + DI - DT
AFOM = SL + TS - NL + DI - DT
Factors that affect Sound in
H2O
• Temperature
• Pressure
• Salinity
It will bend towards
areas of slower speed.
SOUND IS LAZY!!
Speed of Sound in Water
SOUND IS LAZY!!
Variable Effects of:
Salinity
Pressure
Temperature
Depth
Pressure
Depth
Depth
Salinity
Temperature
Typical Deep Ocean
Sound Velocity Profile
Speed of Sound (meters/sec)
1480
1500
1520
Surface Layer
Seasonal Thermocline
Depth of Water (meters)
Permanent Thermocline
1000
2000
Deep Isothermal Layer
3000
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!
Snell’s Law
•Sound will bend TOWARDS the region
of SLOWER sound speed. Sound is lazy!
ISOVELOCITY
Temperature
Range
Transducer
Depth
Maximum Echo Range
Negative Gradient
Direction of Increasing
Temperature and Velocity
Depth
Water Warm
Depth
T
Shadow Zone
C
Water
Cool
Negative Gradient Thermal Structure
Sound Bends Down When Water Grows
Cooler With Depth
Positive Gradient
Depth
Direction of Increasing
Temperature and Velocity
Water
Cool
T
C
Shadow Zone
Water Warm
Positive Gradient Thermal Structure
When Temperature Increases with
Depth, Sound Bends Sharply Up
Layer Depth
Direction of Increasing
Temperature and Velocity
Depth
Depth
Isothermal
T
Shadow
Zone
C
Temperature
Cool
Isothermal Gradient Thermal Structure
Sound Beam Splits When Temperature Is
Uniform At Surface and Cool At Bottom
Sound Channel
Direction of Increasing
Temperature and Velocity
Depth
Depth
Water Warm
T
Shadow Zone
C
Water Cool
Negative Gradient Over Positive
Convergence Zone (CZ)
3-4 deg
T
C
Bottom Bounce
>25 Deg.
Possible Propagation Paths
Surface Direct Isovelocity
Sound
Convergence
Zone
Bottom
Bounce
Channel
Questions?