Principles of
Underwater Sound
Naval Weapons Systems
Detection & Position Fixing
•Active
•Passive
Why do we use SOUND?
• Range of Penetration into the
Medium.
• Ability to differentiate between
objects in the Medium.
• 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 the
medium
• Most concepts already discussed will apply
What a Concept !
The faintest 1000 Hz tone heard in air
has pressure variations of only
2/10,000,000,000 of one
atmosphere of pressure. The
corresponding particle
displacement is smaller than the
diameter of an atom.
Transmission Losses
• Two main types:
• Spreading
– Spherical (omni-directional point source)
– Cylindrical (horiz radiation only, or thermal
layer, or large ranges compared to depth)
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; combined with
refraction and absorption into bottom.
Self Noise
• Machinery Noise
– Pumps, reduction gears, power plant, etc.
• Flow Noise
– High speed causes more noise
– Hull fouling - Animal life on hull (not smooth)
– Want LAMINAR flow
• Cavitation
– 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.
Going deep increases pressure so can go faster
without cavitating.
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
• 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
3000
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!
•Snells 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?