Principles of Underwater Sound

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Principles of Underwater Sound

• Naval Weapons Systems

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Detection Position Fixing

• Active
• Passive

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Why do we use SOUND?

• Range of Penetration into the Medium.
• Ability to differentiate between objects in the
  Medium.
• Speed of Propagation.

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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

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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.

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Transmission Losses

• Two main types
• Spreading
• Spherical (omni-directional point source)
• Cylindrical (horiz radiation only, or thermal
  layer, or large ranges compared to depth)

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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.

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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

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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.
Going deep increases pressure so can go faster
without cavitating.
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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.

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Questions?
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Principles of Underwater Soundcontd
Naval Weapons Systems
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(No Transcript)
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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

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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 gt DT
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Passive Sonar Equation
SL - TL - NL DI gt DT
SL Source level- sound level of targets noise
source. TL Transmission Losses (reflection,
absorption, etc.) NL Noise Level (Ambient
noise) DI Directivity Index DT Detection
Threshold
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SR Maul!!!!!
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Active Sonar Equations
Ambient Noise Limited
SL - 2TL TS - NL DI gt DT
Reverberation Noise Limited (Reverb gt ambient
noise)
SL - 2TL TS - RL gt DT
TS Target Strength, A measure of the
reflectivity of the target to an active sonar
signal.
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DT
Sonar Equipment
SR Hall!!!!!!!
DI
2TL
TS
SL
NL
SL - 2TL TS - NL DI gt DT
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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
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Factors that affect Sound in H2O

• Temperature
• Pressure
• Salinity

It will bend towards areas of slower speed.
SOUND IS LAZY!!
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Speed of Sound in Water
SOUND IS LAZY!!
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Expendable Bathythermograph
Canister Loading Breech
Canister Loading Breech
Launcher Recorder Cable (4-wire shielded)
Stantion
LAUNCHER
Optional Equipment
Alternating Current PowerCable (3-wire)
Terminal Board
Depth/Temperature Chart
RECORDER
Wire Spool
Thermistor
PROBE (XBT)
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SOUND IS LAZY!!
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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!

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ISOVELOCITY
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Negative Gradient
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Positive Gradient
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Layer Depth
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Sound Channel
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Convergence Zone (CZ)
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Bottom Bounce
gt25 Deg.
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Possible Propagation Paths
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Questions?