Principles of underwater sound
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Principles of Underwater Sound. Naval Weapons Systems. Methods of Underwater Detection & Position Fixing. Active Passive. Why do we use SOUND?. Speed of Propagation Range of Penetration Identify Objects. Concepts of Sound. Three (3) elements required for this to work Source Medium

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Principles of Underwater Sound

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Principles of underwater sound

Principles of Underwater Sound

Naval Weapons Systems


Methods of underwater detection position fixing

Methods of Underwater Detection & Position Fixing

  • Active

  • Passive


Why do we use sound

Why do we use SOUND?

  • Speed of Propagation

  • Range of Penetration

  • Identify Objects


Concepts of sound

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


What a concept

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

Transmission Losses

  • Two main types:

  • Spreading

    • Spherical (omni-directional point source)

    • Cylindrical (horizontal radiation only)


Transmission losses cont

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


Self noise

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

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.

Going deep increases pressure so can go faster

without cavitating.


Ambient noise

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

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


Principles of underwater sound

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


Principles of underwater sound

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


Principles of underwater sound

DT

Sonar

Equipment

DI

TL

NL

SL

SL-TL-NL+DI=DT

SR Maul!!!!!


Principles of underwater sound

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.


Principles of underwater sound

DT

Sonar

Equipment

SR Hall!!!!!!!

DI

2TL

TS

SL

NL

SL - 2TL + TS - NL + DI > DT


Principles of underwater sound

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


Factors that affect sound in h 2 o

Factors that affect Sound in H2O

  • Temperature

  • Pressure

  • Salinity

It will bend towards

areas of slower speed.

SOUND IS LAZY!!


Principles of underwater sound

Variable Effects of:

Salinity Pressure Temperature

Salinity

Temperature

Pressure

Depth

Depth

Depth

Speed of Sound in Water

SOUND IS LAZY!!


Principles of underwater sound

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!!


Principles of underwater sound

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!


Principles of underwater sound

Temperature

Range

Transducer

Depth

Maximum Echo Range

ISOVELOCITY


Principles of underwater sound

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


Principles of underwater sound

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


Principles of underwater sound

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


Principles of underwater sound

Direction of Increasing

Temperature and Velocity

Depth

Depth

Water Warm

T

Shadow Zone

C

Water Cool

Negative Gradient Over Positive

Sound Channel


Principles of underwater sound

  • 3-4 deg

T

C

Convergence Zone (CZ)


Principles of underwater sound

Bottom Bounce

>25 Deg.


Principles of underwater sound

Surface Direct Isovelocity

Surface Direct

Negative Gradient

Sound

Channel

Convergence

Zone

Bottom

Bounce

Possible Propagation Paths


Principles of underwater sound

Questions?


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