Lesson 10 - Underwater Sound

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?