1. Principles of Underwater Sound
Naval Weapons Systems
One thing a surface ship hates, and that is SUBMARINES!!
Why?
Very difficult to detect!
Littoral warfare, diesel boats
Subs sit on the bottom
They fight DIRTY!

2. Detection & Position Fixing
Active
Passive
Lead In: Go over the basic concept of Underwater sound and detection by another ship, sub, aircraft
Two ways to derive information from underwater sound:
Active => send out sound wave and listen for it to return (analogous to radar); get a PING off the sub
Passive => listen for sounds of other vessels (analogous to passive EM detection); listen for the sounds of the sub

3. Why do we use SOUND? Range of Penetration into the Medium.
Ability to differentiate between objects in the Medium.
Speed of Propagation.
What are the four types of energy?
Light
Heat IR
Acoustic

4. 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
Source: Vibrates like a speaker in your stereo
Medium: Need a transmission medium, particles excited to vibrate. The particles in the medium move! But not much! KINETIC ENERGY OF PARTICLES IN MOTION!
Detector/Receiver: HEAR the return
Compression: Higher Pressure
Rarefaction: Low Pressure
If students are having a hard time with this concept, consider a series of balls, each connected by a spring. Move one end, and they all will through a series of compressions and rarefactions. See page 178.
Relationship between Frequency and Wavelength is the same as EM waves

5. 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.
Think about how sensitive a device your ear is!

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

7. 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.
Attenuate – 1. To make slender, fine, or small. 2. To reduce in force, value, or amount; weaken 3. To lessen the density of; rarefy.

8. Self Noise Machinery Noise Flow Noise Cavitation
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
1. Background noise interferes with reception
From active noisemakers on the ship/sub
2. Flow Noise {pg. 207, Fig 8-6 – Old Book}
Barnacles on the hull
KEEL HAULING
Navy uses special paints to prevent this
Teardrop hull for submarines (can talk about WWII subs and compare)
Cavitation
High Pressure on leading edge
Low pressure area on the trailing edge causes air bubbles
Tips of the screw!
Bubbles collapse and CLAP back together, very NOISY!

9. Screw Cavitation Going deep increases pressure so can go faster
Blade Tip
Cavitation
Sheet
Cavitation
Water Flow
Water Flow
{Pg 209, fig 8-7, 8-8 – Old Book}
Blade Tip and Sheet cavitation
1. Use graphic top explain how cavitation occurs.
2. Cavitation starts at the blade tips, moving faster so has the lowest pressure.
If speed continues to increase can get sheet cavitation where most of the blades cavitates.
3. To go fast, submarines must go deeper to avoid cavitation.
4. If submarines answer a bell too quickly (to go fast quickly) can get cavitation.
5. Cavitation is very noisy and can be detected on passive sonar.
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.

10. Ambient Noise Hydrodynamic Seismic Biological Ocean Traffic
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.
1. Go over list of ambient noise using graphic.
Lead in:
a. With all that noise, it is sometimes hard to pick out the target.
b. Before going on we need to define some terms

11. Covered the basic principles of Underwater Sound Propagation.
HOW sound moves through the water.
ACTIVE vs PASSIVE
Why do we use sound?
Transmission Losses: Spreading, Attenuation (Absorption/Scattering)
Self Noise: Machinery; Flow Noise; Cavitation
Ambient Noise: Hydrodynamic; Seismic; Biological; Traffic

12. 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, Noise, Hmmm, Signal to Noise ratio?

13. S - N > DT Signal to Noise Ratio (SNR) Detection Threshold (DT)
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.
1. Signal-to-Noise ratio or SNR is essentially the same as with radar.
- High SNR is good.
2. Detection Threshold (DT).
- Go over definition.
- What this means is that if a target is out there the detection threshold is
the SNR level when an operator will detect the target 50% of the time.
(see the target’s return over the noise).
- The DT is a function of equipment and the operator.
3. Bottom line:
To achieve a detection with a specified degree of probability, the signal minus the noise must be equal to or greater than the detection threshold.
S - N > DT (all in dB)
This is the foundation for all sonar equations to predict performance.
S - N > DT

14. 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
{Pg. 217, Fig 8-11 – Old Book}
1. The Passive Sonar Equation represents the ability to detect a target without using active sonar. Just listening for the noise generated by the Target itself.
2. From the basic equation of: S - N > DT
a. . The Source noise level (S) is the Source Noise Level (SL) minus the sound losses due to the water environment (reflection, absorption, scattering , etc.). These losses to the environment are called Transmission Losses (TL)
b. The Noise is the self noise (noise from own ship) and the environmental noise. Together they are the noise level (NL)
c. The Noise portion of the equation is further modified by what is call the Directivity Index (DI) . DI comes into play because the sonar can look in specific directions rather than just 360 degrees. If you are looking the direction of the target you have a better chance of seeing it so the DI increases the Detection Threshold. (it is positive)
(1) Note that N = NL - DI and noise is always positive so DI can never
be more than the Ambient Noise Level (NL).

15. DT Sonar Equipment SR Maul!!!!! DI TL NL SL SL-TL-NL+DI=DT
Recall from previous slide:
From the basic equation of: S - N > DT
a. . The Source noise level (S) is the Source Noise Level (SL) minus the sound losses due to the water environment (reflection, absorption, scattering , etc.). These losses to the environment are called Transmission Losses (TL)
b. The Noise is the self noise (noise from own ship) and the environmental noise. Together they are the noise level (NL)
c. The Noise portion of the equation is further modified by what is call the Directivity Index (DI) . DI comes into play because the sonar can look in specific directions rather than just 360 degrees. If you are looking the direction of the target you have a better chance of seeing it so the DI increases the Detection Threshold. (it is positive)
(1) Note that N = NL - DI and noise is always positive so DI can never
be more than the Ambient Noise Level (NL).

16. Active Sonar Equations
**Ambient Noise Limited:**
SL - 2TL + TS - NL + DI > DT
Reverberation Noise Limited: (Reverb > ambient noise)
SL - 2TL + TS - RL > DT
1. Just like the passive sonar, there are equations that describe the performance of active sonar.
2. Same basic equation applies: S - NL > DT except for:
a. Involves the signal traveling twice the distance. (to the target then back)
So the Transmission losses are double (i.e. 2TL)
b. TS: Target Strength factor is added. Target strength is the amount of
the active sonar signal that is reflected back towards own ship.
c. Involves two equations depending on what noise is the most limiting.
(1) If the ambient noise the the most limiting than use equation 1.
(2) If the reverberation noise becomes stronger than the ambient noise,
then must use the second equation.
- RL: difficult to quantify. A time-varying function resulting from the inhomogeneities in the medium
If the noise level or reverberation level is too high, you won’t detect the target
Lead in: So what good is all of this?
TS: Target Strength, A measure of the reflectivity of the
target to an active sonar signal.

17. DT
Sonar
Equipment
SR Hall!!!!!!! DI
2TL TS SL NL
SL - 2TL + TS - NL + DI > DT

18. 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
1. We can’t rate acoustic gear by range so we use Figure of Merit (FOM).
2. FOM is the ability of a SONAR to detect a level of sound energy.
3. To get the equations just move DT to the other side of the equation.
4. ASK : From the equations, how can we improve the FOM?
a. FOM improves by increasing the source level
(1) Increasing transmitted power for active sonar
(2) Finding a noisier target for passive sonar
b. Improves by decreasing the ambient noise level.
c. Improves by increasing the Directivity Index (DI)
d. Improves by decreasing the detection threshold (DT)
5. FOM is used for measuring sonar capabilities. It is a key tool for deciding detection probabilities and estimated range of detection (if propagation losses are known).
6. Lead in: That is the key. If we can understand and measure the propagation losses of the sound received by the sonar then we can predict the range at which we will detect a target. We previously stated that many things affect the the loss of sound. Those included spreading, scattering, absorption.
a. The speed of sound in the water and the path the sound takes also greatly affect the propagation of the sound energy.

19. Factors that affect Sound in H2O
Temperature
Pressure
Salinity
It will bend towards
areas of slower speed.
SOUND IS LAZY!!

20. SOUND IS LAZY!! Speed of Sound in Water Variable Effects of:
Depth
Salinity
Pressure
Temperature
Salinity Pressure Temperature
Variable Effects of:
Pg 183 in book
1. The speed of sound in water is determined first by the water itself.
a. The elasticity of the medium (for compression and expansion of the sound energy’s longitudinal wave) is the most important factor in determining the speed of sound
b. The effect of medium density is also very important.
2. In addition to the normal density of water, there are several factors which can cause the density of the water to change. They are salinity, pressure and temperature.
3. Salinity (has the smallest affect on sound speed) (an incr in 1 pp thous = an incr in spped of 1.3m/sec)
a. As salinity increases the sound speed increased.
b. Salinity can be a big factor near rivers.
c. Salinity increases with depth
4. Pressure – Biggest factor below ~1500ft (Every 3 feet of depth = m/sec incr in speed)
a. As pressure increases sound speed increases
b. Pressure increase is constant and predictable
5. Temperature (The major factor affecting sound speed above 1500 feet) (The warm spot in the POOL)
a. Below 1500 feet temperature of the ocean is constant, roughly 34F.
b. One degree Celsius increase in temperature will change water speed by
3 meters/sec.

21. Sound Velocity Profile
Typical Deep Ocean
Sound Velocity Profile
Depth of Water (meters)
Speed of Sound (meters/sec)
1500
1520
1480
1000
2000
3000
Surface Layer
Seasonal Thermocline
Permanent Thermocline
Deep Isothermal Layer
Pg 183, Fig 15-4
1. The graphic shows the typical deep ocean sound speed profile. It is the summation of the effects of destiny, salinity, pressure and normal temperatures.
2. Surface layer:
- Sound speed is susceptible to daily and local changes in heating cooling
and wind mixing action.
3. Seasonal Thermocline:
- Thermocline is a layer where the temp. can change rapidly with depth.
- Seasonal can change with the seasons. (wind and storms can cause
mixing of water temperatures)
4. Permanent Themocline
- Affected only slightly by seasonal changes.
5. Deep Isothermal Layer – Sound speed is constant
- Has nearly a constant temperature (about 4 degrees C)
6. Ocean Currents can create an unexpected thermal layer
- This layer can trap sound waves and let the sound travel further.
7. Ocean fronts are boundaries of large masses of different temperature water
(like weather fronts). These can cause large horizontal gradients of
Temperature and pressure.
SOUND IS LAZY!!

22. 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!
Why do we care about these layers? The answer lies in ray propagation theory and how sound behaves when its speed is changed.
1. Just like electromagnetic waves we can depict the travel of the sound wave as an arrow or ray. This will show how the wave front will travel.
2. Snell’s law say the when rays pass through different mediums (different densities) then the ray will bend when passing through the interface.
a. In water the change of densities occurs gradually so the ray is bending
slightly all the time.
b. The effect is that the ray appears to bend.
c. The more the difference the more the bend.
3. Sound rays will always bend towards the area where the speed of sound is
slower.
4. Can explain bending by the following example:
If have a curved surface (wave front) and the bottom of the surface moves slower than the top the surface will have a torque on it which will tend to turn the curved surface towards the slow speed area.

23. ISOVELOCITY Temperature Range Transducer Depth Maximum Echo Range
Pg 186, Fig 15-8
1. Isothermal water is where the water temperature is essentially constant.
2. Rays will travel is a straight line in these waters (very little bending)

24. Negative Gradient Direction of Increasing Temperature and Velocity
Depth
Water Warm
Shadow Zone
Water
Cool
Sound Bends Down When Water Grows
Cooler With Depth
Direction of Increasing
Temperature and Velocity
Negative Gradient Thermal Structure T C
1. Negative gradient profile is when the temperature of the water (and sound speed) decreases with depth.
2. Rays will tend to bend DOWN.
3. This is common near the surface where the sun heats the shallower water.
4. Sound rays can bend so much that a shadow zone exists where the sound can not get to that area.
Shadow zone : Blind spot where you can’t hear any return..

25. Positive Gradient Water Cool Shadow Zone Water Warm
When Temperature Increases with
Depth, Sound Bends Sharply Up
Depth
Direction of Increasing
Temperature and Velocity
Positive Gradient Thermal Structure T C
1. A positive gradient is when the water temperature (and sound speed) increases with depth.
2. Sound in a positive gradient will tend to bend up.
3. Sound will reflect off the surface, good for longer ranges unless the sea is rough, then we get scattering.
Lead in:
4. Unfortunately the sound velocity or temperature curves are not straight lines but are combinations of isothermal and positive and negative gradients.

26. Layer Depth Direction of Increasing Temperature and Velocity Depth
Cool
Shadow
Zone
Isothermal
Sound Beam Splits When Temperature Is
Uniform At Surface and Cool At Bottom
Depth
Direction of Increasing
Temperature and Velocity
Isothermal Gradient Thermal Structure T C
1. Layer depth phenomenon
- When there is a layer of isothermal water over water with a negative
gradient.
- The speed of sound is maximum at the boundary.
Layer depth is the depth of the greatest sound speed above the seasonal Thermocline.
Shadow Zone – p.188 fig

27. Sound Channel Direction of Increasing Temperature and Velocity Depth
Water Cool
Shadow Zone
Water Warm
Depth
Direction of Increasing
Temperature and Velocity
Negative Gradient Over Positive T C
Sound Channels
- Negative gradient over a positive gradient.
- Tends to refract rays back and forth essentially trapping the ray in the
channel
- Rays in a sound channel can travel great distances
Surface Duct is a sound channel near the surface.
- Just below surface and is susceptible to daily and local changes of
heating, cooling and wind direction.
- These are rare and not stable.

28. Convergence Zone (CZ) T C 3-4 deg
Convergence zone (CZ) – p.190 fig
- Negative gradient over a positive gradient in extremely deep water (so rays bend totally before striking the bottom.
- Temperature decreases and the sound bends down when deep pressure causes the beam to go back up.
- About 50 km the beam hits the surface and refracts back towards the bottom.
- Can be multiple CZ zones.
- Can result in very long range detection's.

29. Bottom Bounce >25 Deg. Bottom Bounce
- When the sound bounces off the bottom.
- Depends on the bottom type. Flat and hard ocean floors tend to be the best. Mud and sand tend to be the worst.

30. Possible Propagation Paths
Sound
Channel
Negative Gradient
Surface Direct
Convergence
Zone
Bottom
Bounce
Surface Direct Isovelocity
Pg 238, Fig 8-28
1. Use graphic to summarize the possible paths to receive sound from a target.

31. Questions?