OCEN 201 Introduction
to Ocean Engineering
CHAPTER 8: UNDERWATER ACOUSTICS
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HISTORICAL BACKGROUND
15th-17th Century
•Leonardo da Vinci said, "If you cause your ship to stop and place
the head of a long tube in the water and place the outer
extremity to your ear, you will hear ships at a great distance
from you". The addition of second tube to the other ear and
placing the head of the tube at a different point in the sea
permitted determination of direction.
•Newton published the first mathematical treatment of the
theory of sound "Mathematical Principles of Natural
Philosophy“ in 1687.
18th & 19th Century
•Bernoulli, Euler, LaGrange, d'Alembert, Fourier all contributed
to the theory of sound.
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HISTORICAL BACKGROUND (CONT)
 1827
Colladon & Sturm measured the speed of sound
in water using a light flash coupled with sounding of an
underwater bell to obtain 4707 ft/s at 8°C.
 1840
Joule quantified the magnetostriction effect.
 1877
Rayleigh published "Theory of Sound".
 1880
Curie discovered the piezoelectric effect.
Piezoelectric and magnetostrictive effects are used in
the production of transducers used to generate and
receive underwater sound.
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HISTORICAL BACKGROUND (CONT)
1912
Fessenden developed the first high powered underwater sound source. A
foghorn and underwater bell were used to determine distance from shore.
Also, he designed a moving coil transducer (Fessenden oscillator) for echo
ranging.
1914-18 (World War I)
During World War I (WWI) (1914-18) a system for underwater echo-ranging
was developed under the acronym ASDIC (Allied Submarine Devices
Investigation Committee). The principle of echo-ranging (echo location)
was that a pulse of sound was transmitted into the water, and any reflection
(echo) from a submarine was received by a hydrophone (underwater
equivalent of a microphone).
1915
In the U. S., Hayes pioneered the field of passive sonar arrays. In the U. S.,
the term ASDIC was replaced by SONAR which is an acronym for sound
navigation and ranging. It was coined in search of a name similar to radar.
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HISTORICAL
BACKGROUND
(CONTINUED)
1918
Langevin used the piezoelectric
effect for underwater sound
equipment and detected submarine
echoes at distances as great as
1500 m.
1919
Germans published a paper on the
bending of sound rays due to
temperature and salinity gradients
in the sea.
1925
Fathometers were used by ships in
the US and Great Britain for depth
sounding. Depth sounders were
also used by the fishing industry to
locate schools of fish.
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HISTORICAL BACKGROUND (CONT.)
1935: Adequate sonar systems were developed and
produced in US. US ships were equipped with
underwater listening and echo ranging.
1937 :Spilhaus invented the bathythermograph (BT)
which measures the temperature versus depth of
water. Bathythermographs (BTs) were installed on all
submarines to measure the temperature profile of the
ocean to assist in the determination of characteristics
of sound propagation and sonar detection.
1938: Surface vessels were equipped with underwater
sound equipment that was used for echo ranging. The
operator searched in bearing with headphones and
loudspeaker. Submarines were equipped with
underwater listening devices (line hydrophone array
with headphones).
Bathythermograph
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HISTORICAL
BACKGROUND (CONT)
1945: Significant advances were made in active sonar systems.
Knowledge of underwater acoustic propagation improved and the
ability of detect and measure underwater sound signals in noise
greatly improved. The book "Physics of Sound in the Sea" was
published.
1950: The advent of nuclear powered submarines gave impetus to
being able to detect submarines over vast ocean areas.
1960: Passive sonar systems were advanced to detect submarines
using the advances in digital signal processing and computers.
Sidescan sonars were developed to obtain acoustic images of the
ocean floor.
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HISTORICAL
BACKGROUND (CONT.)
Sidescan sonar
 1970: Multibeam echo sounders were developed. The
multiple echoes allow production of high quality maps of
the seafloor.
 1980: Multibeam echo sounders, sub-bottom profilers,
and sidescan sonars combined to give very detailed maps
of the seafloor and characteristics of the sediments in the
upper layers of the seafloor.
Multibeam echo sounder
 1990: Submarines were able to reduce noise levels and
reversed the trend of developing passive sonars to
improving the range of active sonars. Acoustic Doppler
current meters were developed and quickly
revolutionized the methods for measuring ocean
currents.
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HISTORICAL BACKGROUND (CONT)
ADCP
ADV
 2000: Underwater acoustics continues to
be used to improve field and laboratory
current measurements (ADV & ADCP).
Offshore Industry uses underwater
acoustics for dynamic positioning, ROV
and AUV navigation and tracking, and
deep water pipeline surveys.
2010: Multibeam scanned images.
Images looking like photographs.
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CIVILIAN
ACOUSTIC DEVELOPMENT
1. Fishing aids (locating commercial fish)
2. Hydrographic surveying
3. Ocean Engineering/Oceanography
(telemetry of data, acoustic Doppler current
meter, acoustic release mechanisms, vertical
echo sounders, ocean acoustic tomography)
4. Geophysical research (oil exploration),
seismic exploration
5. Underwater communications (submarines,
divers, remotely operated vehicles)
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CIVILIAN ACOUSTIC DEVELOPMENT
6. Navigation and positioning (depth sounders,
beacons, transponders, acoustic speedometers,
obstacle avoidance sonars)
7. Underwater search and surveying (side scan
sonar and sub-bottom profilers, depth sounders,
multibeam echo sounders).
8. Coastal processes, beach surveys, and dredging
surveys (sediment thickness and characteristics,
acoustic flow meters)
Acoustic beacon
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ACOUSTIC
POSITIONING
SYSTEM FOR
OFFSHORE
PLATFORM
NATURAL SONAR SYSTEMS
1. Porpoise, Dolphins & Whales
(underwater navigation)
2. Bat
(Navigation in air)
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Underwater
Fundamentals
SECTIONS 8-2
UNDERWATER FUNDAMENTALS
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SIDESCAN & MULTIBEAM SONARS
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MULTIBEAM SCAN IMAGE
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Active Sonar Systems
TYPES OF
UNDERWATER
ACOUSTIC
SYSTEMS
• Active echo ranging sonar is used by ships to locate submarine targets.
• Torpedoes use moderately high frequencies to echo range on targets.
• Depth sounders send short pulses downward and time the bottom return.
• Side-scan sonars are used to map the ocean seafloor at right angles to a
ship's
track.
• Fish finding aids are forward looking active sonars for spotting fish schools.
• Diver handheld sonars are for diver location of underwater objects.
• Position marking beacons transmit sound signal continuously.
• Position marking transponders transmit sound only when interrogated.
• Acoustic flow meters and wave height sensors are used.
• Sonobuoy is a link between an aircraft and underwater explosive source.
• Multiple beam echo sounders used to map the seafloor in great detail.
Passive Sonar Systems
• Passive Sonar System is a hydrophone (array) that detects acoustic radiation from
another vessel or object; i.e. hydrophone used by 1960 era submarines.
• Example of Passive Systems
• Acoustic mines - mines explode when acoustic radiation reaches a certain value.
• Torpedoes - home on acoustic radiation of submarine or ship (usually the
propeller).
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reflected wave
Target
Hydrophone
original wave
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https://www.freepik.com/photos/woman>
Woman photo created by freepik www.freepik.com
https://www.rs-online.com/designspark/robot-navigation-with-sonar
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UNDERWATER
ACOUSTIC SYSTEMS
Seismic Systems
https://www.coastalreview.org/2016/05/14318/
• Sub bottom profiles are used to explore
the rocks and sediments making up the
ocean floor. The acoustic pulses used are
basically unidirectional pressure pulses
that are generated by: explosive charges,
underwater arc (sparker),
electromagnetic (thumper), and air guns.
These seismic devices produce results
that show the geological features of the
ocean floor.
Question: Is this an ACTIVE or PASSIVE sonar?
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Underwater Communications and Telemetry
Systems and Navigation
•Underwater telephone(UQC or Gertrude) is a device used
to communicate between a surface ship and a submarine
or between two submarines
TYPES OF
UNDERWATER
ACOUSTIC
SYSTEMS
•Diver communications - diver has a full-face mask which
allows the diver to speak normally underwater and a
throat microphone is used to obtain speech signals. A
transducer is used to transmit the signal. The same
transducer is used to receive, and the signal is passed to
the diver via an earpiece.
•Telemetry systems - data from a submerged instrument is
transmitted to the surface.
•Doppler navigation - pairs of transducers pointing
obliquely downward to obtain speed over the bottom
from the Doppler shift of the bottom returns.
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TYPES OF
UNDERWATER
ACOUSTIC SYSTEMS
Passive ship sonar is a hydrophone array that
detects acoustic radiation from another
vessel or object; i.e. hydrophone used by
1960 era submarines.
Examples of Passive Systems
Acoustic mines - mines explode when acoustic
radiation reaches a certain value.
Torpedoes - home on acoustic radiation of
submarine or ship (usually the propeller).
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Sound is the periodic variation in pressure, particle displacement, and particle
velocity in an elastic medium. Sound is produced by mechanical vibration.
Sound waves are longitudinal waves since the molecules transmitting the wave
move back and forth in the direction of propagation of the wave.
FUNDAMENTALS
OF
UNDERWATER
SOUND
Transmission of sound waves is very complicated so we shall study plane waves of
sound that are the simplest type of wave motion propagated through a fluid
medium.
For a plane wave the acoustic pressures, particle displacement, density changes,
etc. have common phases and amplitudes at all points on any given plane
perpendicular to the direction of wave propagation.
Plane waves are easily produced in a rigid pipe with a vibrating piston. In a
homogeneous medium, plane wave characteristics are attained at large distances
from their source.
A particle of the medium is understood to mean a volume element that is large enough to
contain millions of molecules so that it may be considered a continuous fluid, yet small
enough so that the acoustic variables of pressure, density, and velocity can be considered as
constant throughout the volume element.
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or Condensation
or Condensation
TRANSVERSEWAVESARETHOSEWAVESINWHICH
THEPARTICLESOFTHEMEDIUMMOVE
PERPENDICULARTOTHEDIRECTIONOFTHE
PROPAGATIONOFTHEWAVE. FOREXAMPLE,
RIPPLESFORMEDONTHESURFACEOFTHEWATER
ARETRANSVERSEWAVES
LONGITUDINALWAVESARETHOSEWAVESIN
WHICHTHEPARTICLESOFTHEMEDIUMMOVE
PARALLELTOTHEDIRECTIONOFTHE
PROPAGATIONOFTHEWAVE. FOREXAMPLE,
SOUNDWAVESARELONGITUDINALWAVES
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https://www.geogebra.org/m/SxNZa3Q2
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For the case of a plane wave of sound, the acoustic pressure (p) is
related to the particle velocity (u) by
where
FUNDAMENTALS
OF UNDERWATER
SOUND
(CONTINUED)
p=ρcu
p
- pressure
ρ
- density
c
- propagation velocity of the plane wave
ρc
- is called the specific acoustic resistance
u
- particle velocity
ρcseawater = 1.5 x 105 g/cm2s = 1.5 x 106 kg/m2s ;
ρcair = 42 g/cm2s =420 kg/m2s
Ohm's Law for acoustics
I~u
R ~ ρc
E~p
The energy involved in propagating acoustic waves through a fluid
medium is of two forms:
1. Kinetic Energy - particle motion
2. Potential Energy - stresses set up in the elastic medium
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Decibel Scales
Sound intensities and sound pressures are expressed as logarithmic scales known as sound levels.
Reasons:
1. A very wide range of sound pressures and intensities are encountered in the ocean.
2. The human ear subjectively judges the relative loudness of two sounds by the ratio of their intensities.
The most generally used logarithmic scale for describing sound levels is the decibel (dB) scale.
The intensity level (N) of a sound of intensity I1 and reference intensity I2 is defined by:
Intensity Level (IL)
𝑁𝑁 = 10 log
Sound Pressure Level (SPL)
𝐼𝐼1
𝐼𝐼2
𝑁𝑁 = 20 log
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𝑝𝑝1
𝑝𝑝2
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Decibel Scales (continued)
The level of a sound wave is the number of decibels by which its intensity, or energy flux density,
differs from the intensity of the reference sound wave. In the case of a sound wave with an intensity
of I1 and a reference intensity of I2, the level of the sound wave is equal to:
N dB = 10 log I1 / I 2
For clarity the level should be written:
N dB
re

1 µPa
the intensity of a plane
wave of pressure equal to
If a sound wave has an intensity 500 times that of a plane wave of rms pressure 1 µPa, then
the level N is:
N = 10 log 500/1 = 27 dB re 1 µPa
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Intensity Level
𝐼𝐼1
𝑁𝑁 (𝑑𝑑𝑑𝑑 𝑟𝑟𝑟𝑟 𝑝𝑝2 ) = 10 log
𝐼𝐼2
Sound Pressure Level
𝑝𝑝1
𝑁𝑁 (𝑑𝑑𝑑𝑑 𝑟𝑟𝑟𝑟 𝑝𝑝2 ) = 20 log
𝑝𝑝2
𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑
= 0.1 Pa
𝑐𝑐𝑐𝑐2
1 Pa = 1 × 106 𝜇𝜇 Pa
1 bar = 1 × 105 Pa
1
Converting N:
𝑁𝑁𝑝𝑝2 = 𝑁𝑁𝑝𝑝1 + 20 log
𝑝𝑝1
𝑝𝑝2
or 𝑁𝑁𝑝𝑝2 = 𝑁𝑁𝑝𝑝1 − 20 log
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𝑝𝑝2
𝑝𝑝1
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SONAR EQUATIONS
SECTIONS 8-3
SONAR EQUATIONS (SOUND, NAVIGATION, AND RANGING)
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Practical Functions of Sonar Equations
Performance prediction of sonar equipment with a known or existing design
Sonar design
Total Acoustic Field
Desired portion – signal
Undesired portion – background (noise or reverberation)
Sonar Designer
 Objective – increase the overall response of the sonar system to the signal and decrease
the response of the system to the background (maximize signal to noise ratio)
 A sonar system is just accomplishing its objective when the signal level equals the
background masking level
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Active & Passive Sonar Equation Parameters
Equipment related
Target related
Medium related
* level in decibels relative to the
standard reference intensity of a 1
micropascal (µPa) plane wave
◦ Projector source level (SL*)
◦ Noise level (NL*)
◦ Receiving directivity index (DI)
◦ Detection threshold (DT)
◦ Transmission loss (TL)
◦ Reverberation level (RL*)
◦ Ambient noise level (NL*)
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◦ Target strength (TS)
◦ Target source level (SL*)
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Detection Threshold (DT)
Electronics
Active
Sonar
Equation
Directivity Index (DI)
or
Array Gain (AG)
Receive
Electronics
Headphones
Source Level (SL)
Noise Level (NL)
One-way Transmission
Loss (TL)
Target Strength (TS)
Monostatic – source & receiver are coincident
Bistatic – source & receiver are separated
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Active
• SL-2TL+TS=NL-DI+DT
Sonar
Equations
Active (Reverberation)
• SL-2TL+TS=RL+DT
Passive
• SL-TL=NL-DI+DT
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Example: A passive sonar system is being used to detect an object that has a source level of 80
dB re 0.0002 dynes/cm2 and a directivity index of 12 dB. If the detection threshold is 15 dB and the
transmission loss is 50 dB, determine the noise level which will permit detection of the target.
Given:
◦ SL = 80 dB re 0.0002 dynes/cm2
◦ DI = 12 dB
◦ DT = 15 dB
◦ TL = 50 dB
Find: NL
Solution:
Passive Sonar Equation
SL-TL = NL-DI+DT
Note: 1 μPa = 10-5 dynes/cm2
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Example: A passive sonar system is being used to detect an object that has a source level of 80
dB re 0.0002 dynes/cm2 and a directivity index of 12 dB. If the detection threshold is 15 dB and the
transmission loss is 50 dB, determine the noise level which will permit detection of the target.
Given:
10 −5
N 1µPa = 80 − 20 log
0.0002
◦ SL = 80 dB re 0.0002 dynes/cm2
◦ DI = 12 dB
◦ DT = 15 dB
◦ TL = 50 dB
Find: NL
Solution:
N p 3 = N p 2 − 20 log
N 1µPa = 80 − 20(− 1.3)
N 1µPa = 80 + 26 = 106 dB re 1µPa
SL = 106 dB re 1µ Pa
p3
p2
N 1µPa = N 0.0002 dyne / cm 2 − 20 log
Passive Sonar Equation
1µPa
0.0002 dynes / cm 2
Note: 1 μPa = 10-5 dynes/cm2
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UNDERWATER ACOUSTICS FOR OCEAN ENGINEERS
SL-TL = NL-DI+DT
106-50 = NL-12+15
56 = NL+3
NL = 53 dB re 1 μPa
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Limitations
of
Sonar
Equations
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1. Short pulse sonar requires correction to SL value.
2. Correlation sonar must account for correlation
loss.
3. Medium
a) inhomogeneous medium
b) irregular boundaries
c) one boundary is in motion
The sonar parameters fluctuate randomly with time.
There are unknown changes in equipment and
platform conditions.
Solution of the SONAR EQUATIONS yields a time
averaged result of a stochastic problem.
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TRANSDUCERS AND
ARRAYS
TRANSDUCERS AND ARRAYS
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Underwater sound transducers permit
detection of an underwater sound wave.
Transducers
Transducers - are devices that convert sound and
electrical energy into each other. More generally,
it can be said of any two forms of energy.
Hydrophones - are transducers that convert
sound into electrical energy.
Projector - a transducer that converts electrical
energy into sound energy.
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Transducer
Material
Properties
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The conversion of sound into electrical energy and vice versa
is accomplished with the use of materials that have certain
properties:
1. Piezoelectric - Materials such as quartz, ammonium
dihydrogen phosphate (ADP), and Rochelle salt acquire an
electrical charge between crystal surfaces when placed under
pressure and conversely they acquire a stress when a voltage
is placed across the surfaces.
For example, the electrical potential (voltage) may be varied
periodically at the frequency of the desired sound signal, and
thus, the material vibrates at the desired frequency.
2. Electrostrictive - these materials have the same effect as
piezoelectric materials. However, these materials are
ceramics that have been properly polarized. Examples are
barium titanate and lead zirconate.
3. Magnetostrictive – this material changes dimensions
when it is subjected to a magnetic field and conversely its
magnetic field is changed when it is stressed.
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Transducer Array Design
Transducer arrays consist of a number
of elements spaced in a particular way.
Advantages of arrays:
1. More sensitive
2. Possess directional properties
3. Greater signal to noise ratio than single
elements.
Advantages 1 & 2 are true for projectors also.
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Frequency Range for Acoustic Systems (Coates 1989)
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Example transducers and arrays
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Example Transducers and Arrays
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Example Transducers and Arrays
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TRANSMISSION LOSSES
AND SEA PROFILE
TRANSMISSION LOSS
SEA PROFILE
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Transmission Loss
The flow of acoustic energy from a source to a receiver is described in terms of its
intensity at a distance 1 yd from the source and the reduction in intensity between this
point and the receiver. The transmission (propagation) loss is the reduction in intensity
between the reference point and the receiver.
TL = 10 log I1 / I 2
where I1 is the source intensity referenced to 1 yd and I2 is the intensity at a distant point.
The intensity level (L) at the distant point is
L = SL − TL
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1) Spreading - Acoustic energy becomes diluted as it
Factors
Affecting
Transmission
Loss
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spreads over a larger area and thus the intensity is
reduced.
a) Near the source, the spreading is spherical, and
the loss is proportional to the inverse square of the
distance.
b) At larger distances, the spreading is affected by
refraction (bending of rays or paths along which the
sound waves travel).
2) Attenuation - Loss of energy from the sound wave is
a result of:
a) Absorption - results from the conversion of
acoustic energy into heat (frictional effects).
b) Scattering - is the process whereby objects in
the medium cause some of the sound energy to be
deflected in various directions.
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Table 5-1: Summary of Spreading Laws
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Speed of Sound in the Sea
Speed of sound in water has been determined theoretically and experimentally.
Leroy equation
=
c 1492.9 + 3 (T − 10 ) − 6 ×10−3 (T − 10 ) − 4 ×10−2 (T − 18 ) + 1.2 ( S − 35 ) − 10−2 (T − 18 )( S − 35 ) + Z / 61
2
2
where c is sound velocity, m/s;T is temperature, oC at the depth; S is salinity, ppt; Z is depth, m.
MacKensie (1981)
c = 1448.96 + 4.591T − 5.304x10−2 T2 + 2.374x10−4 T3 + 1.340( S − 35) + 1.630x10−2 d + 1.675x10−7 d 2
− 1.025x10−2 T( S − 35) − 7.139x10−13 Td 3
where c is sound speed (m/s), T is temperature (oC) at the depth, S is salinity (ppt), and d is depth
(m). The range of validity for the MacKensie (1981) equation is: 0oC ≤ T ≤ 30oC, 30‰ ≤ S ≤ 40‰,
and 0 m ≤ d ≤ 8000 m. The MacKensie equation is good for practical work and shows that sound
speed increases with temperature, salinity, and depth.
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Velocity Structure in the Ocean
Surface Layer - sound velocity subject to
daily and local changes in heating and
cooling, and wind action.
Seasonal thermocline - negative thermal or
velocity gradient that varies with season.
Summer-Fall - near surface waters are
warm, and it is well defined.
Winter-Spring - it tends to merge and be
indistinguishable from the surface layer.
Main thermocline - affected only slightly by
seasonal changes. Here the major decrease
in temperature occurs.
Deep isothermal layer - nearly constant
temp of 39oF. Sound velocity increases due
to depth.
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End
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