Underwater Acoustics

OCEN 201

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TYPES OF UNDERWATER ACOUSTIC SYSTEMS

Active Sonar Systems

Active echo ranging sonar is used by ships to locate submarine targets. Depth sounders send short pulses downward and time the bottom return.Side-scan sonars are used for finding objects on sea floor and mapping.Fish finding sonars are forward looking sonars for spotting fish schools.Diver sonars are hand held sonars used for locating 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 .Multiple beam echo sounders used to map the seafloor in great detail.

Seismic Systems

Subbottom profilers are used to explore the rocks and sediments making upthe ocean floor. The acoustic pulses used are basically unidirectional pressure pulses that are generated by air guns. Results show the geological features below the ocean floor.

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SUBMARINE SONAR

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TYPES OF UNDERWATER ACOUSTIC SYSTEMS

3. Underwater Communications and Telemetry Systems and Navigation

a. Underwater telephone is a device used to communicate between a surface ship and a submarine or between two submarines (UQC).

b. 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 ear piece.

c. Telemetry systems - data from a submerged instrument is transmitted to the surface.

d. 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

4. Passive Systemsa. Passive ship

sonar is a hydrophone array that detects acoustic radiation from another vessel or object; i.e. JP or JT hydrophone used by WWII submarines.

b. Acoustic mines - mines explode when acoustic radiation reaches a certain value.Torpedoes - home on acoustic radiation of submarine or ship.

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ACOUSTIC TRANSDUCERS

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ACOUSTIC DOPPLER CURRENT METER

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SIDESCAN SONAR

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Decibel Scales

Sound intensities and sound pressures are expressed as logarithmic scales known assound 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 ratioof their intensities.

The most generally used logarithmic scale for describing sound levels is the decibel scale.

The intensity level (N) of a sound of intensity I1 and reference intensity I2 is defined by:

Intensity Level 2

1

IIlog10NIL)( =

Sound Pressure Level ( ) 21 p/plog20NSPL =

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FUNDAMENTALS OF UNDERWATER SOUND (CONTINUED)

For the case of a plane wave of sound, the acoustic pressure (p) is related to the particle velocity (u) by

p = ρ c uWhere p - pressure

ρ - densityc - propagation velocity of the plane waveρc - is called the specific acoustic resistanceu - particle velocity

ρcseawater = 1.5 x 105 g/cm2sρcair = 42 g/cm2s

The energy involved in propagating acoustic waves through a fluid medium is of two forms:

1. Kinetic Energy - particle motion2. Potential Energy - stresses set up in elastic medium

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FUNDAMENTALS OF UNDERWATER SOUND (CONTINUED)

For a plane wave, the acoustic intensity (I) of a sound wave is the average rate of flow of energy through a unit area normal to the direction of wave propagation. The instantaneous intensity is

I = p2/ ρ c

The average intensity is I = p2 ave/ ρ c

Where p2 ave is the time average of the instantaneous acoustic pressure squared.

Units: p = dynes/cm2

ρ = gm/cm3

c = cm/sI = ergs/cm2s

Since Intensity is also power/unit area and the units are often watts/cm2. One watt is equal to 107 ergs/s then

I = power/area = p2 ave/ ρ c x 10-7 watts/cm2

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Decibel Scales (continued)In general, if we have a quantity x such that

( )a2121 x/xI/I =

then the ratio of the values on the decibel (dB) scale is

( )11 2

210log 10 log /I a x x dBI

⎛ ⎞=⎜ ⎟⎝ ⎠

For a =2, then 10 log (I1/I2) = 20 log (x1/x2) = 20 log (p1/p2)

The reference level must be known to insure proper interpretation of the dB value.(Note that 1 psi x 6895 = number of Pascal). Also 1 Pascal = 1 N/m2

The old reference levels are: 1) 1 dyne/cm2

2) 0.0002 dyne/cm2

The current reference level is: 1 micropascal (1 μPa).

Note: 1 μPa = 10-5 dyne/cm2

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Decibel Scales (continued)To convert from one reference (p2)to another (p3).

Np2 = 20 log (p1/p2)Np3 = 20 log (p1/p3)

Subtract Np3 from Np2,

( ) ( )[ ]21312p3p p/plogp/plog20NN −=−

[ ]21312p3p plogplogplogplog20NN +−−=−

[ ]322p3p plogplog20NN −=−

( )322p3p p/plog20NN +=

Example: Express 125 dB relativeto 0.0002 dyne/cm2 in dB relativeto 1 dyne/cm2. Let

22 dyne/cm 0002.0p =

23 cm/dyne1p =

( )1/0002.0log201253pN +=

dB5174125N 3p =−=

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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:

21 I/Ilog10dBN =For clarity the level should be written:

{ Pa1redBN

toequal pressure of wave plane a ofintensity the

μ

If a sound wave has an intensity 500 times that of a plane waveof rms pressure 1 μPa, then the level N is:

N = 10 log 500/1 = 27 dB re 1 μPa

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Sonar Equations

ActiveSL-2TL+TS=NL-DI+DT

Active (Reverberation)SL-2TL+TS=RL+DT

PassiveSL-TL=NL-DI+DT

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Active Sonar Equation

Headphones

Electronics

Detection Threshold (DT)

Directivity Index (DI)or

Array Gain (AG)

Source Level (SL)

Noise Level (NL)One-way Transmission

Loss (TL)Target Strength (TS)

ReceiveElectronics

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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 dBDT = 15 dB TL = 50 dB

Find: NLSolution:

2

32p3p p

plog20NN −=

2cm/dyne0002.0Pa1 cm/dynes0002.0Pa1log20NN 2

μ−=μ

0.000210log2080N

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Pa1

−

−=μ

( )3.12080N Pa1 −−=μ

Pa1redB1062680N Pa1 μμ =+=

Pa1redB106SL μ=

Passive Sonar Equation

Paμ1redB53NL3NL56

1512NL50106DTDINLTLSl

=+=

+−=−+−=−

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Beam Patterns

Line Array

Circular Plane Array

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Line Array with Equally Spaced Elements

( )b

n d

nd

θ

πλ

θ

πλ

θ=

⎛⎝⎜

⎞⎠⎟

⎡

⎣⎢

⎤

⎦⎥

⎛⎝⎜

⎞⎠⎟

⎡

⎣⎢

⎤

⎦⎥

⎧

⎨⎪⎪

⎩⎪⎪

⎫

⎬⎪⎪

⎭⎪⎪

sin sin

sin sin

2 0

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beam width at -3 db

acoustic axis

db

9

x x x xxxxelements

θ

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Beam Pattern Spreadsheet

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Spherical Spreading and Absorption

Propagation measurements made in the ocean indicate that spherical spreading together with absorption yields a reasonableapproximation to measured data for a wide variety of conditions.Therefore, transmission loss may be expressed by

310rrlog20TL −×α+=

This is a rough approximation but a good rule of thumb.

where r is range in yards, α is absorption coefficient in dB/kyd, and TL is transmission loss in dB.

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Francois & Garrison (1982)

Figure 5-2 shows the variation of the absorption coefficient (α) as a function of frequency from 0.1 to 1000 kHz at zero depth (surface) for a salinity of 35‰ and pH of 8.0.The accuracy of the predicted absorption coefficients is estimated as ±5% for the ranges of 0.4 to 1000 kHz, -1.8 to 30oC, and 30 to 35‰.

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Speed of Sound in the Sea

Speed of sound in water has been determined theoretically and experimentally.

( ) ( ) ( ) ( ) ( )( )2 23 2 21492.9 3 10 6 10 10 4 10 18 1.2 35 10 18 35 /61c T T T S T S Z− − −= + − − × − − × − + − − − − +where c is sound velocity, m/s;T is temperature, oC at the depth;S is salinity, ppt; Z is depth, m.

( )( )

c 1448.96 4.591T 5.304x10 T 2.374x10 T 1.340 S 35 1.630x10 d 1.675x10 d

1.025x10 T S 35 7.139x10 Td

2 2 4 3 2 7 2

2 13 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.

Leroy equation

MacKensie (1981)

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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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Sound Ray Propagation in Ocean

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Ray Tracing Spreadsheet

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Common Sources of Ambient Noise in Deep Water

I. Tides and hydrostatic effects of waves - Pressure fluctuations resulting from tides and waves - very low frequency - not too important at frequencies of interest in underwater sound. Tidal currents can cause flow induced noise.

II. Seismic disturbances - results from earth's constant seismic activity - low frequency < 100 Hz.

III. Oceanic turbulence - caused by turbulencea) induces motion of transducer and causes self noise.b) pressure changes associated with turbulence may be radiated.c) turbulent pressure fluctuations - most significant at low frequency.

IV. Ship traffic - dominant source at 100 Hz; principal noise source 50-500 Hz.V. Surface waves - ambient noise between 500 Hz - 25 kHz correlates well

with sea state or wind speed.Causes - breaking white caps

flow noise - wind blowing over rough sea surface.cavitation - collapse of air bubbles.

Rough sea surface is dominant noise source at 1-30 kHz.VI. Thermal Noise - results from molecular agitation in the sea. Important at

high frequencies (750 kHz).

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Average Deep Water Ambient Noise Spectra

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Intermittent Sources of Ambient Noise

Do not persist over periods of hours or days.

a) Biological sounds - whales, porpoises, dolphins shellfish 10-500 Hz

Snapping shrimp - 500 Hz - 20 kHzCommercially important fish don't make noise

b) Rain 30 dB increase 5-10 kHz heavy rain10 dB increase 19.5 kHz steady rain

c) Seismic explosions - seismic surveys

If shipping and biological noise are absent and wind is the primary contributor, then shallow and deep water noise levels are nearly the same. In general shallow water is a noisy and highly variable environment for most underwater acoustic operations.

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Sounds from the Ocean

http://www.jandaenterprises.com/sounds.htmGoogle “sounds in ocean”

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QUESTIONS