TAMU OCEN 201 - underwater acoustics 2007

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Underwater AcousticsOCEN 2012TYPES OF UNDERWATER ACOUSTIC SYSTEMSz Active Sonar SystemsActive 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.z Seismic SystemsSubbottom 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.3SUBMARINE SONAR4TYPES 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.5TYPES OF UNDERWATER ACOUSTIC SYSTEMS{ 4. Passive Systems{ a. 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.6ACOUSTIC TRANSDUCERS7ACOUSTIC DOPPLER CURRENT METER8SIDESCAN SONAR9Decibel ScalesSound 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 I2is defined by:Intensity Level 21IIlog10NIL)( =Sound Pressure Level()21p/plog20NSPL=10FUNDAMENTALS 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 -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{ ρcair= 42 g/cm2s{{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 elastic medium11FUNDAMENTALS 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 aveis the time average of the instantaneous acoustic pressure squared.{ Units: p = dynes/cm2{ ρ = gm/cm3{ c = cm/s{ I = ergs/cm2s{ Since Intensity is also power/unit area and the units are often watts/cm2. One watt is equal to 107ergs/s then{ I = power/area = p2 ave/ ρ c x 10-7watts/cm2{12Decibel Scales (continued)In general, if we have a quantity x such that()a2121x/xI/I =then the ratio of the values on the decibel (dB) scale is()112210log 10 log /IaxxdBI⎛⎞=⎜⎟⎝⎠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/m2The old reference levels are: 1) 1 dyne/cm22) 0.0002 dyne/cm2The current reference level is: 1 micropascal (1 μPa). Note: 1 μPa = 10-5dyne/cm213Decibel 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,()()[]21312p3pp/plogp/plog20NN−=−[]21312p3pplogplogplogplog20NN+−−=−[]322p3pplogplog20NN−=−()322p3pp/plog20NN +=Example: Express 125 dB relativeto 0.0002 dyne/cm2in dB relativeto 1 dyne/cm2. Let22dyne/cm 0002.0p =23cm/dyne1p =()1/0002.0log201253pN+=dB5174125N3p=−=14Decibel 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 I1and a reference intensity of I2, the level of the sound wave is equal to:21I/Ilog10dBN=For clarity the level should be written:{Pa1redBN toequal pressure of wave plane a ofintensity theμIf 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 μPa15Sonar Equations{ Active{ SL-2TL+TS=NL-DI+DT{ Active (Reverberation){ SL-2TL+TS=RL+DT{ Passive{ SL-TL=NL-DI+DT16Active Sonar EquationHeadphonesElectronicsDetection Threshold (DT)Directivity Index (DI)orArray Gain (AG)Source Level (SL)Noise Level (NL)One-way TransmissionLoss (TL)Target Strength (TS)ReceiveElectronics17Example: A passive sonar system is being used to detect an object that has a source level of 80 dB re 0.0002 dynes/cm2and 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:232p3ppplog20NN −=2cm/dyne0002.0Pa1cm/dynes0002.0Pa1log20NN2μ−=μ0.000210log2080N5Pa1−−=μ()3.12080NPa1−−=μPa1redB1062680NPa1μμ=+=Pa1redB106SLμ=Passive Sonar EquationPaμ1redB53NL3NL561512NL50106DTDINLTLSl=+=+−=−+−=−18Beam Patterns{ Line Array{ Circular Plane Array19Line Array with Equally Spaced


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