THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

432 15. Underwater AcousticsTable 15.3. Sonar Parameters.Parametric symbol Reference DefinitionSource level: SLTransmission loss:TLTarget strength:TSNoise level: NLReceiving: DIdirectivityindexReverberation: RLlevelDirection: DTThreshold1 yard from sourceon its acoustic axis1 yard from sourceand at target orreceiver1 yard from acousticcenter of targetAt hydrophonelocationAt hyrdrophoneterminalsAt hydrophoneterminalsAt hydrophoneterminals10 log (intensity of source/1 μPa)()signal intensity at 1 yard10 logsignal intensity at target or receiver( )echo intensity at 1 yard from target10 logincident intensity( )noise intensity10 log1 μPa⎛⎞noise power generated by an equivalent10 log ⎜isotropic hydrophone⎟⎝ noise power generated by actual ⎠hydrophone[ ](reveberation power at hydrophone terminals)10 log1 μPa⎡ ⎛⎞ ⎤signal power to just at perform⎝ ⎠a certain function10 log ⎢⎣ noise power at hydrophone terminals⎥⎦Note that some of the parameters, viz. SL, TL, TS (and the scattering strength) in Table 15.1 use 1 yardas the reference distance. To obtain their values based on the reference distance of 1 m, 0.78 dB mustbe subtracted from the values of these parameters.level curve at range r n . If the reverberation is high, the echo will be of lesservalue and the range may be considered to be reverberation-limited. But if thenoise-masking level occurs at the level represented by the dashed line in Figure15.10, the echoes will then fade away into a background of noise rather thanreverberation. The new noise-limited r n ′ will be less than the reverberation-limitedTable 15.4. Sonic Parameters: Terminology of Various Combinations.Nomenclature Parameter CommentsEcho level SL – 2TL + TS Intensity of the echo measured in thewater at the hydrophoneNoise-masking NL − DI + DTDetermines minimumlevelReverberationmaskingRL + DTDetectable echo levellevelEcho excess SL − 2TL + TS − (NL − DI + DT) Detection just occurs, when echo excessjust equals zeroPerformancefigureSL − (NL − DI)Difference between the SL and the NLmeasured at the hydrophoneFigure of merit SL − (NL − DI + DT) Equals the maximum allowable two-wayTL for TS = 0 for active sonar, orone-way TL in passive sonars