Composite Training Unit Exercises and Joint Task ... - Govsupport.us

COMPTUEX/JTFEX EA/OEA Final Chapter 4An estimate of 1.6 dB TTS2 per dB increase in exposure EL is the upper range of values from Ward et al.(1958, 1959) and gives the most conservative estimate – it predicts a larger amount of TTS from the sameexposure compared to the lines with smaller slopes. The difference between onset-TTS (6 dB) and theupper limit of TTS before PTS (40 dB) is 34 dB. To move from onset-TTS to onset-PTS, therefore,requires an increase in EL of 34 dB divided by 1.6 dB/dB, or approximately 21 dB. An estimate of 20 dBbetween exposures sufficient to cause onset-TTS and those capable of causing onset-PTS is a reasonableapproximation.To summarize:In the absence of marine mammal PTS data, onset-PTS exposure levels may be estimatedfrom marine mammal TTS data and PTS/TTS relationships observed in terrestrial mammals.This involves:ooEstimating the largest amount of TTS that may be induced without PTS. Exposurescausing a TS greater than this value are assumed to cause PTS.Estimating the growth rate of TTS – how much additional TTS is produced by anincrease in exposure level.A variety of terrestrial mammal data sources point toward 40 dB as a reasonable estimate ofthe largest amount of TS that may be induced without PTS. A conservative assumption isthat continuous-type exposures producing TSs of 40 dB or more always result in someamount of PTS.Data from Ward et al. (1958, 1959) reveal a linear relationship between TTS2 and exposureEL. A value of 1.6 dB TTS2 per dB increase in EL is a conservative estimate of how muchadditional TTS is produced by an increase in exposure level for continuous-type soundsThere is a 34 dB TS difference between onset-TTS (6 dB) and onset-PTS (40 dB). Theadditional exposure above onset-TTS that is required to reach PTS is therefore 34 dB dividedby 1.6 dB/dB, or approximately 21 dB.Exposures with ELs 20 dB above those producing TTS may be assumed to produce a PTS.This number is used as a conservative simplification of the 21 dB number derived above.Derivation of Effect ThresholdsThe TTS threshold is for cetacean is based on TTS data from Schlundt et al. (2000). Since these testsused short-duration tones similar to sonar pings, they are the most directly relevant data. The meanexposure EL required to produce onset-TTS in these tests was 195 dB re 1 μPa 2 -s. This result iscorroborated by the short-duration tone data of Finneran et al. (2000, 2003a) and the long-duration noisedata from Nachtigall et al. (2003 a, b). Together, these data demonstrate that TTS in cetaceans iscorrelated with the received EL and that onset-TTS exposures are fit well by an equal-energy line passingthrough 195 dB re 1 μPa 2 -s.The TTS threshold for pinnipeds is based on TTS day from Kastak et al. (1999; 2005). Although theirdata is from continuous noise rather than short duration tones, pinniped TTS can be extrapolated usingequal energy curves. Continuous sound at a lower intensity level can produce TTS similar to shortduration but higher intensity sounds such as sonar pings.The PTS threshold is based on a 20 dB increase in exposure EL over that required for onset-TTS. The 20dB value is based on estimates from terrestrial mammal data of PTS occurring at 40 dB or more of TS,4-29 February 2007