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Abstract: Helicopter Long Range Active Sonar (HELRAS), used by lowering a transducer and receiver array into water from helicopters, produces ~1.3 to 1.4 kHz signals within the hearing range of many marine animals, including seals. The distance at which the HELRAS signals can be heard by seals is unknown but partly depends on the hearing thresholds of seals for the signals. The hearing thresholds of two adult harbor seals (Phoca vitulina) for HELRAS signals were quantified by means of a psychoacoustic technique. Hearing thresholds were obtained for five 1.25-s simulated HELRAS signals varying in harmonic content and amplitude envelopes. Hearing thresholds (50% detection rates) were similar: mean 51 dB re 1 μPa, root-mean-square (rms; broadband sound pressure level, averaged over the signal duration). Harmonic distortion in three of the five signals had no influence on audibility, as the harmonics were ≥ 20 dB below the hearing thresholds for the fundamental frequencies of the signals. The results of this study, combined with information on the source level of the signals, the propagation conditions, and ambient noise levels, allow the calculation of distances at which harbor seals can detect HELRAS signals. Under similar conditions, harbor seals are able to detect HELRAS signals at greater distances than harbor porpoises (Phocoena phocoena).
Key Words: acoustics, anthropogenic noise, detection, dipping sonar, hearing, helicopter, navy, phocids
Page Numbers: 349-355