Recent mass strandings of beaked whales (Ziphiidae, Cetacea) coinciding with the use of midfrequency range (1–10 kHz) active sonar have caused speculation about the potentially adverse effects of these sound sources. Particular questions of the research and regulatory communities concern whether beaked whale sensitivity to midfrequency sound exposure is influenced by oceanographic characteristics present at the time of the mass stranding events. This study investigated the interaction between beaked whale habitat characteristics and the nature of a midfrequency signal by analyzing the oceanographic factors affecting underwater acoustic propagation. Three types of model sites were selected from five specific geographical locations where beaked whales have been regularly recorded or where a mass stranding event has been reported. A ray-trace acoustic propagation model was used to generate transmission loss for a 3-kHz signal over a representative 60-km transect at each locality. Model outputs visually demonstrated how the combination of site/event-specific oceanographic characteristics affects the sound propagation of a moving source. A parametric sensitivity comparison and statistical analysis were conducted to identify influential factors between environmental parameters, source depth, and the resulting transmission loss. Major findings of this study as well as future research direction are discussed. [Research supported by NAVSEA.]

To study the effects of acoustic masking from anthropogenic noise, bottlenose dolphin (Tursiops truncatus) echolocation performance was assessed in the presence of different masking noise types using Navy relevant source transmissions. Echolocation clicks produced by the dolphin were detected with a hydrophone, then digitized within a phantom echo generator (PEG). The PEG converted the received clicks into echoes, delayed appropriately for the simulated target range. The echoes were then broadcast to the dolphin via a sound projector while masking noise transmissions were held constant. Using an acoustic response and a modified method of constants procedure, the echolocation performance of the dolphin was computed as a function of range between 3 and 17 m. Comparative echolocation performance to different masking noise type categories was analyzed between intermittent and continuous noise, direct path transmissions and multipath exposure, and mid frequency versus high frequency bands. These results expand the limited understanding of biosonar processing capability and signal characteristic alterations used to discriminate and resolve changes in small scale features while exposed to potential noise interference types. [Work supported through the Office of Naval Research.]

Although spatio–temporal links exist between some military active sonar testing and cetacean mass stranding events, the underlying causal mechanism(s) of the strandings are not well understood. It has been demonstrated that cetaceans may experience in vivo gas bubble/emboli development; however, the basis for cavitation formation remains unknown. Acoustically mediated in vivo cavitation formation in cetaceans exposed to high-intensity sound sources has been proposed. The distance of these exposure effects from midfrequency sound sources can be influenced by a combination of factors affecting underwater acoustic propagation. To investigate these issues, this experiment considered rectified diffusion as a possible mechanism for acoustically mediated in vivo cavitation formation in cetaceans. Gelatin samples were placed under water and exposed to an SPL of 205 dB re: 1 μ Pa at 4.5 kHz for up to 1000 s. The sound field was generated by a Massa transducer similar to the transducer element used in the United States Navy AN/SQS–23 shipboard high–power sonar system. Exposure time threshold of cavitation formation was determined to be 600 s for the given sound field. Experimental results were compared with previously developed theoretical models for acoustically enhanced cavitation growth. The applicability and limitations of this approach as well as future research direction are discussed.

Scientific literature states that anthropogenic sound, such as mid-frequency sonar, may cause a behavioral response in marine mammals. The degree of response is highly variable and dependent upon many factors, including how sound transmission is influenced by environmental features. The physical parameters of the ocean medium, such as sound speed profile and bathymetry, are important controls of underwater acoustic propagation. Determining the acoustic propagation loss of the ocean environment is an application used to identify and correlate influential environmental factors. This study investigates the sensitivity of acoustic propagation loss based on specific physical characteristics found in five different sites representing beaked whale environments. These sites were chosen with regards to existing data on beaked whale distribution, historical mass stranding records, and presence of mid-frequency sonar activity. A range-independent, ray-tracing acoustic propagation model was used to generate a two-dimensional sound field over a range of 30 km. From the results of this experiment, the acoustic importance of bathymetry and sound speed profile of the five beaked whale environments were identified. Preliminary results from the experimental study will be presented.

Research and regulatory communities have raised questions about the influence of oceanographic conditions on propagation of midfrequency sound produced by naval sonars prior to beaked whale mass stranding events. Resolving these questions requires detailed study of acoustic properties at locations known to support beaked whale populations potentially at risk of exposure. To address these concerns, this study investigated sound propagation at two known beaked whale habitats: the North Atlantic Frontier to the west of Scotland and Sagami Gulf in eastern Japan, by computing the range dependent propagation loss for an omnidirectional 3.5 kHz signal with a source level of 235 dB re 1 μPa at 1 m. From this, a method was developed to reduce the three-dimensional sound field to yield a source-receiver range at which there was a high probability that a given received sound pressure level would not be exceeded. To investigate the uncertainty in data for sound speed profiles and seabed acoustic properties, the sensitivity of this method to typical variations of these factors was evaluated. Understanding the physical oceanographic properties affecting sound propagation can provide critical science for development of conservation management and mitigation practices.