Naval sonar, marine mammal strandings, and the bubble hypothesis

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.