Radiated noise measurements from two unusual sources

As part of ongoing programs involving the measurement and assessment of ambient noise, data typical of two diverse but important environments have been acquired: (1) wide-band ambient noise in the neighborhood of deep-moored open-ocean buoys (typical of the NOAA 12-m discus buoys), and (2) wide-band noise generated by cycloidal propulsion systems (Voith-Schneider propellers). The extended use of the data buoys as platforms of opportunity and their presence in the ocean as noise sources recommend the acquisition and publication of their general acoustical characteristics. In connection with an acoustic system currently being developed, it became necessary to evaluate the cycloidal propeller as a noise source. No information regarding noise spectra and levels produced by such propulsion systems was found in the literature. Noise data were recently obtained on cycloidally-propelled ferries, the GALENA and NEEDLES, operating on the Arrow Lakes of British Columbia. Analysis of the data from these sources is presented. The noise character of both sources is quite different from the usual ship and machinery noise observed in the oceans. Since our observations were made over a relatively small range of environmental situations and platforms, further sampling is indicated.