Recommendations for the collection of multifrequency acoustic data
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Traditionally, collection of acoustic data has been optimised for a single frequency, while requirements for the optimal combination of multifrequency data has been given less attention. Even though data manipulation can enable single-frequency data to be combined to some extent, optimal multifrequency comparisons cannot be made from a system if the input data are not collected properly. Multiple single-frequency [s(f)] data may be collected in various ways onboard research and fishing vessels. For detailed analysis, the physical and spatial characteristics of acoustic data should be as similar as possible. While direct comparability is impossible in all aspects, ideal data is defined as a reference point for the collection and analysis of multiple s(f) data. Acoustic data from several single frequencies are defined as ideal in this context if they can be used to generate combined frequency [c(f)] data at the same resolution as the original. This requires comparable physical measurements, carried out simultaneously from identical volumes, limited only by the effective range of the higher frequencies. Requirements necessary for recording ideal multi-frequency acoustic data are presented. Consequences of combining acoustic data originating from beams of different widths and separation distances are also analysed and illustrated.