Gauging the Reliability of Acoustic Instruments for Fisheries Surveys

When acoustical methods for quantitative measurement of fish stock size were developed around 1970, the need arose for reliable methods to calibrate the instruments. Both standard targets, such as steel spheres, and wideband hydrophones were utilized, but with unsatisfactory results. The advantage of using hydrophones is that transmitted Source Level and Receiving Sensitivity can be measured separately. Gradually, it was realized that hydrophones, which are secondary standards, were not suitable because their performance fluctuated more than that of the instruments being calibrated. Steel spheres were more stable, but their acoustical properties could vary considerably, depending on the composition of the alloy and the hydrography of the medium, e.g., temperature and salinity. Not until optimised reference targets were introduced around 1980 was it possible to monitor the stability of survey equipment with an acceptable degree of confidence. For the first time, one could prove the real long-term stability of transducers and demonstrate ageing effects. While acoustic technology has developed, the electronics has become more stable, and transducer manufacturers have improved their production methods. In the recent years, time series have been obtained showing that the performance of scientific echo sounders varies within a few hundredths of a decibel. After multi frequency technology for identifying species was introduced, calibration over a wide frequency spectrum became very important. In an example, time series of the performance of scientific echo sounders with frequencies from 18 to 200 kHz are shown. A new multi-beam sonar with 500 beams and a frequency range 70-112 kHz presents new challenges in calibration. An example of how problems with the new sonar are being solved relatively simply is given