Monitoring of equivalent beam angles of hull-mounted acoustic survey transducers in the period 1983-1995

By 1983 a method for measuring the equivalent beam angles of hull-mounted transducers had been developed at the Marine Research Institute in Iceland. The technique has since been used to measure and monitor the equivalent beam angles of old and new transducers used for acoustic surveying of fish by the MRI. The transducers are all manufactured by SIMRAD, Norway. Some were installed more than 25 yrs ago, others as recently as 1993. The transducers cover a frequency range from 12 kHz up to 200 kHz and the equivalent beam angles range fi-om - 14 to - 25 dB. Significant differences have been observed between the equivalent beam angles as measured after hull mounting and the values supplied by the manufacturer. Significant differences in the performance of transducers of the same type have been observed as well. This is not always the case. An important exception is the current line of transducers made by SIMRAD where the difference is generally less than 5%. The results indicate that the transducers are inherently stable with time. (C) 1998 International Council for the Exploration of the Sea.