Optimal acoustic beam pattern corrections for split beam transducers
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Precise in situ target strength estimates of fish can only be made when the effect of the transducer directivity is totally removed from the recorded target amplitudes. By guiding a standard calibration target through the acoustic beam while simultaneously recording the amplitudes and angular positions of the target, a precise reconstruction of product of transducer transmit and receiving directivity can be made. From several thousand point measurements taken in a cross section through the acoustic beam, the recorded data have been fitted to a generalized three dimensional model by the use of non-linear estimation. The suggested model functions yield very low residual beam correction error with 95% of the data within ± 0. 5 dB. Within the half power points of the beam, the residual beam correction error is close to the observed ping to ping system stability of ± 0.1 dB. The precision and repeatability of the method is demonstrated through data from four split beam transducers mounted on four different research vessels. These are three ES-400 transducers and one EK-500 transducer, all working on 38 kHz.