We consider the peculiarities of echolocation discrimination of targets by dolphins in the presence of interfering reflection from steel cylinders, at angles less than the width of the transmission beam pattern. New data are presented that when the animal performs an echolocation task his spatial hearing is provided by the monaural mode of reception, which represents the best (practically ideal) noise-immunity from interfering refection, compared with the possibilities of binaural hearing. Using monaural reception, during performance of an echolocation task, the dolphin finds the optimum position of the head (transmitter and receiver) in relation to targets and interfering cylinders? providing the best condition of mutual suppression interfering echo (destructive interference). Modelling the destructive interference has shown that 1) the resultant energy of interfering reflection decreases four fold; 2) at the frequencies of maximum hearing sensitivity in dolphins there is a complete suppression component. The most probable mechanism of monaural perception of echo seems to be provided by passing through the lower jaw "acoustic window'' (Norris 1964, 1968, 1969, Brill 1988, 1991). In this case, the base of reception apparently becomes much less than the distance between the external hearing meatus, which essentially reduces the interaural difference of intensity and phase in the inner ear. It has- also been shown that dolphins use the slopes of the beam patterns in both radiation and reception to increase the difference between intensities of echo from the targets and interfering cylinders.
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