An experimental study of the directivity gain of underwater transducers with the attachment of horns was made, and factors influencing such directionality were established for exponential, tractrix, and conical horns. The influence of steel and aluminum for construction materials and the effects of polyurethane foam or air backings were determined. The directivity index was found to show sudden drops at discrete frequencies, and it was shown that this behavior was related to transverse modes of propagation inside the horn by the mouth of the horn for acoustic pressure and phase variations. The effect on directivity of various cell arrangements in a conical horn was studied. [This work is supported by the Office of Naval Research.]