An arrangement for finding the direction of a sound in a medium has a thin shell structure having a circular symmetry. The structure can flex in response to incident sound energy from the medium to perturb the form of the structure. A membrane of a piezo-electric material with an anisotropic piezo-electric response to distortion is arranged to be distorted by the perturbation of the thin shell structure. The response of the membrane to this perturbation permits the direction of the incident sound to be indicated. The membrane can be of polyvinylidene difluoride with anisotropic properties. The membrane may be formed into a "dome" for example by fluid pressure inside a hemispherical shell closed by the membrane.