Development of a perforated plate underwater acoustic ground cloak.

One of the commonly investigated transformation acoustic device is the ground cloak, which conceals a scattering object on a reflecting surface. Multiple studies have numerically simulated acoustic ground cloaks, but because of the challenges associated with realizing a homogeneous anisotropic metamaterial, only two acoustic ground cloaks have been built and tested. Perforated plastic plates in air were used to construct two and three dimensional ground cloaks and alternating layers of brass and water were used to construct an extended area ground cloak underwater. With underwater mass density anisotropy previously demonstrated for perforated steel plates, the primary focus of this article is to build and evaluate an underwater ground cloak with perforated steel plates. The cloak was evaluated at a water-air pressure release reflecting surface. The cloak successfully concealed the scattering object over a broad frequency range of 7-12 kHz.

Divergence of a stereotyped call in northern resident killer whales.

Northern resident killer whale pods (Orcinus orca) have distinctive stereotyped pulsed call repertoires that can be used to distinguish groups acoustically. Repertoires are generally stable, with the same call types comprising the repertoire of a given pod over a period of years to decades. Previous studies have shown that some discrete pulsed calls can be subdivided into variants or subtypes. This study suggests that new stereotyped calls may result from the gradual modification of existing call types through subtypes. Vocalizations of individuals and small groups of killer whales were collected using a bottom-mounted hydrophone array in Johnstone Strait, British Columbia in 2006 and 2007. Discriminant analysis of slope variations of a predominant call type, N4, revealed the presence of four distinct call subtypes. Similar to previous studies, there was a divergence of the N4 call between members of different matrilines of the same pod. However, this study reveals that individual killer whales produced multiple subtypes of the N4 call, indicating that divergence in the N4 call is not the result of individual differences, but rather may indicate the gradual evolution of a new stereotyped call.