Underwater Sound Localization: ICE and Helmholtz resonators

Internally coupled ears (ICE), where an interaural cavity of some shape acoustically couples the eardrums, is an anatomical trait present in more than half of the terrestrial vertebrates. The superposition of outside and internal pressure on the two eardrums results in internal time and level differences, which are keys to sound localization. Although ICE is primarily a low-frequency terrestrial adaptation, the African clawed frog Xenopus laevis is a fully aquatic species with a distinct air-filled connection between the ears. Unlike terrestrial animals with ICE, the Xenopus interaural cavity is also medially connected to the lungs. By modeling the inflated lungs as a Helmholtz resonator, we demonstrate their effect on improving hearing in a low-frequency regime, while simultaneously enhancing sound localization in a disjoint high-frequency regime, corresponding to the frequency range of male advertisement calls. In conjunction with its unique plate-like eardrums, we show how Xenopus uses its ICE-like interaural coupling to generate considerable internal level differences between eardrum vibrations and thus overcomes the challenges of underwater sound-localization.