Hearing in Two Worlds: Theoretical and Actual Adaptive Changes of the Aquatic and Terrestrial Ear for Sound Reception

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Abstract

Sound in both water and air has two physical properties, the near-field particle motion generated by any moving object and the far-field pressure simultaneously generated by these objects (39). Based on the physical properties of the medium (the so-called characteristic impedance, which is about 3500 times larger for water than for air) and the steep loss of energy over distance (dipole source: 1/distance3; monopole source: 1/distance2) in the particle or direct sound in a frequency specific fashion (63), this component of sound carries enough energy to stimulate a receptor only over a very short range at low frequencies. In particular in air, this range is too short to play a role in terrestrial hearing in vertebrates and is used only by some insects (52). While sound pressure reception opens up a wider range over which sound can be received as it falls off less steeply (1/distance), it cannot be extracted easily without specializations external to the inner ear. These adaptations to sound pressure reception, the way terrestrial and some aquatic vertebrates hear, essentially have to funnel the limited energy present in the far field with minimal loss to the appropriate receptor organs in the inner ear.