Comparative Analysis of Directivity of a Single Vector Hydrophone in Near-Field and Far-Field

In underwater acoustic detection, vector hydrophones, which can simultaneously acquire multi-dimensional sound pressure and particle velocity information, are a core direction. Existing research mainly focuses on their combined directivity under far-field conditions, where they show "8"-shaped dipole characteristics independent of frequency, suitable for unambiguous target positioning. However, in near-field scenarios like microphone arrays or small-target detection, the plane wave model fails. Phase differences from sound ray bending and near-field acoustic impedance alter the directivity of sound pressure-velocity combinations, making them distinct from far-field characteristics. This study finds: ① The normalized directivity of a single particle velocity or its deformed form remains consistent in far/near fields, unaffected by distance; ② Sound pressure-velocity combinations have different directivities in far/near fields due to phase differences from near-field acoustic impedance; ③ Positioning with vector hydrophones (arrays) requires compensating for both propagation time differences from the source to array elements and phase differences between sound pressure-velocity channels. This research provides theoretical support for near-field acoustic detection and positioning.