Wave-number space response of a near periodically ribbed shell

Employing near-field acoustic holography measurements, the wave-number space response of a ribbed shell to a point drive is investigated. The data show the dominance of flexural vibration in the mid-frequency range, and demonstrate the prominent role played by the eigensolutions of the periodic structure. The supersonic region of frequency–wave-number space is examined in detail and the fast membrane waves, although suppressed in level relative to the subsonic bending waves, provide a clear highlight in this domain at frequencies below the first radiating flexural passband. The results are also examined in kz−kφ space for several frequencies. This representation enables an interpretation of the results in terms of the slowness surface of the flexural waves and provides a simple explanation for the various complex beaming and focusing phenemena which occur in this system.