Efficient design of nearfield focused beamformer with frequency invariant patterns for vertical line array

This paper proposes an efficient design method of nearfield focused beamformer with frequency invariant patterns for underwater radiated noise measurement, where the impinging of the wideband radiated source on the acoustic array is restricted by the angular and range conditions of the Fresnel approximation. With the Fresnel approximation, the received signal can be first amplitude-compensated and phase-shifted according to the signal attenuation and the wave curvature before the beamforming operation, and then the problem of nearfield wideband beamforming can be readily transformed to a corresponding farfield issue, and the well-established farfield design method of frequency invariant beamformers (FIBs) can be used. Representative example is presented to demonstrate the reduced design complexity and the encouraging beamforming performance under nearfield condition.