This paper investigates the applicability of calibrating a broadband acoustic system in the near field. The calibration was performed on a single transducer with a mono-static configuration using a single standard target, a 25-mm tungsten carbide sphere in the nearfield of both the transducer and the sphere. A theoretical model was developed to quantify the nearfield effect. Numerical simulations revealed that the frequency responses at different distances varied significantly, the null positions were essentially invariant-a unique characteristic for determination of the compressional and shear wave speeds in the calibration sphere. The calibration curves obtained in the near field could be applied to farfield once the nearfield effects were accounted for. Since the transducer was located in the near field, the signal-to-noise ratio was high, resulting in a much wider useable bandwidth than the nominal bandwidth. The resultant calibration uncertainty, i.e., root-mean-square uncertainty over the entire usable frequency band was 1.05 dB and reduces to 0.33 dB when the regions corresponding to nulls were excluded. The methods reported here could potentially be applied to the calibration of multibeam and broadband echosounder/sonar systems since it is difficult to meet the farfield condition for outermost beams when shipboard calibrations are needed.

When a smooth curved object is lowered into water, the initial specular contribution to high frequency backscattering of sound is weak before the specular point on the object becomes illuminated by incident sound. The associated transition in reflection from a metallic cylinder viewed at grazing incidence was previously studied [Baik and Marston, IEEE J. Ocean. Eng. 33, 386-396 (2008)]. The present research involves analogous measurements of backscattering performed using short tone bursts facilitating improved temporal resolution of distinct mechanisms contributing to the backscattering. The measurements reveal the presence of a delayed contribution to the backscattering that evolves in time in a way consistent with a scattering contribution of an acoustic Franz wave. The wave appears to be partially reflected at the free surface after having been excited on the cylinder by the incident acoustic wave. For slightly exposed cylinders viewed at grazing incidence, the Franz wave mechanism dominated the observed backscattering.

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