Considerations for Choosing Sensitive Element Size for Needle and Fiber-Optic Hydrophones II: Experimental Validation of Spatial Averaging Model.

Acoustic pressure can be measured with a hydrophone. Hydrophone measurements can underestimate incident acoustic pressure due to spatial averaging effects across the hydrophone sensitive element. The spatial averaging filter for a nonlinear focused beam is a low-pass filter that decreases monotonically from 1 to 0 as frequency increases from 0 to infinity. Experiments were performed to test an analytic model for the spatial averaging filter. Nonlinear pressure tone bursts were generated by three source transducers with driving frequencies ranging from 2.5 MHz to 6 MHz, diameters ranging from 19 mm to 64 mm, and focal lengths ranging from 38 mm to 89 mm. The nonlinear pressure fields were measured using four needle hydrophones with nominal geometrical sensitive element diameters of 200, 400, 600, and 1000 μm. The average root-mean-square difference (RMSD) between theoretical and experimental spatial averaging filters was 5.8 ± 2.6%.