Pressure Pulse Distortion by Needle and Fiber-Optic Hydrophones due to Nonuniform Sensitivity

Abstract

Needle and fiber optic hydrophones have frequency-dependent sensitivity, which can result in substantial distortion of nonlinear or broadband pressure pulses. A rigid cylinder model for needle and fiber optic hydrophones was used to predict this distortion. The model was compared with measurements of complex sensitivity for a fiber optic hydrophone and 3 needle hydrophones with sensitive element sizes (d) of 100, 200, 400, and 600 μm. Theoretical and experimental sensitivities agreed to within 12 ± 3% (RMS normalized magnitude ratio) and 8 ± 3 degrees (RMS phase difference) for the four hydrophones over the range from 1 – 10 MHz. The model predicts that distortions in peak positive pressure can exceed 20% when d/λ0 < 0.5 and SI > 7% and can exceed 40% when d/λ0 < 0.5 and SI > 14%, where λ0 is the wavelength of the fundamental component and SI (spectral index) is the fraction of power spectral density contained in harmonics. The model predicts that distortions in peak negative pressure can exceed 15% when d/λ0 < 1. Measurements of pulse distortion using a 2.25 MHz source and needle hydrophones with d = 200, 400 and 600 μm agreed with the model to within a few percent on the average for SI values up to 14%. This work 1) identifies conditions for which needle and fiber optic hydrophones produce substantial distortions in acoustic pressure pulse measurements and 2) offers a practical deconvolution method to suppress these distortions.

Directivity and Frequency-Dependent Effective Sensitive Element Size of Needle Hydrophones: Predictions From Four Theoretical Forms Compared With Measurements

“…did not provide experimental validation for their model, Wear et al . recently provided experimental validation that the RP model can accurately predict sensitivity for some needle and fiber-optic hydrophones [31]. …”

“…Although directivity of needle hydrophones has been investigated previously [25, 33–39], it makes sense to revisit this issue because 1) the RP model has not been tested experimentally for needle hydrophone directivity (to the authors’ knowledge) but is a promising approach given its recent success in modeling needle hydrophone sensitivity[31], 2) some previous investigations were conducted decades ago[25, 33–35], raising concerns about their relevance to modern hydrophone technology, 3) all the investigations cited above except one [25] considered only the common RB model [33, 34, 36–38] or no theoretical model[35, 39], 4) one investigation considered only a single frequency [39], 5) some investigations considered only a single sensitive element diameter [34, 36, 38, 39] or a limited range of 300 – 600 μm [35], and 6) some investigations involved a limited range of angles up to a maximum of 40 degrees or less [33, 38]. …”

Directivity and Frequency-Dependent Effective Sensitive Element Size of a Reflectance-Based Fiber-Optic Hydrophone: Predictions From Theoretical Models Compared With Measurements

“…While sensitivity of reflectance-based fiber-optic hydrophones has been considered in previous publications[1, 11, 28, 29], directivity has received less attention. Directivity corresponds to voltage output for quasi-planar pressure waves as a function of angle of incidence.…”

Correction for Spatial Averaging Artifacts in Hydrophone Measurements of High-Intensity Therapeutic Ultrasound: An Inverse Filter Approach

“…Sensitivities M L1 (f) and M L2 (f) in (9) were provided by the manufacturer. Sensitivities for needle [75,76] and fiber optic [76] hydrophones have been shown to be accurately predicted by the rigid piston model [73,74]. The sensitivity deconvolution bandwidth was limited by the maximum frequency for which the HNA-0400 sensitivity was calibrated, which was 60 MHz.…”

“…A formula for the frequency-dependent effective sensitive element size [50], which is required for (5), can be derived from a rigid piston model [73,74] that has been previously validated for sensitivity [75,76] and directivity [67] of needle hydrophones and sensitivity [76] and directivity [68] of fiber optic hydrophones.…”