On the steering of sound energy through a supercritical plate by a near-field transducer array

Abstract

The ability to direct sound energy through the flexural vibrations of a submerged plate at various angles of incidence using a near-field transducer array is investigated. An alumina bar is placed in front of a one-dimensional, eight-element transducer array, between the array and the water. Operating in a receive mode, data were taken as a function of angle of incidence and compared to data taken without the presence of the alumina bar. The array was also operated in transmit mode and results were compared to corresponding receive mode data, showing that reciprocity holds. Results show that in fact sound energy can be steered through a plate, and that the measurement method used provides a convenient method of measuring the angular dependence of transmission through a plate, including measurements at frequencies above the plate's critical frequency. Experimental results of sound transmission versus angle of incidence of finite sized plates agree qualitatively with theoretical results from an analysis of the transmission through an unbounded flexible partition.

Grating lobe reduction in transducer arrays through structural filtering of supercritical plates

“…Another convenient way to display the directivity pattern data is to plot the directivity patterns for various steer angles as slices in a three-dimensional surface plot. 34,35 This display format allows one to analyze the effect of the inser- tion of the bar/plate across many steer angles at the same time. Figure 7 displays directivity pattern surface plots for the alumina bar array, at frequencies of 45 and 50 kHz.…”

“…33 A previous paper written by the authors showed that an array may steer sound energy through a supercritical plate without refraction and without widening the main lobe of energy, meaning that the plate does not alter the intended electronic phasing required to steer an array. 34 Sound energy passes through the plate at normal incidence and radiation angles less than the coincidence angle are attenuated somewhat due to the mass of the plate. However, sound energy at large radiation angles, greater than the coincidence angle, will be attenuated due to the stiffness of the plate.…”

On the measurement of airborne, angular-dependent sound transmission through supercritical bars

“…Anderson et al 7 found that it was possible to steer a beam from an array of transducers through a supercritical bar in the near field. They suggested that the array standoff distance be small.…”

“…Anderson et al 7 used the pulse approach for finite length bars submerged in water. (A bar, in this case, has only one dimension much longer than a wavelength.)…”

Ultrasonic airborne insertion loss measurements at normal incidence (L)

“…The purpose of this letter is to present preliminary measured IL levels at ultrasonic frequencies that will serve as a benchmark for other techniques that the authors are currently exploring to more fully investigate the angular dependence of plates excited above their critical frequencies, specifically the coincidence angle and the departure of experiments 1,4 from theory 1 at grazing incidence.…”