Benjamin A. Cray

A scanning laser Doppler vibrometer acoustic array

Experiments confirm that a laser Doppler vibrometer can be used to detect acoustic particle velocity on a fluid-loaded acoustically compliant, optically reflective surface. In these experiments, which were completed at the Acoustic Test Facility of the Naval Undersea Warfare Center, Scotchgard™ reflective tape was affixed to the interior surface of a standard acoustic window. The polyurethane array window had a thickness of 0.9525cm (0.375in.) and a material density of 1000kg∕m3. The surface velocity measured, using a commercial scanning laser vibrometer system (SLVS), was beamformed conventionally and flawlessly detected and localized acoustic signals. However, the laser Doppler vibrometer used in the experiments had relatively poor acoustic sensitivity, presumably due to high electronic noise in the photodetector, speckle noise, standoff distance, and drifting laser focus. An improved laser Doppler vibrometer, the simplified Michelson interferometer laser vibrometer sensor (SMIV), is described in brief....

Acoustic and vibration performance evaluations of a velocity sensing hull array

Acoustic and vibration measurements were conducted at the Naval Undersea Warfare Center’s Seneca Lake Facility to investigate the in situ signal response of a linear array of velocity sensors (sensors that measure either acoustic particle acceleration, velocity, or displacement have generically been denoted as velocity sensors) on a coating. The coating used at Seneca Lake consisted of air‐voided elastomeric tiles with an overall coating thickness of approximately 3 inches. The accelerometer array and coating were mounted on the Seneca Lake Hull Fixture, which measures 33 feet lengthwise with an arc length of 20 feet. The fixture weighs approximately 30 tons. Specifically, measurements of in situ sensitivity, velocity reduction, reflection gain, array beam response, and equivalent planewave self‐noise levels are presented.

Experimental verification of acoustic trace wavelength enhancement

Directivity is essentially a measure of a sonar arrays beamwidth that can be obtained in a spherically isotropic ambient noise field; narrow array mainbeam widths are more directive than broader mainbeam widths. For common sonar systems, the directivity factor (or directivity index) is directly proportional to the ratio of an incident acoustic trace wavelength to the sonar arrays physical length (which is always constrained). Increasing this ratio, by creating additional trace wavelengths for a fixed array length, will increase array directivity. Embedding periodic structures within an array generates Bragg scattering of the incident acoustic plane wave along the arrays surface. The Bragg scattered propagating waves are shifted in a precise manner and create shorter wavelength replicas of the original acoustic trace wavelength. These replicated trace wavelengths (which contain identical signal arrival information) increase an arrays wavelength to length ratio and thus directivity. Therefore, a smaller array, in theory, can have the equivalent directivity of a much larger array. Measurements completed in January 2015 at the Naval Undersea Warfare Centers Acoustic Test Facility, in Newport, RI, verified, near perfectly, these replicated, shorter, trace wavelengths.

Response of infinite length bars and beams with periodically varying area

This talk develops a solution method for the longitudinal motion of a rod or the flexural motion of a beam of infinite length whose area varies periodically. The conventional rod or beam equation of motion is used with the area and moment of inertia expressed using analytical functions of the longitudinal (horizontal) spatial variable. The displacement field is written as a series expansion using a periodic form for the horizontal wavenumber. The area and moment of inertia expressions are each expanded into a Fourier series. These are inserted into the differential equations of motion and the resulting algebraic equations are orthogonalized to produce a matrix equation whose solution provides the unknown wave propagation coefficients, thus yielding the displacement of the system. An example problem of both a rod and beam are analyzed for three different geometrical shapes. The solutions to both problems are compared to results from finite element analysis for validation. Dispersion curves of the systems a...

Enhanced directivity with array gratings

A planewave, incident on a panel, produces an acoustic trace wavelength that propagates along the surface of the panel. The trace wavelength excites the panel into vibration, creating structural waves within the panel that propagate. These structural waves can be purposely Bragg scattered, creating replicas of the trace wavenumber. The replicas are shifted in wavenumber precisely by the inverse of the periodic separation distance l. Hence, in principle, it should be possible to resolve the acoustic trace wavelength from one of the shifted replicas of the panels response. The incident angle can then be ascertained from the replicated trace wavelengths.

Achievable performance of a novel Bragg-scattered acoustic receiving array

The merits of a novel beamforming technique, based on generating Bragg-scattered acoustic wavelengths along the surface of an array, will be described. The technique, denoted acoustic trace wavelength enhancement, relies on embedding periodic structures within an array, chosen to precisely replicate and shift an incident acoustic wavenumber into higher wavenumber regions. Thus, shorter trace wavelengths are created over the aperture surface. The enhancement technique is documented in two recent publications: enhanced directivity with array grating [J. Acoust. Soc. Am. 136, 2014] and experimental verification of acoustic trace wavelength enhancement [J. Acoust. Soc. Am. 138, 2015). These references dealt, however, solely with high signal-to-noise ratios. Specifically, we will investigate the noise characteristics of this new array by calculating its array gain and Cramer-Rao lower bounds on bearing estimation error for plane wave signals embedded in an isotropic Gaussian noise field. Of particular interest...

The response of an acoustic trace wavelength enhanced array to an isotropic noise field and optimum processing

Recently, a new array based on generating Bragg-scattered acoustic wavelengths along the surface of an array was proposed (Enhanced directivity with array grating (J. Acoust. Soc. Am. 136 (2), 2014) and Experimental verification of acoustic trace wavelength enhancement (J. Acoust. Soc. Am. 138 (6), 2015)). The technique, denoted acoustic trace wavelength enhancement, relies on embedding periodic structures within an array, chosen to precisely replicate and shift the incident acoustic wavenumber into higher wavenumber regions. In principle, an important advantage of this array over conventional hydrophone-based arrays of the same size is its greatly improved directivity. A prototype array was built and demonstrated in a test tank under idealized high signal-to-noise ratio conditions. However, two important open questions which remain are how this array responds to ambient noise and how to optimally beamform it when multiple signals are present. Using idealized analytical models for the arrays plane wave r...

Vibrational response of a simulated 1‐3 composite array.

The objective of this work was to identify structural waves (and their corresponding parameters) that propagate in plates that consist of periodic elements embedded in filler materials. To this end, wavevector decompositions were made of highly‐sampled (in time and space) measurement data, using two different plates, one of which was isotropic and the other fabricated so that its mechanical structure emulates a typical 1‐3 composite array plate. Knowledge of the various wave types that propagate and attenuate will reveal the lateral deformation of the elements (sensors) along with the normal stresses and shear stresses that are applied by the filler material onto the sensors. These stresses produce secondary voltages in the sensors which are detrimental to the primary voltage output and thus degrade array performance. A secondary objective therefore will be to determine values of the parameters (e.g., distance between sensors and filler material) that minimize these detrimental effects in the frequency ba...

An autonomous moored ocean profiler

An unconventional set of environmental and acoustic data, measured from an autonomous moored ocean profiler, is presented. The profiler is a programmable robotic underwater winch and data acquisition system that cycles vertically through the water column. The system profiles at pre‐determined time intervals, surfaces to transmit data via RF links, and can be deployed for up to 180 days. Built by WETLabs Incorporated (Western Environmental Technology Laboratories) for the Naval Underwater Systems Center (NUWC), the system is outfitted with three environmental sensors and two acoustic vector sensors. The environmental sensors include; a Nortek Vector velocimeter which samples current components and characterizes surface wave velocities, a Seabird FastCAT Conductivity, Temperature, Depth (CTD) sensor; and a Wetlabs Fluorometer‐Turbidity optical sensor that measures both chlorophyll‐a and turbidity. The Wilcoxon acoustic vector sensors measure acoustic pressure and the three components of acoustic particle ac...