Karl B. Washburn

Generalized nearfield acoustical holography for cylindrical geometry: Theory and experiment

From the measurement of the acoustic pressure on a cylindrical, two‐dimensional contour located close to the surface of an underwater, vibrating cylinder, the complete three‐dimensional sound field can be reproduced (reconstructed) with the aid of a computer. This reconstruction technique, called GENAH (generalized nearfield acoustical holography), is unlike conventional holography because it provides a super resolution image of the sound‐pressure field from the surface of the cylinder to the farfield. At the same time, GENAH reconstructs, from this two‐dimensional measurement, the vector velocity and the vector intensity fields (energy flow) in the nearfield of the source, and identifies modes of surface vibration of the cylinder. Experimental results are provided and the accuracy of GENAH is demonstrated by comparison with the two‐hydrophone technique.

Vibration of two concentric submerged cylindrical shells coupled by the entrained fluid

The vibrational characteristics of a point‐driven ‘‘double shell’’ (two concentric submerged cylindrical shells coupled by the entrained fluid) are investigated theoretically and experimentally. Of particular interest are the shielding effects, if any, of the outer shell upon the inner shell. The theory on the double shell is based on Flugge’s infinite‐shell equations, the Helmholtz wave equation, and boundary conditions at the fluid–structure interfaces. This theory is used to model a finite double‐shell structure in wave number‐frequency space. Experiments are carried out in which generalized near‐field acoustical holography (GENAH) is employed to provide the experimental vibration characteristics in wave number‐frequency space of the finite double shell. It is confirmed theoretically and experimentally that the outer shell of the double shell exhibits two separate dispersion curves: A higher‐frequency dispersion curve exhibits in‐phase vibrations with respect to the inner shell, and a second lower‐freq...

Time-domain analysis of the energy exchange between structural vibrations and acoustic radiation using near-field acoustical holography measurements

The vibrational energy in a structure and the radiated acoustic energy are analyzed in the time domain using data from near‐field acoustical holography measurements. A signal processing method is described that uses data from a single broadband acoustical holography measurement to determine the structural and acoustic responses for synthetic forces different from the original measurement force. The processing is based on the assumption that the measured acoustic pressure and particle velocity are related to the drive force by a linear shift invariant transfer function. Finite time length synthetic forces are examined for measurements from a fluid‐loaded, point‐driven, finite cylindrical shell. Comparisons of the amount of energy input to the structure by the driver, the energy injected into the fluid near field, and the energy radiated to the fluid far field show large amounts of energy that enter the fluid near field while the drive force is on, but then re‐enter the structure where it is damped once the...

Computational aeroacoustic modeling of rotary mower noise

The primary source of noise from rotary mowing machines is the aeroacoustic sound generated by the cutting blades. The requirements for maintaining high quality of the cut contradict those for reducing noise from the blade. The key to balancing these competing requirements is understanding the aeroacoustics of blades within the cutting environment. Until recently, there has been no reliable way to predict the noise generated by a configuration of blades and housing decks. To address this, John Deere and its partners have undertaken a program in Computational Aero‐Acoustics (CAA). By leveraging existing computational fluid dynamics and parallel processing capabilities, the participation of several software vendors, and the cooperation of industrial and academic partners, this project has provided a capability to accurately predict the sound generated by low Mach number flows in industrial systems. This presentation reviews the results of applying a nascent CAA capability. The noise generated by a single‐bl...

Holographic sound‐field imaging as a diagnostic tool

Approaching its third decade, sound‐field imaging using near‐field acoustical holography has matured as an analysis technique. It has been established as the tool of choice for source motion mapping in structural acoustics and for source identification in noise control. The strength of near‐field holography lies in its complete description of the sound field, from source to far field. This opens a door to using sound‐field imaging as an acoustical diagnostic tool. Several current trends in applying holography, including moving sources, reverberant spaces, and lower‐cost systems, are surveyed. New diagnostic applications of sound‐field reconstruction in audio‐source imaging, transmission path analysis, and boundary material characterization are described.

Experiences in three‐dimensional visualization of near‐field acoustical holography data sets

Data collected and processed in near‐field acoustical holography are inherently multi‐dimensional. The data span two spatial dimensions (or their transform‐space counterparts) and a dimension in either time or frequency, and the quantities of interest within those spaces also represent dimensions. Various processes, such as multiple projections or time‐frequency transforms, can increase the dimensionality even further. When powerful, inexpensive graphics display systems became available, they brought the capability for viewing at least four dimensions simultaneously (two in the screen space, one implicit in space via depth cues, and one in color). A public domain package called ‘‘BoB’’ (for ‘‘Brick‐of‐Bytes’’), provided by the Army High Performance Computing Research Center (AHPCRC), was used to view large blocks of holographic data in four dimensions. This work examines our experiences in trying to explore holographic data using this form of visualization. It was found to be inferior in comparison to cur...

Projection of near‐field measurements of scattered pressure to the far field

NRL has successfully produced the first scattered hologram of an underwater shell, ensonified by a bow‐incident plane wave. The total acoustic pressure is measured as a hologram on a cylindrical surface concentric to and in the near field of the shell. The scattered pressure is constructed by subtracting a hologram of the incident field, measured without the shell, from the total field hologram, with the shell in place. Using generalized near‐field acoustical holography (GENAH), the scattered hologram is processed to reconstruct the pressure and radial velocity on the surface coincident with the shell exterior. The Helmholtz integral equation (HIE) is then applied to the pressure and velocity to compute the far‐field directivity pattern. The shell used in the holographic measurement had its far‐field response directly measured as well: the two measurements will be compared. Sources of error, including scattered energy escaping from the ends of the unclosed cylindrical contour used in the HIE, will be exam...

Theory and experiment of two concentric submerged cylindrical shells coupled by the entrained fluid

In this paper theory and experiment for the coupled vibration of two concentric cylindrical shells (double shell) are described; the inner shell contains air, the outer shell is surrounded by water, and water exists in the annular space between them. A point force is applied to the inner shell, whose displacement produces acoustic pressure in the annular space and in turn this pressure wave drives the outer shell. The nature of acoustic field in the annular fluid and its coupling effect on the shells is investigated. Using Flugges shell equations and the Helmholtz equation, the normal modes of an infinite double shell are calculated. The theory includes the effect of initial prestress (uniform axial compression). The results of these numerical calculations in wave‐number space will be compared with the results from generalized near‐field acoustical holography (GENAH) experiment on a simply supported finite double shell. The wave‐number/frequency representation of this double shell will be also compared w...

Modal analysis of fluid‐loaded, axisymmetric shells using nearfield acoustical holography data

Because typical methods of performing experimental modal analysis on structures in air do not apply well to those in water, an alternative, noncontact method was sought to measure vibration of thin, axisymmetric, submerged shells. Generalized nearfield acoustical holography (GENAH) provides a method for computing the normal velocity on the surface of these shells. The reconstructed velocity responses along the axis of the shell for each nth circumferential order form a series of “sensors.” The multiple reference Ibrahim time domain technique (MRITD), a high‐resolution modal decomposition, is applied to the data from these “sensors.” The MRITD is an eigenanalysis method that allows for multiple inputs and outputs. Its eigenvalues yield the mode frequencies and damping factors; its eigenvectors are the mode shapes. Repeated iterations and cluster analysis are used to locate true mode poles in the presence of noise. A least‐squares technique is further applied to generate participation factors for each mode....

Time‐based energy analysis of acoustic radiation and structural vibration using generalized near‐field acoustical holography measurements

A time‐based energy analysis of structural vibration and radiation is carried out with near‐field acoustical holography measurements. By using the force driving the structure as a reference for the measured pressure, the impulse response can be determined over the frequency range of the force signal. Assuming that the signals are linear and shift invariant in time, the structural vibration and radiated acoustic field due to a synthetic force are determined. This is done by multiplying the frequency domain descriptions of the measured impulse response and the synthetic force. The signal processing, which will be outlined, occurs in the frequency domain. The results are then transformed into the time domain via discrete Fourier transforms where the analysis is done. Thus, from a single measurement, the response to force time histories other than the original measurement force can be studied, assuming that the new force signal can be described with the frequency range of the original measurement. Results for...