bluuebunny/crossref_metadata_2025 huggingface dataset:
Rayleigh-Ritz analysis and finite element descriptions derived from variational principles of diffraction by a circular disk
Authors: Ginsberg J. H. , Pierce A. D. , Wu X.-F. , DiMarco J. S.
Published: October 2005

The Kirchhoff-Helmholtz integral equation yields a variational principle for the pressure on the surface of a vibrating body [A. D. Pierce and X.-F. Wu, J. Acoust. Soc. Am. Suppl. 1 74, S107 (1983)]. A comparable principle is derived here for wave diffraction by a rigid body. Both versions offer the prospect of a more efficient treatment of diffraction effects than other methods, such as T matrices and the doubly asymptotic approximation. As a first application, algorithms for the evaluation of the pressure on the surface of a vibrating disk are derived and developed for constant frequency excitation and an axisymmetric surface pressure. For radiation, this situation arises when the vibrational pattern is axisymmetric. Axisymmetry for the reflection problem is obtained for low values of ka at arbitrary incidence, or else for normal incidence at arbitrary ka. The variational principles are used to derive system equations associated with (1) assumed mode functions and (2) finite elements. In the former, a sequence of admissible functions are selected to represent the surface pressure, and the variational principle provides the best combination of (complex) amplitudes describing the overall contribution of each mode. The finite element version yields simultaneous equations for the pressures at the nodes of the finite element mesh. In both techniques the system equations are simultaneous linear algebraic equations. The only difference in the treatment of the radiation and reflection problems is the inhomogeneous terms in the equations. [Work supported by ONR.]
Long wavelength acoustic properties of laminated plates
Authors: Schoenberg Michael
Published: October 2005

A propagator matrix formulation is used to study the elastodynamics of laminated plates. For a free plate in the low-frequency limit, the only mode is an extensional wave. The speed of this wave is usually found by a quasistatic analysis (first the extensional modulus for uniaxial strain is found and then the speed is calculated using the thickness weighted average density of the plate). This quasistatic result may be obtained from the exact dynamic relation for waves in a laminated plate (found by using the propagator matrix approach) and then taking the limit as frequency tends to zero. This free laminated plate extensional wave speed is αpl = 2[(〈μ〉 − 〈γμ〉)/〈ρ〉]1/2, where μ is the lamina shear modulus, ρ is the lamina density, and γ denotes the square of the ratio of shear speed to compressional speed for each lamina. 〈〉 signifies a thickness weighted average over the constituent laminas. The propagator approach can then be used to find the leading term (as wavelength becomes long) of the reflection coefficient of the plate immersed in an ideal fluid and to find the long wavelength slowness relation for compressional wave propagation in a system of periodically spaced laminated plates immersed in an ideal fluid. These results are shown to be identical with analogous results found for a homogeneous transversely isotropic plate with elastic moduli given by the set of effective moduli for layered medium [G. E. Backus, “Long-wave elastic anisotropy produced by horizontal layering,” J. Geophys. Res. 67, 4427 (1962)] and density 〈ρ〉. This means that for long wavelengths, a laminated plate is indistinguishable from an equivalent homogeneous transversely isotropic plate.
Sound reflection from a rigid concave/convex disk and its application to a spherical concave source
Authors: Ukigai Masahiro , Miida Yoshiro
Published: October 2005

This paper presents an analytical technique for calculating the amplitude of reflected sound waves from a rigid concave/convex disk. The formulas for the reflectivity are derived approximately under certain geometrical restrictions using Fresnel-Kirchhoff's diffraction formulas. Moreover, we identify the reflected sound pressure from a concave disk when a point source is located at the center of curvature with the radiated sound pressure from a spherical concave source, whose surface vibrates at the same time. Then we analyze the radiated sound pressure from a spherical concave source under this condition. To verify the validity of the approach, some experiments were performed in air. Experimental results showed good agreement between the calculated and the measured values and this confirmed the usefulness of the present technique to calculate the amplitude of reflected sound waves from a concave/convex disk.
The Green's functions for an acoustic source of arbitrary shape
Authors: Rudgers Anthony J.
Published: October 2005

When either the velocity or the pressure is specified on the surface of an acoustic source, there is an appropriate Green's function that can be used to describe the radiated pressure field of the source. It is demonstrated how to construct an analytic expression for each of these two Green's functions for a source having an arbitrary shape. The distribution of velocity or of pressure need not be uniform on the source surface. The resulting Green's functions, which are expressed in terms of the geometry of the source and the free-space Green's function for the Helmholtz equation, are calculable, in principle, by performing a series of elementary mathematical operations. An iterative-operator technique, based upon the Neumann-series method for solving linear integral equations, is used to construct the Green's functions.
Reflection and refraction of ultrasonic waves on a plane interface between two generally anisotropic media
Authors: Rokhlin S. I. , Bolland Ken , Adler Laszlo
Published: October 2005

Based on the Fedorov theory, we developed a unified approach for numerical solutions for the problem of the reflection-refraction of elastic waves on interface between two generally anisotropic solids. In contrast to other studies we consider Kristoffel equations and boundary conditions for both anisotropic media in coordinate systems formed by incident and interface planes rather than in crystallographic coordinates. This makes it possible to write simple unified algorithms for finding slowness refraction vectors and unit displacement vectors. Special care was taken to include in the general algorithm the cases of propagation in directions of acoustics axes. We illustrate our results by calculation of energy reflection and refraction coefficients for two cases: (1) anisotropic stainless-steel weld and (2) nickel-nickel bicrystal interface. [This work is supported by the Department of Energy.]
Within-subject repeatability of high-frequency thresholds
Authors: Imaoka Keiko , Cullen John K.
Published: October 2005

Reliability of high-frequency thresholds was studied in normal-hearing listeners ranging in age from 20–48 years. Using the PEST (Parameter Estimation by Sequential Testing) procedure, coupler-referenced thresholds were measured in ten subjects at frequencies 1, 2, 4, 8, 10, 12, and 14 kHz using circumaural earphones. Ten replicates of the measurements were made in all subjects over several weeks. The results indicate a general tendency for standard deviations of thresholds to increase as a function of stimulus frequency in all subjects. Nevertheless, the standard deviations did not exceed 4.0 dB in any of the subjects, suggesting high-frequency thresholds can be reliably obtained within subject, at least up to 14 kHz. Our findings are in disagreement with earlier results [E. A. G. Shaw, J. Acoust. Soc. Am. 46, 1502–1514 (1969)] which indicated that threshold variability is disproportionately high above 12 kHz in comparison with that below 12 kHz. [Work supported by the EENT Foundation of Louisiana.]
Relationship of auditory threshold to on-time and off-time
Authors: Young I. M. , Lowry L. D.
Published: October 2005

The effect of varying the on-time and off-time of interrupted pure tones on threshold of Bekesy tracings was discussed for subjects showing sensorineural hearing loss with abnormal adaptation. In the majority of subjects, with a fixed long on-time, threshold increased as the off-time was shortened. When the off-time was shorter than a critical value, threshold approached that for continuous tones. There was insignificant change in threshold when the on-time was varied with a fixed long off-time. However, some subjects showing abnormal adaptation demonstrated no observable change in threshold as the off-time was shortened with a fixed on-time. In the differential diagnosis of these cases, organic hearing loss due to multiple sclerosis or some other unknown causes and nonorganic hearing loss should be considered. Above finding was compared and discussed with other audiological test results obtained in these cases.
Comodulation masking release in notched noise
Authors: Hall Joseph W.
Published: October 2005

The notched noise masking method, in which the threshold of a signal is determined as a function of the width of a notch centered on the signal frequency, is an effective means of measuring the frequency selectivity of the auditory system [R. D. Patterson, J. Acoust. Soc. Am. 55, 802–809 (1976)]. Except for small deviations which have been noted at narrow notchwidths, threshold improves as a spectral notch centered on the frequency of the signal is increased. In modulated noise the deviation at narrow notchwidths is accentuated: with regard to a “no notch” condition, threshold increases appreciably for a notchwidths from 25–50 Hz wide centered on 500 Hz. For notchwidths greater than about 100 Hz, the function for modulated noise is not distinguishable from the function for random noise. The results will be discussed in terms of comodulation masking release [J. W. Hall, M. P. Haggard, and M. A. Fernandes, J. Acoust. Soc. Am. 76, 50–56 (1984)] and of suppression.
Can ABR latency measures predict psychoacoustic performance?
Authors: Stager Sheila V. , Langford Ted L.
Published: October 2005

The possibility of a predictive relationship between an electrophysiological measure—the auditory brainstem response—and a psychoacoustic measure of the ability to discriminate interaural level differences was explored. Thirty-seven subjects, from 15 to 51 years, participated in both parts of the study. Brainstem responses were simultaneously recorded in the vertical and horizontal derivations using both condensation and rare-faction clicks at a rate of 21.1 per second at 70 dB nHL. Six measures, the absolute latencies of waves I, III, and V and their interpeak durations, were determined. A “same/different” procedure was used to measure the discriminability of noise bursts with interaural level differences of 1.5, 3, and 6 dB. The interaural difference which produced a 75% level of performance was determined from the resulting psychometric functions, and used in subsequent calculations. A predictive relationship was assumed if a significant linear relationship existed between any latency measure and psychoacoustic performance. Age and sex were included as variables. Fourteen ABR latency measures, primarily absolute latency measures, predicted or were predicted by performance on interaural level discrimination. Ear of stimulation was a key parameter of the predicted latencies. [Work supported in part by NIH grant ♯NS16396.]
Studies of the auditory steady-state response II. Masking
Authors: Galambos Robert , Makeig Scott , Stapells David R.
Published: October 2005

By definition, continuous wideband noise acts as a masker when it raises the threshold of a signal delivered to the contralateral ear. In our physiological studies the “signal” (a train of suprathreshold monaural clicks or tonepips) evokes brain potentials (the 40-Hz or steady-state response) that are continuously recorded and analyzed for amplitude and phase. When wideband noise is introduced into the contralateral ear at levels below that which raises the threshold of the signal, the evoked potential amplitudes typically drop monotonically with increase in noise intensity. Whether this physiological central suppression is related to “central masking” is unclear; certainly no perceptual correlate for it has yet been uncovered. [Work supported by NIH.]
Industrial exposure of the hand to vibration
Authors: Brammer A. J.
Published: October 2005

Vibration transmitted to the hand-arm system from a power tool, machine, or work piece may cause a complex of neurological, vascular, and musculoskeletal disturbances. The early neurological symptoms, periods of tingling and numbness, are probably associated with the nerve endings, while there is some evidence that the sensory loss reported in severe cases is related to mechanoreceptor degeneration. The vascular signs, which consist initially of episodes of finger blanching, appear to be associated, at least in part, with thickening of the digital arterial walls. In time, the consequent reduction in blood flow is believed to be responsible for nutritional deficiencies and ulceration at the finger tips. In the absence of a clear understanding of the disease processes, some success in modeling the onset of vasospasms has been obtained from epidemiological studies of industrial workers exposed to vibration. The range of exposure parameters is not sufficient, however, to derive directly tolerable occupational exposure limits. The various attempts to provide this information based on current knowledge of the mechanisms causing damage to the hand-arm system will be discussed.
Hand-arm vibration models
Authors: Reynolds Douglas D.
Published: October 2005

Several mechanical models of the human hand and arm have been developed by different investigators. These models typically treat the hand and arm as a series of mass, spring, and damping elements. The complexity of the models have ranged from systems with just one degree of freedom to systems with four degrees of freedom. These models have been very useful for gaining an understanding of the transmission of hand-induced vibration into the hand and arm and in the designing of test stands for measuring the vibration levels of selected hand-held tools. This paper traces the development of some of the mechanical hand-arm vibration models and discusses the relevant results that have been obtained from these models. It also presents information on how the results of these models can be used to develop simple mechanical analog models of the hand that can be used in machines that are designed to test and measure the vibration levels of hand-held power tools.
Biodynamic models for injury prediction
Authors: von Gierke Henning E. , Kaleps Ints
Published: October 2005

Various types of models have been used for years to explain human body response to vibration or impact and to understand injury or performance decrement. This paper reviews recent developments in this area. Although lumped parameter models are extremely useful to understand the basic dynamics occurring in the body structure, the advent of large computers allows the realistic modeling of geometric and mechanical properties of the human body and detailed calculation of body motions and deformation under external forces. Internal stresses and tissue displacement, in turn, can be correlated with hard and soft tissue injury. Two such specific models to be discussed will be the Articulated Total Body Model (ATBM) and the Head-Spine Model (HSM). The ATBM is used to predict the gross motions of the human body and limbs under impact and sustained acceleration; the HSM addresses the stresses, deformation, and injury probability of the human spine under impact and whole body impedance under vibration. Application of these models to automotive crash and aerospace medical problems will be reviewed and demonstrated by motion pictures.
Performance effect of vibration
Authors: Guignard J. C. , Bittner A. C.
Published: October 2005

This review attempts to develop a simple theory of decrements in task performance associated with whole-body vibration and low-frequency motion. Postulated mechanisms are twofold: (1) direct mechanical interference acting at the man/task interface, and (2) centrally acting perceptual and cognitive effects that are mediated physiologically. Mechanical interference (for example, with locomotion, manual, and visual tasks) is usually heavily dependent on the vibration spectrum and largely independent of time; whereas the central, physiologically mediated effects are less clearly related to the vibration spectrum, are apt to exhibit a latent period before any impairment is measurable, and tend to worsen as a complex function of vibrational acceleration and the passage of time. The importance of this distinction to research methodology and to biodynamic standards development is addressed.
A method for predicting speech sound recognition performance by the hearing impaired
Authors: Pavlovic Chaslav V. , Studebaker Gerald A. , Sherbecoe Robert L.
Published: October 2005

We have reported earlier [J. Acoust. Soc. Am. 74, 1606–1612 (1983); J. Acoust. Soc. Am. Suppl. 1 75, S84 (1984)] the details of an articulation index (AI) scheme that was found satisfactory for individual normal-hearing listeners. We have also reported on the difficulties associated with applying the AI procedure to individuals with hearing impairment [J. Acoust. Soc. Am. 75, 1253–1258 (1984)], as well as the results of our earlier attempts to introduce modification in the procedure to account for hearing loss [J. Acoust. Soc. Am. Suppl. 1 74, S104 (1983)]. We now present the theoretical framework of an approach that we found adequate for a number of hearing impaired subjects with different audiometric configurations. The procedure is based on relating various psychoacoustical variables to each other and to the speech processing capacity of the auditory channel. The method was tested and found satisfactory under various conditions of noise distortion, speech filtering, and low presentation levels. [Work supported by NINCDS.]
Statistical considerations in predicting hearing for speech from the audiogram
Authors: Bilger Robert C.
Published: October 2005

The earliest work in relating sensitivity to tones to hearing for speech was conducted at the Bell Telephone Laboratories by Fletcher (1929, 1950) and by French and Steinberg (1947). Fletcher dealt with predicting the threshold for speech from the audiogram and French and Steinberg with predicting the “articulation” score from the frequency response of the transmission system. In both instances, their approach was instrumental, and they were predicting mean scores from frequency response data. The problem of predicting hearing from speech from an individual's audiogram is a more complicated problem, one that requires a statistical approach to regression analysis in order to account for errors of measurement and individual differences. The nature of this statistical approach and the problems that must be dealt with will be described here. Illustrations of each point will be drawn from data accumulated during standardization of a test of speech perception in noise (SPIN). [Work supported by NIH.]
Firing patterns, synaptic connections, and potentials
Authors: Young Eric D.
Published: October 2005

This talk will review four approaches that have been taken to the problem of the genesis of auditory neural firing patterns. These approaches will be illustrated by examples from the cochlear nucleus. The first approach is extrapolation from morphology. Cells with relatively simple synaptic input, such as the bushy cells of the AVCN, should have response characteristics more similar to their input than should cells with more intricate synaptic input, such as the principal cells of the DCN. The second approach is theoretical consideration of the way in which membrane potentials summate spatially and temporally. Much phenomenological work has been done in this area, both in predicting the nature of output patterns and in trying to recover input patterns from output patterns and assumptions about cells' characteristics. The relationships between cellular morphology and regularity of discharge is an area in which this approach has proven useful. The third approach is to record intracellular potentials and to attempt to correlate them with neurons' activities. Recent improvements in this technology have led to some interesting new information about the membrane properties of cells in the cochlear nucleus as well as direct confirmation of correspondences between firing patterns and morphology which had been inferred from indirect evidence. The fourth approach is cross-correlation of simultaneously recorded discharge patterns. This method has been applied to the study of neurons in several areas of the auditory system. Although it suffers from problems of interpretation, the cross-correlation method offers the possibility of studying functional relationships among neurons which cannot otherwise be tested easily.
Passive source localization with intersecting spherical surfaces from time delay estimates
Authors: Schau H. C. , Robinson A. Zed
Published: October 2005

The problem of passive localization of an acoustic source is a classic area of research in underwater acoustics. While the three-dimensional problem presents difficulties, most past investigations have been able to make suitable approximations to reduce the localization to one in a plane involving two dimensions. However, acoustic source localization in anechoic tanks, pressure vessels, or other similar enclosures often requires a truly three-dimensional formulation. This paper presents the equations describing source localization in three dimensions, given time delays between four receiving channels. Numerical difficulties associated with their solution are discussed, particularly those involved in the nonlinear solution of sets of intersecting hyperboloids. An alternate formulation is presented which makes use of intersecting spheres rather than hyperboloids. This reduces the numerical complexity markedly since finding intersections of closed surfaces such as spheres is inherently more stable than for open surfaces like hyperboloids. Several possible algorithms for implementing this approach are presented, and the practical limitations of each for use with nonideal data are contrasted. Results indicate that the proposed method of intersecting spheres is numerically simpler and provides a more accurate solution for passive source localization than the method of intersecting hyperboloids. [Work partially supported by NAVSEA PMS 409 and by NAVSEA 63X5.]
A modified time delay spectrometry technique for underwater acoustic measurements
Authors: Brekke Bjoern , Wilson O. B.
Published: October 2005

A measurement technique, based on the time delay spectrometry (TDS) method, has been developed which makes possible the determination of acoustic transmission loss for direct, surface- and bottom-reflected paths between a source and receiver at moderate range in water. It appears to have some advantages compared to the normal TDS technique. The linear frequency modulated sweep, normally used in the TDS method, is retained. The rapid multiple sampling and spectral estimation capabilities of the Hewlett Packard 3561A dynamic signal analyzer, controlled by a desktop computer, are used to resolve signals having different arrival times and frequencies. An interactive computer program which permits the determination of the level of signal at various frequencies for resolvable multipaths will be described. Some of the limitations of the technique will be discussed and the results from an example of its use will be presented. [Supported by the Naval Undersea Warfare Engineering Station, Keyport, WA.]
Bottom-bounce time-spread functions in monostatic and bistatic active sonar
Authors: Brill Michael H. , Zabal Xavier
Published: October 2005

The time-spread function for a single-bottom-bounce path is the time histogram of energy returned from an incident impluse of acoustic power [M. H. Brill, X. Zabal, and S. L. Adams, J. Acoust. Soc. Am. 75, 1062–1070 (1984)]. More fundamentally, the time-spread function is a second-order statistical moment of the channel impulse-response function (in pressure). The present work develops time-spread functions for an active-sonar geometry: a sound wave from an omnidirectional source reflects once from a rough bottom, reflects omnidirectionally from a target, and reflects once again from the bottom on its way to an omnidirectional receiver. Under these conditions, our earlier geometric-acoustic model leads to the following theorem: when source and receiver are separated (bistatic case), the total time-spread function is the convolution of the time-spread functions from source to target and from target to receiver. When the source and receiver are coincident (monostatic case), this simple theorem does not apply, for the channels to and from the target are not statistically independent. Methods are explored for connecting monostatic and bistatic expressions for time spread, by relaxing some of the assumptions of the earlier model to accommodate the correlation distance of the rough bottom.
The dependence of adaptive array beamforming on the covariance matrix estimate
Authors: Penner A. Raymond
Published: October 2005

An investigation into the dependence of adaptive array beamforming on the covariance matrix estimate was carried out. The beamforming method used is based on the Cholesky decomposition of the covariance matrix. Both adaptive beam patterns and bearing estimates were obtained and used to study the adaptive properties of different planar array structures. Specifically the dependence of these outputs on the number of samples used to calculate the covariance matrix was investigated. Beamforming was performed on both real and simulated data; both linear and exponential averaging of the covariance matrix were considered.
Interference cancellation for uniform line arrays
Authors: Stockhausen J. H. , Farrell Joseph B.
Published: October 2005

The effect of strong directional interference in an acoustic field is to mask small signals at nearby bearings. This paper presents a technique for reducing the effect of such interference on the performance of a uniform line array. A smoothed estimate of the interference bearing is obtained and a reference beam is steered in that direction. A single point inverse Fourier transformation of the reference beam signal results in a set of element excitations which are used to characterize the interference. These excitations are subtracted from the original excitation data, nulling the interference in subsequent beamforming. The null has an effective angular width (related to beamwidth and noise directionality), and the structure of the field map near the interference direction is obscured by the steep slopes of the null. Normalization of the map with respect to the ideal null shape restores much of the hidden structure and at the same time substantially reduces the bias of signal direction estimates.
Experimental test of acoustic localization in shallow water
Authors: Bucker Homer P. , Schey Philip W.
Published: October 2005

Passive shallow water narrow-band systems may be designed to locate a known source, or to detect and track an unknown source in shallow water. An experiment was done in 66-ft deep shallow water off Mission Beach, CA with signals recorded at eight sensors in a vertical line array. A 3-kHz cw source was moved in a predetermined manner to simulate a target at various ranges and depths. Analysis has been completed on the first question. From the available data, good localizations can be made at ranges out to at least four times the water depth. The second problem (detecting and tracking an unknown target) is, of course, more difficult and only a preliminary report can be given at this time. Because of many dimensions in the problem (e.g., correlation versus range, depth, and time), it seems necessary to calculate sequential probability ratios for all tracks consistent with target motion characteristics. This is a variation of the old “track before detect” philosophy used in active sonar systems. Several examples will be shown where a likelihood ratio is calculated as a function of target range and depth.
A critical overview
Authors: Loy Gareth
Published: October 2005

The talk will attempt to characterize the standardized Musical Instrument Digital Interface (MIDI) from several directions: (1) what it is physically; (2) the control paradigm it implements; (3) the implications of this paradigm for two specific categories of control: (a) performance capture, and (b) synthesizer control; (4) conclusions regarding its usefulness for the variety of different tasks for which it could find application. A description of ways MIDI has been incorporated in the computer music workstation project of the Computer Audio Research Laboratory will be included.
Perceptual controls for synthesizers: Relating synthesis parameters to perceived timbre
Authors: Martens William L.
Published: October 2005

When faced with the task of producing a given range of timbres on a commercially available synthesizer, users must learn how the manipulation of the controls of the synthesizer affects the timbres produced by the synthesizer. The process of developing the desired palette of timbres usually involves making educated guesses at how the synthesis parameters must be varied and then proceeding with a trial and error search for the desired sound qualities. This process may be described as a method for the psychophysical scaling of synthetic timbres. With the advent of the standardized Musical Instrument Digital Interface (MIDI), an avenue has been opened for a more systematic approach to the problem of relating synthesis parameters and perceived timbre. At the Northwestern University Computer Music Studio, we have developed a software environment which applies the methods of modern psychoacoustics to determining this relation for MIDI-controlled commercial synthesizers. The software supports the automatic perceptual scaling of synthesis parameters and is capable of providing control over synthetic timbres which is based strictly upon the perceptual features of the desired timbres.
Measurement of impulse noise propagation over natural impedance surfaces
Authors: Lewis Nelson , Raspet Richard
Published: October 2005

The peak sound pressure level decay with distance of the impulse noise from hand grenade simulators has been measured over the following types of surfaces: water, swamp, rocky ground with small bushes, dead grass with sparse snow cover, and dense evergreen forest. The ground impedance is estimated for each surface using the calculation of R. Raspet. H. E. Bass, and J. Ezell [J. Acoust. Soc. Am. 74, 267–1274 (1983)] and predictions of the effect of each type of ground on lower frequency pulses are estimated.
A source cancellation method of noise control
Authors: Koopmann G. H. , Chen W. , Neise W.
Published: October 2005

The control of the blade tones generated by a centrifugal fan is the subject of this paper. Blade tones are produced when the flow leaving the blades of a fan's impeller impinges on the nearest part of the fan casing called the cutoff. Measurements of the surface pressure on the cutoff at the blade tone frequencies reveal a dipolelike sound source. Reductions in the sound levels radiated as blade tones can be achieved by introducing external sources directly at the cutoff. The exterior sources are generated with loudspeakers contained within a heavy metal enclosure which has an open end made of a curved, perforated plate, which replaces part of the original cutoff. The loudspeakers are driven with signals that are synchronized to the fan's rotation. When an optimum cancellation condition is reached, pressure measurements on the cutoff surface indicate that the altered phase distribution of the blade tone component closely matches that of the convection velocity of the passing fluid flow. [Work supported by Electric Power Research Institute.]
Extraction of helicopter-radiated noise by frequency domain processing
Authors: Dwyer Roger F.
Published: October 2005

Helicopter-radiated noise manifests itself in the time domain as impulses due to blade vortex interactions and in the frequency domain as harmonic components from both the main and tail rotors. But these components are not pure sinusoids. In addition, they have non-Gaussian probability distributions. They appear to be caused by frequency modulation due to the rotating blades. The paper discusses a method to extract helicopter-radiated noise components by implementing a nonlinearity in the frequency domain. It is shown with real helicopter-radiated noise data that autocorrelation estimates of a signal, being interfered with by additive helicopter-radiated noise, can be improved by extracting some of the interfering components. The extracted components are also available as an enchanced time domain representation.
Propeller noise pressure-time history characteristics from quasistationary flyby-tests
Authors: Kallergis M.
Published: October 2005

Wing-mounted microphone tests and flyby-tests employing a “quiet” microphone-carrying glider, flying in formation with a test propeller aircraft, were conducted to study the dependencies on several source characteristics (such as helical blade-tip Mach number, advance ratio, and blade-pitch setting), on the emission angle of the propeller-noise pressure-time histories (PTH) close to the propeller, and at greater distances. Characteristic changes, both in PTH shape and relative contribution of positive and negative pressure signature portions, were obsevered, which duplicate findings obtained in wind-tunnel model tests on small-scale propellers.
Further comparisons of wind tunnel and airplane acoustic data for advanced design high-speed propeller models
Authors: Dittmar James H.
Published: October 2005

The noise of high-tip-speed propellers at cruise has been identified as a possible cabin noise problem for advanced turboprop airplanes. Comparisons were made between the SR-2 and SR-3 model propeller noise data taken in the NASA 8×6 wind tunnel, in the United Technologies Research Center (UTRC) Anechoic Tunnel, and with boom and fuselage microphones on the NASA Jetstar airplane. Plots of peak blade passage tone noise versus helical tip Mach number generally showed good agreement. The curve shapes were similar except for the UTRC data, which was flatter than the other sets. This was attributed to the UTRC data being taken at constant power while the other data were at constant advance ratio. Directivity shape comparisons at the cruise condition were also similar. The generally good comparisons of the data from four configurations using three facilities indicates the ability to use any of these facilities for measuring the blade passage tone noise of these types of propellers.
Suppression of the flow induced acoustic environment in an irregularly shaped cavity with an opening exposed to subsonic flow
Authors: Shaw Leonard L.
Published: October 2005

A wind tunnel test was performed on an irregularly shaped cavity with an opening exposed to airflow at Mach numbers from 0.30 to 0.80. Methods to suppress the acoustic levels in the cavity consisted of three different leading edge spoilers, three different trailing edge shapes, and internal acoustic treatment. The effect of spoiler porosity was also investigated. The results indicated that all the spoilers were effective in suppressing the flow induced levels while the trailing edge shape variations, in conjunction with the leading edge spoilers, had very little effect on the levels. The internal acoustic treatment was shown to be very effective in suppressing the levels.
A comparison of the noise environment in open-plan and traditional single family dwellings
Authors: Holger David K.
Published: October 2005

The noise environment within two single family dwellings was investigated using a sound power source and sound level meter. The dwellings have three levels and are roughly the same size. One of the dwellings is an open-plan design, both within and between levels, and the other is a traditional multilevel design. Both dwellings are representative of common residential construction and neither incorporate special noise control features. Octave band pressure levels from 125 Hz to 8 kHz were measured at eight locations in each dwelling for each of three source locations. Average NC values in living areas of the traditional dwelling were found to be 7 dB less than in the open-plan dwelling. In bedroom areas, the average NC values were more than 20 dB lower in the conventional dwelling. These results are in good agreement with those of a previous study [D. K. Holger and M. S. Pickett, J. Acoust. Soc. Am. Suppl. 1 72, S91 (1982)] which concluded that boundary absorption is very important in open-plan residential design. [Work supported by the Engineering Research Institute.]
Representative noise levels of snow making, slope grooming, and other ski area equipment
Authors: Kleinschmidt Klaus
Published: October 2005

In the process of producing the environmental noise assessment for a major expansion of the Mt. Wachusett (MA) ski area, noise data for ski area equipment were collected. Modern ski areas are highly mechanized. Snow making equipment includes large air compressors and snow guns or other air/water dispersal devices. Slopes and trails are groomed by special tractors towing various rollers, plows, scrapers, etc. to provide well-packed mogul-free surfaces. Much of the snow making and grooming activities take place at night. By day snowmobiles are used to transport the ski patrol to serious accidents and mechanics to repair stalled lifts. Construction of new ski trails requires additional, typically loud equipment such as chain saws, log skidders, wood chippers, bulldozers, etc. In the rural environment of ski areas these activities can exceed ambient noise at considerable distances. Furthermore, the trend to overnight accommodations for skiers directly on ski hills requires consideration of such equipment noise.
Target strength and identification: Wave fronts, resonances, and hybrid forms
Authors: Felsen L. B.
Published: October 2005

The scattered radiation from a target illuminated by an incident transient acoustic field contains different information in the portions of its high low-frequency spectrum. The high frequencies synthesize wave fronts that arrive at the observer along ray paths and thereby convey aspect-dependent information about local features of the target encountered along these paths. The wave fronts form useful identifiers of structural detail as long as they can be individually resolved, i.e., at early response arrival times. The low frequencies synthesize complex resonances that encompass the target as a whole and thereby convey global information. Resonances are aspect independent and dominate the late time response; they are formed by collective superposition of wave fronts that have made multiple traversals of the scatterer. From these physical attributes, it is evident that identification algorithms based on wave fronts alone or on resonances alone are inadequate to cope effectively with late time and early time data, respectively. Moreover, for low-Q targets, the resonances may be so strongly damped as to be ineffective altogether. A hybrid algorithm that combines relevant wave fronts and resonances self-consistently within a single framework can optimize analysis of the scattered field for all observation times. This is illustrated for canonical target shapes such as cylinders, strips, layered structures, etc. The presentation emphasizes the physical mechanisms that synthesize the responses, with numerical results confirming observations concerning the respective roles of wave fronts and resonances. An important finding is that multiple diffraction effects calculated by ray acoustics, when summed collectively, can synthesize quite accurately even the lower-order (i.e., low frequency) complex resonances. This makes ray theory relevant not only for wave front construction but also for determination of target resonances. Modifications when the target is in a shallow ocean environment are indicated. [Work supported by ONR Underwater Acoustics.]
Scattering from spheroidal elastic shells at various incident angles
Authors: Werby M. F. , Gaunaurd G. , Davey Kent
Published: October 2005

An analytical and computational study of acoustical scattering from spheroidal elastic shells is presented for arbitrary angles (relative to the axis of symmetry) of incident plane waves. The calculations are performed using a modified T-matrix approach in which backscattered cross sections are examined. The asymptotic backscattered cross sections (form functions) are plotted as a function of the nondimensionalized frequency KL/2 (K = 2pgr;/λ, where λ is the wavelength and L the length of the object) for angles relative to the axis of symmetry ranging from 0°–90° in increments of 15°. The aspect ratio (ratio of length to width) varies between 1.25 and 2.0 in increments of 0.25 and KL/2 varies from 3 to up to 15. The material under study is steel. We are, in particular, interested in yielding insight into the nature of resonances for such objects including the classification and physical characteristics. In this study we observe two classes of resonances. The first class corresponds to standing waves excited along the axis of symmetry and the second to standing waves that are excited perpendicular to the axis of symmetry. For plane waves that are incident at angles other than 0° and 90° relative to the axis of symmetry one can observe both classes of resonances; they occur at the same values of KL/2 as if they were excited at 0° and 90°.
Hyperbolic umbilic diffraction catastrophe in the scattering from a penetrable spheroid at short wavelengths
Authors: Marston Philip L.
Published: October 2005

There has been recent progress in understanding the scattering patterns from spheroids at long wavelengths. At short wavelengths, however, partial wave sums are difficult to evaluate, while geometric optics is not applicable near angular foci where the scattering is enhanced. Consequently, the discovery and classification of angular foci is important to the understanding of short wavelength scattering. A novel focus or “diffraction catastrophe” was recently observed in the light scattered from oblate spheroidal drops of water in air [P. L. Marston and E. H. Trinh, Nature 312, 529–531 (1984)]. Hyperbolic umbilic focal sections were characteristic of drops having D/H≃1.31, where the drop's diameter is D in the equatorial plane and is H along the axis of rotational symmetry. This critical axis ratio was subsequently calculated [J. F. Nye, Nature 312, 531–532 (1984)] to be related to the refractive index μ by (D/H)2 = 3μ2/4μ2 − 1). This expression is in agreement with the observed D/H≃1.31 for water drops having μ = 1.332. In the present research, this expression was confirmed using a general ray-tracing formalism. Nontrivial diffraction catastrophes should also be present in the farfield acoustical scattering from nonspherical penetrable objects. [Work supported by ONR.]
Rayleigh-mode backscattering from an elastic sphere: Sommerfeld-Watson transformation and experimental confirmation
Authors: Williams Kevin L. , Marston Philip L.
Published: October 2005

The acoustical scattering form function of fluid-loaded bodies such as cylinders and spheres may be written as a partial wave series. The Sommerfeld-Watson transformation (SWT) has been used previously to rewrite the form function of elastic cylinders with ka⩾1. The SWT allows one to understand the form function in terms of reflected, transmitted, and surface waves. In the present discussion we carry out the SWT on a fluid-loaded elastic sphere. We concentrate on the specular reflection and Rayleigh wave contribution to scattering at small backscattering angles. We previously measured and modeled the angular dependence of the Rayleigh contribution to near backward scattering [K. L. Williams and P. L. Marston, J. Acoust. Soc. Am. Suppl. 1 76, S8 (1984)]. The SWT confirms the physical picture used and, for the first time, predicts the absolute Rayleigh contribution associated with one or more circumnavigations of the sphere. To test the SWT, tungsten carbide spheres in water were ensonified by tone bursts having central frequencies in the range 30 < ka < 80. Measurements were made of the first and second Rayleigh contributions to the backscattered pulse train. Plots of these measured distinct Rayleigh amplitudes as a function of ka confirm the results of the SWT and illustrate the significance of radiation damping and axial focusing. [Work supported by ONR.]
Scattering of a spherical wave by an elastic solid cylinder
Authors: Piquette Jean C.
Published: October 2005

The results of a theoretical study of the scattering of a spherical wave which is incident on a lossless elastic cylinder of infinite length are presented. The solution is obtained by solving the relevant differential equations, subject to boundary conditions requiring continuity of stress and displacement at the scatterer's surface. The results of a numerical implementation of the solution for 1-kHz spherical wave, incident on a 2-cm-diam metallic scatterer, are described. Results are presented for a variety of metals. The differences between the solution presented and the solution to the equivalent problem involving an incident plane wave are substantial in the nearfield. For example, an attempt to determine the relative effectiveness of a lead sample to that of a tin sample at scattering of a 1-kHz plane wave would be in error by approximately 20 dB if evaluated by using a spherical source located 5 m from the scatterer. The extent of the nearfield associated with an incident spherical wave is also much greater than that associated with an incident plane wave. The most significant conclusion of this work is that the actual wave geometry which is used in implementing acoustic nondestructive evaluation cannot be ignored.
The Resonance Scattering Theory for stacks of bonded elastic plates
Authors: Jackins P. D. , Gaunaurd G. C.
Published: October 2005

We have formulated the Resonance Scattering Theory (RST) for the general case of acoustic reflection from (and transmission through) a stack of arbitrarily many elastic flat plates. The whole multilayered system separates two possibly dissimilar outer fluids. Incident waveforms impinge on the structure at arbitrary angles of incidence from one of the outer fluids, and are subsequently reflected and transmitted. The clear and simple form of the RST formulation permits a very easy interpretation of the scattering process taking place in all the elastic layers, since each shear or compressional waveform inside each layer can be constructed from the shear or compressional resonances excited in each layer. According to the RST, these resonances and the scattered waves they cause are described by a series of simple poles which yield meromorphic expressions for the transmission and reflection coefficients. These coefficients are numerically computed and graphically displayed in many multilayered cases of interest with relative ease, in view of the simple-pole behavior of the system, which is exploited by the RST. We show how the multilayered formulation can be put in the same simple resonance form we had for a single or double layer [i.e., P. D. Jackins and G. C. Gaunaurd, J. Acoust. Soc. Am. 73, 1456–1463 (1983)]. The RST predictions compare favorably to exact but cumbersome calculations based on the traditional Thomson-Haskell approach, and we recover many of our earlier results for single or double layers [i.e., P. D. Jackins and G. C. Gaunaurd, J. Acoust. Soc. Am. Suppl. 1 74, S24 (1983)]. The present RST results are ideally suited to allow the solution of the inverse scattering problem for the stack.