bluuebunny/crossref_metadata_2025 huggingface dataset:
Ambient ULF/VLF ocean-bottom noise 200 km west of San Francisco: Motion and pressure 0.002 to 20 Hz
Authors: Sutton G. H. , Carter J. A. , Barstow N.
Published: October 2005

Data recorded from long-period and short-period, three-component seismometers and hydrophones of the Columbia-Point Arena ocean-bottom seismic station (OBSS) are being analyzed in the frequency range 0.001 to 40 Hz to determine the characteristics of ULF/VLF ocean-bottom noise. The OBSS, located at 38° 09.2′ N, 124° 54.4′ W at 3903-m depth, operated for over 6 years, from May 1966 to September 1972. Selected portions of essentially continuous FM tape data are being digitized to obtain ULF/VLF spectra and covariances during quiet and noisy times and during passage of vessels and earthquake wavetrains. It is planned to compare OBSS data with available source information on sea/ swell conditions, tidal currents (measured by a current meter on the OBSS), earthquakes, and shipping. Spectral levels appear to be above system noise for most frequencies above about 0.002 Hz. The pressure spectra agree with those obtained more recently [Cox et al.,Atmos. Oceanic Tech. 1, 237–246 (1984)], showing a strong minimum between about 0.03 and 0.1 Hz. The motion spectra from the OBSS seismometers are similar. Coherency between vertical motion and pressure is well above random for most of the spectrum below 0.5 Hz. There is clear evidence that ambient noise maxima rise and fall with weather conditions and water wave activity and that ULF noise, especially on the horizontal seismometers, has a tidal dependence. [Research supported by ONR.]
Finite difference modeling of acoustic pulse propagation in the atmosphere
Authors: Sparrow Victor W. , Raspet Richard
Published: October 2005

In a previous presentation [J. Acoust. Soc. Am. Suppl. 1 80, S104 (1986)], the use of a finite difference approximation of the time-dependent acoustic equations for arbitrary axisymmetric pulses was described. In that paper, however, only the application of these equations for a time harmonic simple source was demonstrated. In the presentation, the initial condition requirements on the acoustic variables' values composing an acoustic pulse will be addressed and results for the numerically simulated propagation of such a pulse will be shown. Further, an extension to the above model for atmospheric propagation currently being tested will be described that includes both classical and molecular relaxation absorption effects. Space permitting, the implementation of our programs on a CRAY X-MP supercomputer will be commented upon.
An on going study of the use of the boundary element method to solve some of the more commonly encountered industrial noise and vibration control problems
Authors: Mourad Kassem M. , Ciskowski Robert D. , Royster Larry H.
Published: October 2005

A study is under way to establish the advantages or disadvantages of using the boundary element method (BEM) to obtain solutions to some of the more commonly encountered noise and vibration control problems encountered in general industry. Initial investigations include predicting the steady-state and transient room-volume SPL response from internal and boundary acoustic sources for various boundary impedances. The numerical code developed to obtain the solutions of such problems was designed to run on the IBM family of personal computers and workstations.
Effect of background noise on sound power estimates using the sound intensity technique
Authors: Shirahatti U.S. , Crocker Malcolm J. , Raju P. K.
Published: October 2005

The presence of a background noise source affects the accuracy of the estimate of the source sound power level. In this paper, the results of a systematic experimental study on the accuracy of estimation of sound power levels in the presence of a background noise source are reported. A calibrated sound power source was used in these experiments. The sound power measurements were made using the two-microphone sound intensity technique. The errors in the estimates of the sound power level of the calibrated sound source were determined for different levels of background noise. Also, a systematic study was made on the relationship between the number of points of measurement on the enclosing surface defined around the source and the accuracy of the sound power estimate. It has been demonstrated that the accuracy of sound power estimates is greatly improved by continuous hand scanning of the enclosing surface instead of by making measurements at a small or moderate number of fixed points. The local sound pressure minus intensity index and the global sound pressure minus intensity index are used as indicators of the data quality. [Work supported by IBM, Charlotte, NC.]
Acoustic radiation from plates driven by multiple point random forces
Authors: Peng H. , Keltie R.F.
Published: October 2005

The problem of acoustic radiation from plates driven by multiple random point forces is analyzed in this study. First, the formulation of the sound power radiated by infinite thin plates (one dimension) under the action of multiple random point forces is derived. The reaction of the acoustic medium on the vibratory response of plates is taken into account. With the input spectrum being assumed as bandlimited white noise, the effects of input force correlation on the radiated sound power level are examined. The analysis is then extended to finite thin plates with four simply supported edges. The fluid loading effects are neglected. The exact expressions for the surface acoustic intensity and the radiated sound power are also derived. Assuming light damping, approximate solutions for acoustic intensity and sound power are obtained. An experimental study is carried out to measure the surface acoustic intensity patterns and compare with the analytical results. The effects of input force correlation on the acoustic intensity patterns and the sound power level are discussed.
Effect of a semicircular diffuser on the sound field in a rectangular room
Authors: Jie Pan , Bies D. A.
Published: October 2005

Previous experimental work has indicated the influence of a rotating diffuser upon the diffusion of a reverberation field, upon the radiation impedance of a sound source in a reverberation room, and upon the boundary absorptions. Some interesting results of this work need a quantitative interpretation while others suggest an effective investigation to obtain a detailed physical insight. In an effort to find an insight into and an interpretation of this work, an analytical approach is presented. This paper is an interim report on continuing research, and it reviews an investigation of the effect of a semicircular diffuser upon the two-dimensional sound field in a rectangular room. The numerical relationship between the resonance frequencies of the acoustical modes in the room and the diffuser orientation is given. The sound-pressure distributions of the acoustical modes are also given as the function of the diffuser orientation. A three-dimensional cavity is used for experimental verification. In this cavity, only the sound waves in the horizontal plane will be affected by a diffuser. The experimental and numerical results agree closely.
A generalized theory for acoustical holography and acoustical diffraction tomography
Authors: Gan W. S.
Published: October 2005

A generalzied theory is proposed to combine acoustical diffraction tomography (ADT) with acoustical holography (AH) to take account of the diffraction and scattering during transmission of sound waves through an inhomogeneous medium. This is usually neglected in AH but must be considered if the medium is inhomogeneous. In this generalized theory, AH is a nearfield solution of the Helmholtz wave equation and ADT is a farfield solution. The nonlinear case of strong scattering in the inhomogeneous medium is considered. The Feynman path integral is applied to the problem, and this can give the solution in closed form. Previous works [M. Slaney and A. C. Kak, paper at the 1985 IEEE Ultrason. Syrup.; and Z. Q. Lu, IEEE Trans. Ultrason. Ferroelectrics and Frequency Control UFFC-33 (6), 722-–30 (1986)] cannot give the solution in closed form. The path integral is based on the action principle (variational principle). Here, the Schrödinger equation will be used. The scattering integrals, perturbation expansion of the scattering potential, and the wavefunction are all expressed in terms of the path integrals. The algorithm of the imaging process for the generalized theory is given.
Harmonic singing and the Harmonic Choir
Authors: Hykes David
Published: October 2005

In harmonic singing, the singer emphasizes a selected upper harmonic of the vocal pulse. In this way, he can sing two notes at once, the fundamental and the selected harmonic. Members of the Harmonic Choir have developed techniques for the following effects: (1) The fundamental is constant while the selected harmonic varies; (2) the fundamental varies in a melody while the selected harmonic remains constant, leading to parallel harmony; (3) both the fundamental and the selected harmonic vary, either in converging or in diverging directions. The Choir together performs unaccompanied works which are composed but not scored. Different performances follow a common path but differ in many details.
Modal analysis of a Caribbean steel drum
Authors: Hansen Uwe J. , Rossing Thomas D.
Published: October 2005

Modes of vibration and coupling between adjacent note areas in a double-second steel drum are compared by several complementary techniques, including impact modal analysis, holographic interferometry, and recording sound spectra under varying conditions of damping. Each note has at least one overtone mode tuned to a harmonic of the fundamental frequency. Striking the A4b note area, for example, excites not only a second harmonic A5b in that same note area but also the strategically located A5b and A6b note areas adjacent to it. The coupling depends upon amplitude in a nonlinear way.
Physical correlates of perceptual similarity among synthesized approximations to selected targets
Authors: Gibian G. L. , Clements D. R. , Harnden E. N. , Williams H. E. F. , Massaro R. K.
Published: October 2005

Experiments are described to evaluate the relative merits of several measures for predicting the perceived closeness of synthesized approximations to selected steady-state portions of musical tones (“targets”). Approximations were synthesized by means of audio-rate frequency modulation [Chowning, J. Audio Eng. Soc. 21, 526–534 (1973)] using a computer program developed in a previous paper [Gibian et al., J. Acoust. Soc. Am. Suppl. 1 81, S46 (1987), and Audio Engineering Society Pre-print ♯2380]. The importance of including level- and frequency-dependent error weightings according to the Fletcher-Munson curves will be assessed. This information can be useful to composers who are blending electronic and traditional instruments in mixed ensembles.
Comprehensive study of analysis and synthesis of tones by spectral interpolation
Authors: Dannenberg Roger , Serra Marie-Helene , Rubin Dean
Published: October 2005

A new approach to the real-time generation of digital sounds uses a completely automated analysis/synthesis technique for natural sounds. This approach leads to a more efficient implementation than classical additive synthesis; moreover it allows dynamic spectral variations to be controlled with only a few high-level parameters. Additive synthesis devices require a large number of oscillators (one for each partial). This technique gives excellent results; however, it requires a large amount of computation, and a large amount of control data. On the other hand, fixed-waveform synthesis uses only one oscillator, but the results are of poor musical quality since there is no dynamic evolution of the spectrum. A new technique has been investigated in which spectral variation is achieved through spectral interpolation. The research shows that spectral interpolation provides high-quality synthesis including controlled timbral variation at little more than the cost of a table-lookup oscillator. The task of analyzing different kinds of instrumental sounds to produce control information for this technique has been automated.
Nonuniformity in timbre of string instruments
Authors: Krokstad Asbjørn
Published: October 2005

Mechanical musical instruments, may show great variation of all physical properties that correlate to timbre, even over intervals of a semitone. To examine preferences and limits of acceptability, timbre variations have been studied in a melodic context. A short melody of 13 tones covering a decime Was played by two professional musicians on three violins and a viola, and recorded at two microphone positions in an anechoic room. Each tone was processed digitally to reduce differences in loudness and in transients. In the first of two listening tests, dissimilarities in timbre between pairs of successive tones were evaluated. From these differences, integrated differences for different versions of the whole melody are calculated, and differences between versions. The possibility of identifying a certain player, a certain instrument or a certain microphone position may be given as relative numbers. The dissimilarity data are used for the synthesis of new versions of the melody with a range of nonuniformity in timbre. In the second listening test, acceptability of nonuniformity were evaluated. To study the dimensions and physical variabilities of timbre by sustained tones, multidimensional analysis of the subjective differences are performed and several spectral variables for describing the differences are tested.
Theoretical relationship between bow pressure and vibration amplitude, derived from a sticking-sliding force-velocity characteristic
Authors: Boutilion Xavier , Weinreich Gabriel
Published: October 2005

The Helmholtz motion for bowed strings may be described with the help of a characteristic between the bow's frictional force and the string velocity at the bowing point. Although this characteristic has never been dependably measured, some of its features are implied by players' common experience:. In general, the vibration amplitude changes with bow pressure or bow velocity. The relationships between these changes and the local evolution of the characteristic will be established and discussed. The case of perfect stickiness, in which the amplitude becomes independent of bow pressure, follows as a special case. [Work supported in part by NSF and the French Ministry of Culture.]
Impedance characteristics above cutoff for clarinet-like structures
Authors: Strong William J.
Published: October 2005

The cutoff frequency determined by the open tone holes on a clarinet-like structure is a useful means for characterizing the structure's musical properties. [See, for example, the discussion and data in A. H. Benade, Fundamentals of Musical Acoustics (Oxford U.P., London, 1976), Chaps. 21–22; note Sec. 22.5 in particular.] When impedance peaks above the cutoff frequency have amplitudes that are significant fractions of those below cutoff, they may influence the clarinet reed vibration. The input impedance of a clarinet-like structure (a cylindrical tube in the present case) is influenced by its input and output terminations. Three input terminations are considered: (1) rigid cap, (2) rigid taper, and (3) rigid taper with reed. Several output terminations are considered including (1) open end, (2) “infinite” tone hole lattice, (3) “short” tone hole lattice, and (4) short lattice plus bell. Input impedances are calculated for various combinations of the input and output terminations to demonstrate their effects on the amplitudes of the input impedance peaks both above and below the cutoff frequency.
Spectral characteristics of the Helmholtz resonator driven by an air jet
Authors: Khosropour Roset , Millet Peter
Published: October 2005

Motivated by an interest in the action of air jets on resonant systems in woodwinds, preliminary measurements have been made on the frequency spectra of several air jet driven Helmholtz resonators. Over the range of air jet velocities examined, stable oscillations dominated by a single frequency (under certain conditions harmonics are in evidence) occur in several intervals of jet velocity. Within each such interval the dominant frequency increases with increasing jet velocity. The span of frequencies associated with each interval includes the calculated Helmholtz frequency and, depending on the resonator and the interval, can be as much as an octave. The spectrum of the zone between the intervals is formed from an overlap of the extrapolated spectra of the adjacent intervals, i.e., it is marked by the appearance of two frequencies, where one is above the maximum frequency from the lower interval and the other is below the minimum frequency from the higher interval.
Acoustical behavior of a bass drum
Authors: Rossing Thomas D.
Published: October 2005

The bass drum is one of the loudest instruments in the orchestra. Its two large membranes may be set to equal tension, but more often the batter head is tuned to a higher tension. The sound spectrum contains many peaks that can be related to the vibration modes of a membrane. The low tension of the heads plus the strong coupling via the enclosed air lead to a large frequency difference (about 112 octave) between the two (01) modes associated with in-phase and out-of-phase motion of the heads. Sound decay rates for most modes are slightly greater when the two heads are at the same tension.
Geoacoustics of the shallow-water Arctic
Authors: Matthews J. E.
Published: October 2005

From the perspective of geology, the shallow-water regions of the Arctic Ocean can be divided into two provinces of approximately equal size. One is characterized by unglaciated, thick, prograding sedimentary deposits, which are found on the continental shelves of the Beaufort, Chukchi, and East Siberian Seas. The other is characterized by a glaciated bank-trough terrain, and includes the continental shelf of the Canadian-Greenland Arctic and the Barents, Kara, and Laptev Seas. From a low-frequency geoacoustics point of view, the prograding shelf province consists of a few to tens of meters of poorly sorted elastic sediment (1.5–1.6 km/ s) that overlies an acoustic basement of consolidated coarse sediment (1.7 km/s). The bank-trough province has two distinctly different environments: shallow bands that consist of zero to several meters of coarse sediment (1.6–1.7 km/s) overlying an acoustic basement of sedimentary rock (3.5 km/s), and deep troughs floored by a few tens of meters of poorly sorted sediment (1.5–1.6 km/s) over an acoustic basement of consolidated sediment (1.9 km/s). These generalizations must be considered tentative because of the critical scarcity of data; however, plane-wave reflection and normal-mode calculations are presented for these generic environments to demonstrate their acoustic similarities and differences.
Stiffness of the tectorial membrane in live Mongolian gerbils
Authors: Zwislocki J. J. , Serafini M. W. , Cefaratti L. K.
Published: October 2005

The classical model of hair-cell stimulation in the mammalian cochlea, according to which the tectorial membrane acts as a stiff anchor for the cells' sterocilia, requires the stiffness of the membrane in the radial cochlear direction to be greater than that of the sterocilia. This stiffness was measured by means of flexible, calibrated micropipettes inserted deep into the membrane at the location of the outer hair cells. They were introduced into the cochlea through an opening in the lateral wall of the scala media, as described previously [J. J. Zwislocki, S.C. Chamberlain, and N. B. Slepecki, J. Acoust. Soc. Am. Suppl. 1 81, S6 (1987)]. The stiffness was determined by moving the micropipettes radially and by measuring their bend and the resulting displacement of the tectorial membrane. According to the experiments, the radial stiffness of the tectorial membrane is several times smaller than the corresponding stiffness of the stereocilia determined by Strelioff and Flock (Hearing Res. 15, 19–28). This contradicts the classical model of cochlear hair-cell stimulation. The tectorial membrane appears to act on the stereocilia as a mass load rather than their stiff anchor.
Three-dimensional analysis of fluid flow between the tectorial membrane and the organ of Corti
Authors: Boulet Joseph A.M. , Holt J. Barney
Published: October 2005

Three-dimensional solutions for propagating waves in the fluid-filled gap between the tectorial membrane (TM) and the organ of Corti (OC) are obtained and included in an existing analytical model of the cochlea. The three-dimensional solutions give essentially the same coupling between TM and OC as that previously found with two-dimensional solutions. However, including the three-dimensional solutions gives rise to a new propagating mode. With two-dimensional waves in the gap, the model exhibits three ducts (two scalae and the inner sulcus) and two propagating modes. With three-dimensional solutions, the gap functions as a fourth duct. Consequently, a third propagating mode appears. The flow patterns associated with this mode are described, and the mode's significance is discussed [Work supported by NSF.]
Quantification of cerebrospinal fluid (CSF) contamination of scala tympani (ST) perilymph samples
Authors: Hara Akira , Salt Alec N. , Varghese Jogy , Thalmann Ruediger
Published: October 2005

Previously it has been reported that levels of most amino acids in small perilymph samples (< 100 nL) taken from ST through the round window (RW) of guinea pigs are distinctly different from those in CSF [A. N. Salt and R. Thalmann, Adv. Otorhinolaryngol. (in press)]. The composition of samples of larger volume more closely resembles CSF. Now an analysis of amino acid content has been performed as the sample volume withdrawn through the RW was systematically varied from 100 nL to 5 μL. Glycine, serine, glutamine, and alanine were measured by HPLC (Physiological Pico-Tag, Waters). Up to 200 nL, the glycine content of ST perilymph was similar to that of scala vestibuli. With larger volumes, the glycine content rapidly declined toward that of CSF. These data suggest that perilymph samples taken through the RW are contaminated with CSF if more than 200 nL is withdrawn. If ST perilymph is sampled through a small fenestra in the third turn, larger volumes (up to 2μL) can be withdrawn without significant CSF contamination, evidently because of the distance from the cochlear aqueduct. The data also suggest that glycine content can be used as an indicator of CSF contamination of ST perilymph samples.
Two-microphone measurement of the power-reflection coefficient in human ears
Authors: Murphy William J. , Tubis Arnold , Long Glenis R.
Published: October 2005

For each of eight normal hearing and eight abnormal hearing subjects, the acoustic intensity and power reflection coefficient spectra are measured using a two-microphone probe with probe tips ≈ 8 mm downstream from the point of sound delivery [W. J. Murphy, A. Tubis, and G. R. Long, J. Acoust. Soc. Am. Suppl. 1 81. S75 (1987)]. The tube of an Etymotic Research ER-2 sound delivery system passes through one port and the probe tips from a pair of ER-7 microphones pass through the second port of a latex ear mold. Results in the frequency range of 4–12 kHz are compared with previous measurements by Stinson et al. [J. Acoust. Soc. Am. 72, 766–773 (1982)], Hudde [J. Acoust. Soc. Am. 73, 242–247 (1983)], and Stinson [J. Acoust. Soc. Am. 77, 386–393 (1985); 79, 1003–1009 (1986)]. Hearing thresholds are also determined in order to identify any significant correlations in the objective and psychophysical measure. [Work supported by a grant from the Deafness Research Foundation.]
A receptor-coded cochlear implant: Speech discrimination and comprehension results
Authors: Wasserman Gerald S. , Miyamoto Richard T.
Published: October 2005

Cochlear implants are, among other things, artificial receptors. Like hair cells, they transduce acoustic energy into bioelectric signals that activate auditory nerve fibers. The contribution of receptor coding to perception in adult and juvenile deaf patients surgically fitted with cochlear implants has therefore been investigated. A filter-feedback system was used to simulate the signal processing of natural receptors. This system permitted controlled within-patient tests of the effect of receptor coding on speech perception; this effect was examined in two different ways: Speech discrimination was tested with CVC monosyllables that differed only in their final phoneme. Speech comprehension was tested with a modification of the SPIN test. All tests were run as two-alternative forced-choice tasks. Patients were completely alone in a windowless acoustic booth during testing; stimulation and data collection were under computer control. In every case, receptor coding produced a significant improvement in phoneme discrimination. The sentence comprehension task was more difficult and most patients were at or near chance in both experimental and control conditions; however, receptor coding can produce clearly significant performance in a patient whose control sentence comprehension is at chance.
Evaluation of a multichannel cochlear implant
Authors: Soli Sigfrid D. , Kirby Virginia M. , van den Honert Christopher , Widin Gregory P.
Published: October 2005

A wearable multichannel signal processor for stimulation of single-electrode cochlear implants has been field tested with two patients. Each channel in the processor, which is implemented in a digital signal processing chip, consists of a resonator followed by an instantaneous compressive nonlinearity. The channel outputs are digitally mixed for use with single-electrode implants. The resonators perform a spectral-to-temporal transformation of the input signal and the nonlinearities limit output level to emulate the response characteristics of normal auditory neurons. The resonator and nonlinearity parameters are adjusted to accommodate both the acoustic properties of speech sounds and the electrical dynamic range of the patient. Several processor configurations with different resonator and nonlinearity designs have been evaluated. The results of psychophysical tests, used to fit each processor configuration to the patient and measure speech performance in quiet and noise with each configuration, will be reported.
Channel interactions measured by forward-masked “place” tuning curves with multichannel electrical stimulation
Authors: Kirby Virginia M. , Nelson David A. , Soli Sigfrid D. , Fortune Todd W.
Published: October 2005

Simultaneous stimulation of multichannel intracochlear electrodes can give rise to peripheral and central channel interactions. The ability to eliminate or predict and control the interactions produced by a given electrode geometry is a processing goal for optimizing performance with a multichannel cochlear implant. Previous studies have used loudness summation and forward-masking pattern techniques to estimate interactions between channels of electrical stimulation. In this study, interactions between bipolar channels of analog electrical stimulation were estimated using a forward-masking paradigm with a fixed-level, fixed-location probe. By varying the electrode location of a 200-Hz, 300-ms sinusoidal masker and determining the level of the masker at each location necessary to just mask a 200-Hz, 10-ms probe, a “place” tuning curve was derived. The level of masker required at a given location to mask the probe depends on the amount of excitation produced by the probe and reflects, in part, the degree to which there is overlap of neural populations responding to each stimulus. These “place” tuning curves, which display interactions as a function of masker location were determined for several probe levels and probe locations. Results and implications for speech processing strategies will be discussed.
Effects of measured mode nonorthogonality on conventional matched field processing
Authors: Ioup Juliette W. , Ioup George E. , Rayborn Grayson H. , Del Balzo Donald R. , Feuillade Christopher
Published: October 2005

As discussed by Rayborn et al. (preceding abstract), the potential improvement achievable from the use of modal filtering in matched field processing is greatest when the measured modes are least orthogonal. To assess the ability of matched field processing using a conventional cross-correlation estimator to realize this potential, ambiguity surfaces for a Pekeris waveguide have been constructed employing (a) the pressure fields at the hydrophones and (b) the amplitudes resulting from modal filtering. The quality of the surfaces is quantified utilizing measures that compare the height of the source peak above the mean and the height of the source peak above the standard deviation of the surface. The results indicate that modal filter processing, for a given array of hydrophones, offers the most improvement when bottom depth, bottom type, and frequency combine to produce measured modes that are the least orthogonal, and yields no improvement when the measured modes are orthogonal.
Effects of correlated noise on the cross-spectral matrix in modal composition space
Authors: Smith George B. , Feuillade Christopher , Del Balzo Donald R.
Published: October 2005

Computer simulations of hydrophone cross-spectral matrices in a shallow-water waveguide were generated for signals with different mixtures of, correlated and white noise. These matrices were then mapped to modal composition space, a space populated by vectors whose elements are the amplitudes for the trapped modes. It was found that, in modal composition space, the cross-spectral matrix is not sensitive to the difference between correlated and white noise, but is sensitive to the difference between noise and signal. While the distribution of signal and white noise among the elements of the cross-spectral matrix is similar before and after the mapping to modal composition space, the distribution of correlated and white noise is not. Temporally discrete noise sources, which are correlated at the hydrophones, but not from sample to sample, make little or no contribution to the off-diagonal elements of the cross-spectral matrix in modal composition space. This fact has significant implications for matched field processing in low signal-to-noise situations.
Adaptive beamforming or matched field processing in media with uncertain propagation conditions
Authors: Baggeroer A. B. , Schmidt H. , Kuperman W. A. , Scheer E. K.
Published: October 2005

Adaptive beamforming or matched field processing provides high resolution with sidelobe control if accurate replica fields can be generated. The generation of these replica fields is a formidable problem requiring knowledge of the complex ocean propagation environment. On the other hand, the detection problem in the ocean may not require high resolution, whereas sidelobe control is still an important issue. Relaxing resolution requirements suggest that a certain tolerance incorporating uncertainty of the propagation conditions is permissible, or even desirable, because of the difficulties both in specifying the medium exactly and in identifying global peaks. This possibility of lowering the requirements on our knowledge of the environment is investigated with two methods: (1) by constructing multiple beam (constraint) algorithms and (2) by considering stochastic blurring by the medium. These two approaches are applied to plane-wave beamforming and matched field processing.
Matched catastrophe deconvolution with application to the inversion of marine seismic refraction data
Authors: Brown Michael G. , Bullwinkel Paul E.
Published: October 2005

The problem of extracting accurate estimates of the travel times of unresolved arrivals (e.g., the second and third arrivals within a triplication) from a set of noisy bandlimited measurements of a time-dependent acoustic wavefield is addressed. The method of solution presented is based on the assumption that the underlying caustic structure (or, equivalently, the travel time curve structure) of the wavefield is known. Because generic caustics associated with causal wavefields take on only certain forms, this is a weak assumption. Additionally, it is assumed that the medium structure varies as a function of depth only. This further restricts the class of observable caustic sections. These constraints are incorporated into a matched field processing approach to the deconvolution problem. The technique is demonstrated using a measured marine seismic refraction data set. A Herglotz-Wiechert inversion of the resulting kinematic data T(R) agrees well with previously published waveform inversions of the same data set. Advantages of the new approach over linearized waveform inversions are discussed.
Effects of nearby soft screens on idealized array reception
Authors: Martinez Rudolph , Westagard Ann
Published: October 2005

Arrays of sensors must often function in the neighborhood of diffracting and reflecting surfaces; these, in turn, represent effective passive sources that add to the incoming signal. The result is a computed wave-number spectrum contaminated by the presence of spurious targets in which the true sea source may appear shifted off its actual azimuth. Here, such effects are demonstrated for the fully analyzable situation of an unshaded array near a thin semi-infinite soft screen, taken either as a colinear aft baffle or a perpendicular backstop to the incident wave. Exact and asymptotic expressions have been derived to predict perceived strengths of virtual targets and associated azimuthal distorsions; their reported numerical evaluations could serve as benchmark results against which one might gauge the effectiveness of sophisticated processing methods under similar array installation conditions.
Improved time delay estimates of underwater acoustic signals using beamforming and prefiltering techniques
Authors: Ferguson Brian G.
Published: October 2005

The cross-correlation method for estimating the differential time delay for an underwater acoustic signal to arrive at two spatially separated receivers is described. The output amplitudes of the two receivers are cross correlated and the lag time at which the cross-correlation function peaks provides an estimate of the time delay. Often the time delay estimates of the signal are corrupted by the presence of noise. By replacing each of the (omnidirectional) receivers with an array of receivers and then cross correlating the beamformed outputs of the arrays, it is shown that the effect of noise on the time delay estimation process is substantially reduced. Both conventional and adaptive beamforming methods are implemented and the advantages of array beamforming (prior to cross correlation) are highlighted using both modeled data (for tutorial purposes) and experimental data. The performance of the cross correlator is further improved by using various prefiltering techniques to process the experimental data. Each of the prefilters—Hannan-Thomson, smoothed coherence transform, and phase transform—consists of a frequency-domain weighting function that is applied to the cross-spectral density function before the cross-correlation function is computed using the inverse Fourier transform. The prefilters are observed to enhance the estimation of the time delay by reducing the ambiguity associated with detecting the peak value of the basic (unweighted) cross-correlation function, which is oscillatory in nature.
Stepped FM signals for acoustic remote sensing of geoacoustic properties of bottom sediments
Authors: DeFerrari Harry A. , Nguyen Hien B.
Published: October 2005

Signals used for sub-bottom profiling are usually high-intensity transients generated by explosions, air guns, sparkers, etc. Each transient has a unique complex frequency spectrum and serial transmissions cannot be coherently averaged. Coherent acoustic signals generated by electromechanical transducers are not widely used, owing to the relatively low intensity or bandwidth limitations, but even weak signals can be averaged for long periods if the propagation medium is stable, to produce very high equivalent signal levels. Likewise, it appears possible to synthesize very broadband signals by coherently averaging narrow-band signals. Several narrow-band signals transmitted serially can be processed as though they were transmitted simultaneously to synthesize signals of any desired bandwidth—a method called stepped FM. A system has been developed and tested for transmitting and receiving pseudorandom code stepped FM signals. Signals with a bandwidth of 1700 Hz and intensity levels greater than 210dB after processing are used to probe the first few tens of meters of the bottom sediments. The resulting pulse duration, approximately 0.5 ms, allows for a separation of reflected from refracted paths. Travel times of refracted rays can then be used for tomographic inversions to yield compressional velocity profiles of the bottom sediments.
Signal design for Doppler processing
Authors: Haddad Ziad , Parkins Bowen
Published: October 2005

The Doppler sonar processing requires that the illuminating signal have a spectrum with low sidelobes over the range of frequencies at which Doppler shifts are expected. This is conventionally achieved by “windowing” the driving sinewave. Uncertainties in the amplitude response (threshold onset and linearity) of typical transducers make it desirable to have alternate means of obtaining low sidelobes. One approach is to transmit a series of nonuniformly spaced short pulses of constant duration and amplitude. Pulse start times are specified to yield the desired spectral shape using the approach of A. Ishimaru [IEEE Trans. Ant. Propag. AP-10, 691-702 (1962)] developed for space tapering antenna array elements. The resulting spectrum is not affected by nonzero threshold onset or limited linear range. The sidelobe reduction that can be achieved depends on the number of pulses, hence, on the amplifier bandwidth. Amplitude shading and varying pulse durations allow the design of a second type of signal, a windowed cw signal that does not depend on threshold onset and is less sensitive to amplifier bandwidth. The sidelobe reduction achievable with this second design depends on the linear range.
Interpretation of coherence estimates determined for time series of limited length
Authors: Leybourne Allen E.
Published: October 2005

Coherence of multichannel time series data has been calculated by the method of G. C. Carter and J. F. Ferrie [A Coherence and Cross Spectrum Estimation Program, Programs for Digital Signal Processing (IEEE,New York,1979), No. 2.3-1-18]. The properties of this statistic has been contrasted through simulation studies at known noise levels for short and long time series through a range of 40 data segments. Results at less than eight data segments are shown to be difficult to interpret; however, modest improvements in the coherence estimate can be made by adjustments indicated by these studies. A method of calculating coherence estimates through a technique referred to as frequency bin averaging is described. The results obtained are contrasted with the conventional method of Carter. Although reliable estimation of coherence for short time series data (transient data) is difficult to obtain in the conventional sense, the comparisons made should provide a basis for interpretation of coherence estimates as well as suggest corrections that may be applied to relatively short time series sequences. [Work supported by NORDA.]
The effect of multipath and sensor location offsets on the performance of a cross-correlation system
Authors: Bilazarian P.
Published: October 2005

The combined influence of multiple ray arrivals and sensor location offsets on the performance of a broadband cross-correlation system is investigated. The system consists of three sensors that are nominally positioned on a horizontal line. Source location parameters are determined from time-delay estimates obtained by cross-correlation techniques. An analytical model for the prediction of localization bias errors due to multipath and to offsets from nominal sensor positions is discussed. This model is applicable to an oceanic medium with a depth-dependent sound-speed profile. Results are presented for a variety of multipath conditions, such as those associated with direct path, surface duct, bottom bounce, and convergence zone types of propagation. The sensitivity of localization errors to variations in environmental parameters, such as sea state or bottom-reflection properties, is also investigated. It is demonstrated that multipath, especially when combined with sensor position offsets, can have a significant impact on system performance.
A fast bistatic reverberation and systems model
Authors: Kewley D. J. , Bucker H. P.
Published: October 2005

The performance prediction of bistatic active sonar systems requires the calculation of the reverberation and noise background along with the signal propagation characteristics. To consider the general case of a three-dimensional beam pattern for both source and receiver, and three dimensional noise fields, the prediction model tends to be expensive in computer time and memory requirements. The ASONAR model presented here incorporates the RUMBLE bistatic reverberation model [H. P. Bucker, J. Acoust. Soc. Am. Supp. 1 80, S 63 (1986)] and the DUNES directional noise model [R. W. Bannister, A. S. Burgess, and D. J. Kewley, J. Acoust. Soc. Am. Suppl. 1 80, S65 (1986)]. These newly developed models incorporate fast execution speeds and can run on IBM PC type microcomputers. By using SALT (Sound-Angle-Level-Time) tables, the propagation loss and reverberation to the many scattering areas are quickly determined. The scattering areas are increased in size as the ranges increase to keep the computation time within reasonable limits. The noise calculations are made using simplified propagation laws. ASONAR combines the outputs of RUMBLE and DUNES to calculate bistatic signal excess versus range from the receiver. Comparisons with other models and some experimental data are made. [Work supported by the NORDA NOP Program and NUSC.]
The design and performance analysis of barrel stave projectors
Authors: Jones D. F. , McMahon G. W.
Published: October 2005

There is growing interest in low-frequency, light-weight sound sources for underwater acoustics research and for potential future sonar applications. A design being studied at DREA is the barrel stave projector, which consists of a piezoceramic cylinder driving a set of annular staves in flexure. An electroelastic finite element model has been used to analyze the linear vibrations and acoustic radiations of projectors having outer diameters of about 8 cm and fundamental resonance frequencies below 1600 Hz in water. Prototype projectors that operate at electroacoustic efficiencies greater than 50% at resonance have been assembled and their measured acoustic properties have been compared to those predicted by the model. In addition, a brief discussion of the model parameters and limitations is given.
Seismic, acoustic, and ionospheric wave kinematics associated with moderate to large earthquakes
Authors: Warshaw S. I. , Followill F. E. , Mills J. M. , Albee P. R.
Published: October 2005

Unusually detailed Doppler radar records of ionospheric disturbances following two moderate-to-large dip-slip earthquakes were reported in 1984. These indicate that earthquakes can perturb the ionosphere by means of acoustic waves launched by the moving ground surface. For one earthquake {magnitude 6.5, Coalinga, CA, 2 May 1983 [J. H. Wolcott et al., J. Geophys. Res. 89, 6835-6839 (1984)]}, a seismic-aeroacoustic-ionospheric kinematic wave analysis was performed to explain radar signature timing. Surface Rayleigh waves were modeled and interpolated from main and after-shock seismic records, acoustic wave trajectories by acoustic ray tracing in a standard atmosphere were established, and radar detection altitudes were determined from radio rays traced through an ionosphere calculated from an ionogram measured close by. Our calculated results agree well with Doppler signature times observed on several 5- and 10-MHz radar beam paths 160–300 km from the epicenter. Similarities and differences were identified in the other earthquake {magnitude 7.1, Urakawa-Oki, Japan, 21 March 1982 [T. Tanaka et al., J.Atm. Terr. Phys. 46, 233–245 (1984)]} and its signatures were studied to assess commonalities in perturbation mechanisms. It is tentatively proposed that nonlinear acoustic propagation effects could be significant in shaping observed Doppler signatures. [Work performed under the auspices of USDOE by LLNL under contract W-7405-Eng-48.]
Acoustic-to-seismic coupling under winter conditions
Authors: Peck Lindamae
Published: October 2005

A field program to investigate acoustic-to-seismic coupling under winter conditions took place in Maine during December 1985-January 1986. The test area was packed sand with a top layer (30 cm) of loose sand. Site conditions were: 20-cm frost with no snow cover, 6-cm snow or 18-cm snow; and 45-cm frost, 28-cm snow. Microphones were at heights of 1/2–2 m or at the sand (snow) surface. Geophones were placed at the snow surface, the sand surface, and several depths in the sand layer. Acoustic-to-seismic coupling through frozen sand was investigated under controlled conditions using geophones in a sandbed at the USACRREL Frost Effects Research Facility. Test conditions were: dry sand, frozen or unfrozen; saturated sand, frozen or unfrozen; and thawing sand. The acoustic source for both investigations was blank pistol fire. Results on acoustic-to-seismic coupling through a snow layer and/or frozen sand are presented and contrasted with summer conditions for the frequency band 5–500 Hz. [Work supported by Directorate of Research and Development, U.S. Army Corps of Engineers Project 4A762730AT42.]
Low-frequency acoustic-to-seismic coupling in the summer and winter
Authors: Albert Donald G.
Published: October 2005

Experiments conducted in northern Vermont investigated acoustic-to-seismic coupling in the 5- to 500-Hz frequency band for ranges between 1 and 300m. The strongest coupling into the ground occurs as the air wave passes, with a measured ratio of about 0.001 cm s−1/Pa. The P waves are induced into the ground immediately under the source, and are the first arrivals, since they travel at the higher seismic wave velocity, but their amplitudes are about three orders of magnitude less than the motion coupled via the later-arriving air wave. A comparison of the summer and the winter recordings shows two major effects of a 25-cm-thick snow cover. Frequencies above 100 Hz are strongly attenuated, with signals recorded by geophones above and below the snow yielding a value of around 2 for the Q of snow at 30 Hz; the snow produces a strong waveguide effect that enhances the air-coupled Rayleigh waves. [Work supported by Directorate of Research and Development, U.S. Army corps of Engineers Project 4A762730AT42.]