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  • 1
    Online Resource
    Online Resource
    Ovid Technologies (Wolters Kluwer Health) ; 2016
    In:  JONA: The Journal of Nursing Administration Vol. 46, No. 1 ( 2016-01), p. 49-54
    In: JONA: The Journal of Nursing Administration, Ovid Technologies (Wolters Kluwer Health), Vol. 46, No. 1 ( 2016-01), p. 49-54
    Type of Medium: Online Resource
    ISSN: 0002-0443
    Language: English
    Publisher: Ovid Technologies (Wolters Kluwer Health)
    Publication Date: 2016
    detail.hit.zdb_id: 2007563-7
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  • 2
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2007
    In:  The Journal of the Acoustical Society of America Vol. 121, No. 5_Supplement ( 2007-05-01), p. 3064-3064
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 121, No. 5_Supplement ( 2007-05-01), p. 3064-3064
    Abstract: In atmospheric acoustics, formulation of a reactive, time-domain boundary condition (TDBC) at the ground surface is a challenging problem since many commonly used models of the ground are noncausal and have a slowly decaying response. A correctly formulated TDBC can significantly simplify finite-difference time-domain simulation of outdoor sound propagation, which is a rapidly developing field in computational acoustics. In the present paper, approaches are developed that enforce causality and improve computational efficiency of TDBCs. First, an approach is developed that allows one to derive a causal TDBC for any impedance model of the ground. A Pade approximation is used to represent the characteristic admittance. Then, using fractional derivatives, a causal TDBC is formulated. This method is illustrated by formulation of TDBCs for the Zwikker-Kosten (ZK) and Attenborough models of the ground. Furthermore, by approximating the fractional derivatives as a summation of decaying exponentials, an effective recursive algorithm for implementation of these TDBCs is outlined. Second, it is demonstrated that approximating the known TDBC for the ZK model as a summation of decaying exponential functions significantly improves the computational efficiency of the method.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2007
    detail.hit.zdb_id: 1461063-2
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  • 3
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2017
    In:  The Journal of the Acoustical Society of America Vol. 141, No. 5_Supplement ( 2017-05-01), p. 3885-3885
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 141, No. 5_Supplement ( 2017-05-01), p. 3885-3885
    Abstract: In this work, the Green’s function of an atmospheric acoustic propagation channel is extracted from experimental measurements. Of particular interest, is the accuracy of Green’s function retrieval methods in ideal and non-ideal situations. To this end, an emitting acoustic source (impulsive and audible) is placed at a distance of more than 90 meters from receiving tri-axis microphone arrays in open (ideal) and wooded areas (non-ideal) in southern Maryland. Green’s function retrieval methods are employed and investigated on the collected data. Green’s function retrieval by crosscorrelation has been successful in various applications despite the limitations of its lossless medium and equipartitioned wavefield assumptions. To overcome the violation of these assumptions, the multidimensional deconvolution has been proposed. Comparisons of the results between impulsive and audible sources using the two methods will be presented. The effects of multipath, source-receiver distance, temperature, and wind will be discussed.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2017
    detail.hit.zdb_id: 1461063-2
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  • 4
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2004
    In:  The Journal of the Acoustical Society of America Vol. 115, No. 5_Supplement ( 2004-05-01), p. 2596-2596
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 115, No. 5_Supplement ( 2004-05-01), p. 2596-2596
    Abstract: Acoustic travel-time tomography of the atmosphere allows one to retrieve the virtual temperature and wind velocity fields, and to monitor their evolution in time. The temperature and wind velocity fields in a horizontal slice near the ground have been successfully retrieved by several research groups. One future direction for acoustic tomography of the atmosphere is the retrieval of detailed time-varying volumetric wind and temperature fields. The ability to retrieve these fields is strongly dependent upon the system configuration, e.g., the number and placement of sources and sensors, the geometry of the array, the environmental effects, and the type of the source. A sensitivity analysis for the retrieval of the temperature and wind velocity fields as a function of sensor configuration has been performed using an acoustic tomographic array simulator. We discuss the simulation model and results of the sensitivity analysis to some baseline propagation cases.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2004
    detail.hit.zdb_id: 1461063-2
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  • 5
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2003
    In:  The Journal of the Acoustical Society of America Vol. 113, No. 4_Supplement ( 2003-04-01), p. 2271-2271
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 113, No. 4_Supplement ( 2003-04-01), p. 2271-2271
    Abstract: There are many physical situations in which a received signal may be modeled as a complex Gaussian random variable. One such situation occurs when an acoustic wave is strongly scattered as a result of propagation through a random medium such as the atmosphere or ocean. However, there are also many conditions under which this model fails to provide an adequate representation. A specific example is the geometric acoustics regime, in which diffraction and scattering by the medium are both weak and the variance of the phase fluctuations is much larger than the variance of the log-amplitude fluctuations. The reduced wavefunction is the wavefunction of a signal propagating in an inhomogeneous medium normalized by the wavefunction in free space. A statistical model is developed for a reduced wavefunction whose real and imaginary components are Gaussian random variables with unequal covariances. A linear transformation is performed and the probability density function of the received signal at a passive array is calculated. The Fisher information is calculated for special conditions of the transformation that are of interest to acoustic beamforming. The coupling of the Cramer–Rao lower bounds on the parameter estimates (e.g., the angles of arrival, source phase, and medium parameters) is addressed.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2003
    detail.hit.zdb_id: 1461063-2
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  • 6
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2298-2298
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2298-2298
    Abstract: The performance bounds of an atmospheric acoustic array operating in a turbulent medium with fluctuations described by von Kármán’s spectrum are investigated. This treatment considers a single monochromatic source and no multiple-path effects. The Cramer–Rao lower bounds of the azimuthal and zenith angles of arrival (AOAs) are calculated. The normalized range (to wavelength), turbulence parameters, and signal-to-noise ratio are introduced as additional unknowns. It is found that the Cramer–Rao lower bounds of the AOAs increase significantly in strong turbulence at large values of the normalized range. For weak turbulence and moderate values of the normalized range, the signal-to-noise ratio is the dominating factor. For an incident plane wave, the estimates of the AOAs will decouple from the estimates of the other parameters with the appropriate choice of array geometry; however, the estimates of the azimuth and zenith are coupled to one another due to the constraints of the turbulence model.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
    detail.hit.zdb_id: 1461063-2
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  • 7
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2003
    In:  The Journal of the Acoustical Society of America Vol. 113, No. 4_Supplement ( 2003-04-01), p. 2312-2312
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 113, No. 4_Supplement ( 2003-04-01), p. 2312-2312
    Abstract: In the geometric acoustics regime, a propagating wave is weakly diffracted and weakly scattered by the medium. The variance of the real component of the signal is much less than the variance of the imaginary component, thus the signal may not be modeled as a complex Gaussian random variable (whose real and imaginary components have equal variance), as is often done in the Rytov extension region, where both scattering and diffraction are strong. A statistical model for a signal in the geometric acoustics regime has been previously developed [S. L. Collier and D. K. Wilson, J. Acoust. Soc. Am. 111, 2379 (2002)] and its properties have been further investigated [S. L. Collier and D. K. Wilson, ASA April 2003 Meeting on Signal Processing (submitted)] . This statistical model is applied here to an acoustic wave propagating in a random medium with fluctuations described by von Kármán’s spectrum. Additive white Gaussian noise is also considered. The correlation functions of the phase and log-amplitude fluctuations for a von Kármán spectrum are derived in the geometric acoustics limit. The Cramer–Rao lower bounds (CRLBs) on the angle-of-arrival estimates are calculated assuming multiple unknown parameters. The range dependence of the CRLBs is studied in detail.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2003
    detail.hit.zdb_id: 1461063-2
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  • 8
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2010
    In:  The Journal of the Acoustical Society of America Vol. 127, No. 3_Supplement ( 2010-03-01), p. 1779-1779
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 127, No. 3_Supplement ( 2010-03-01), p. 1779-1779
    Abstract: Refraction, turbulent scattering, and other atmospheric propagation effects complicate the performance of outdoor microphone arrays used to infer source bearings. In principle, with good knowledge of the atmospheric profiles, one can compensate for the refraction effects. However, the random angle-of-arrival variations induced by atmospheric turbulence limit array performance even when the signal-to-noise ratio is high. The turbulent scattering effect can be viewed as a coherence loss between elements of the beamforming array. Most research to date on acoustic signal coherence in the atmosphere has dealt with line-of-sight (LOS) propagation paths. A number of complications characteristic of real turbulence have been introduced; these include statistical inhomogeneity, anisotropy, and intermittency of the turbulent eddies. The LOS theory has also been extended to include sensor displacements longitudinal to the propagating wavefronts as well as transverse displacements. Formulations for the Cramer–Rao lower bound (CRLB) on the angle-of-arrival accuracy have been developed from the LOS theory. Based on limited experimental data and modeling, actual performance appears to be substantially worse than the CRLB. Recently, numerical methods have been introduced for solving second-moment parabolic equations, which offer the possibility of incorporating non-LOS effects into coherence calculations.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2010
    detail.hit.zdb_id: 1461063-2
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  • 9
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2010
    In:  The Journal of the Acoustical Society of America Vol. 127, No. 3_Supplement ( 2010-03-01), p. 1780-1780
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 127, No. 3_Supplement ( 2010-03-01), p. 1780-1780
    Abstract: It is well known that atmospheric turbulence can negatively impact the performance of acoustic beamformers. While many beamformers, especially adaptive ones such as minimum variance distortionless response (MVDR), may be robust when the turbulent fluctuations are mild to moderate, they fail when the fluctuations are large. Other methods, such as maximum likelihood estimation, may be used to mitigate the effects of turbulence by directly incorporating the physics of the propagation medium into the assumed model of the acoustic signal (through the covariance and mean). When using synthesized data, the previously developed maximum likelihood estimator (MLE) for the azimuthal angle of arrival was found to outperform classical beamformers. However, in reality the atmosphere does not exhibit the exact behavior of the assumed atmospheric model, or all the required input parameters, such as the meteorological data, for the atmospheric model are not available. Therefore, we compare the performance of methods such as the MLE to that of classical and parametric methods, such as MVDR, multiple signal classification, and matched subspace detector, for data collected during a variety of atmospheric conditions. We critically examine the expense of gained accuracy over computational speed.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2010
    detail.hit.zdb_id: 1461063-2
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  • 10
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2004
    In:  The Journal of the Acoustical Society of America Vol. 116, No. 4 ( 2004-10-01), p. 1889-1892
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 116, No. 4 ( 2004-10-01), p. 1889-1892
    Abstract: A general set of time-domain equations describing linear sound propagation in a rigid-frame, gas-saturated porous medium is derived. The equations, which are valid for all frequencies, are based on a relaxational model for the viscous and thermal diffusion processes occuring in the pores. The dissipative terms in the equations involve convolutions of the acoustic fields with the impulse response of the medium. It is shown that the equations reduce to previously known results in the limits of low and high frequencies. Alternative time-domain equations are also derived based on a Padé approximation.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2004
    detail.hit.zdb_id: 1461063-2
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