摘要:

Based on the warping method and the generalized eigenvalue problem the range‐independent solution is extended to that of range dependence. In this method, coupling in range is analytical and may be either one way or two way. Since the range steps are not dependent on finite‐difference schemes, the optimal range step at each point is determined. This leads to two advantages: (1) There is no need for input of range step; and (2) the optimal range step is predetermined by several conditions, thus avoiding overkill and reruns to determine stability. Comparisons with other range‐dependent methods are presented. [Work sponsored by ONR, the Naval Res. Lab., and the Univ. of New Orleans.]

ISSN:0001-4966    

DOI:10.1121/1.419227

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Berenger’s perfectly matched layer (PML) has recently been proved to exist for elastodynamic equations [Chew and Liu, Schlumberger‐Doll‐Res. Rep. (1995)]. This fictitious material absorbs all waves with an arbitrary incident angle and an arbitrary frequency without giving rise to any reflections. Therefore, when used as an absorbing boundary condition (ABC) at the computational edge in a finite‐difference method, the PML provides orders of magnitude higher absorption than other existing ABCs. In this work, the PML is further extended as an ABC for a finite‐difference simulation of acoustic waves in viscous media. For such attenuative media, an additional term involving the time‐integrated wavefield is introduced to account for the coupling between the attenuation from the PML and the normal viscous attenuation. This ABC is highly effective in absorbing outgoing waves at the computational edge even when a dipping interface intersects the outer boundary. This new material ABC is ideal for parallelization on multiprocessor computers. The algorithm is validated by analytical and numerical solutions. Various two‐ and three‐dimensional numerical simulations will be shown to demonstrate the applications in underwater acoustics and other areas. [Work supported by a Presidential Early Career Award for Scientists and Engineers through EPA and by Sandia National Laboratories.]

ISSN:0001-4966    

DOI:10.1121/1.419314

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Standards for vibration exposure promulgate different limits for hand–arm vibration (HAV) and for whole‐body vibration (WBV). The discrepancy between the two should be explainable on some rational basis [Griffin,HandbookofHumanVibration(1990)], but there is no obvious difference in the mechanism of pathogenesis to the skin at each of the sites. The difference may arise because the endpoints chosen as the basis of each of the standards are different. One approach to estimating dose response [Jarabek, Toxicol. Lett.79, 171–184 (1995)], characterizes the steps in developing a model as proceeding from a protective to a predictive paradigm as data progress from qualitative to quantitative. To develop vibration standards along this continuum, certain data must be developed: Among these are identification of performance effects of HAV, identification of long‐term effects of WBV, and clarification of the impedance of the affected biological structures. Some of these areas have been addressed in recent revisions of ISO TC108/SC4 standards. Other activities will be proposed to further the evolution of human biodynamic assessment.

ISSN:0001-4966    

DOI:10.1121/1.419177

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Consider a two‐dimensional waveguide consisting of a rigid wall and a parallel wall having uniform specific impedance. When the impedance is purely imaginary, the characteristic equation is real. In the springlike case, the eigenvalue of the fundamental mode (defined as the mode that varies most slowly in the transverse direction) is imaginary, whereas all other eigenvalues are real. This type of mode does not exist if the impedance is masslike. Morse and Ingard [TheoreticalAcoustics(McGraw‐Hill, New York, 1968), p. 496] fail to recognize the differing behavior of the fundamental mode for either type of lossless impedance, and subsequently no one seems to have rectified this oversight. The fundamental mode in the springlike case behaves much like the plane mode in a hard‐walled waveguide in that it has no cutoff frequency, but the phase speed of this mode is subsonic at all frequencies. Introducing a resistive part to the impedance, corresponding to dissipation at the wall, moves the respective eigenvalues off the real or imaginary axis. The presentation will discuss the phase and group speeds, and the transverse mode shapes, in ideal and lossy cases.

ISSN:0001-4966    

DOI:10.1121/1.419181

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Of concern are distortionless surface stress waves propagating in a medium which may be nonuniform relative to depth. A similar problem was posed by Burkeetal. [Q. Appl. Math. 183–194 (1976)] where a unidimensional electrical ladder network model, nonuniform along the principle axis, was used in the investigation. Presently, a two‐dimensional development is given based on Hooke’s law and Newton’s law leading to the well‐known Rayleigh‐waves formulation in a uniform medium. It will be seen that broadband high‐pass propagation modes exist along the principle surface. As these propagate along the surface, they are accompanied by a phase change, linear with distance, in the frequency domain, and an associated symmetry change, periodic with distance, in the time domain. Applications may be found in high‐frequency acousto‐optics,insituultrasonic monitoring of elongated bodies, high‐frequency SAW filters, and whenever surface acoustic waves are employed in an environment of high precision or relatively large distances. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.419187

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The rotorcraft noise model (RNM) is a multiplatform computer program being developed for NASA‐Langley Research Center which calculates sound levels at receiver positions on the ground from helicopter and tiltrotor operations. RNM calculates levels at positions on a uniform grid or a specific defined locations. The basic computational model written in FORTRANcalculates maximum A‐weighted sound level, sound exposure level (overall, C‐ and A‐weighted), and effective perceived noise level (EPNL). Contained within the program are sound propagation and EPNL algorithms that are the same as, or equivalent to, corresponding algorithms in NASA’s Aircraft Noise Prediction Program (ANOPP). Acoustic properties of the noise source are defined by a set of sound‐pressure noise spheres. One set of noise spheres provides broadband data in the form of one‐third octave band sound‐pressure levels. The other set of noise spheres provides narrow‐band data in the form of pure‐tone sound‐pressure levels and phase. RNM outputs a file that is input to NMPLOT, a noise contouring/smoothing program used by the Department of Defense (DOD) and other government agencies. This paper will describe the basic element of the noise model and compare the noise models prediction with XV‐15 noise data collected by NASA‐Langley.

ISSN:0001-4966    

DOI:10.1121/1.419250

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A program for predicting the sound power level from earth‐moving equipment is under development in SHI. Fan noise is one of the major sources, whose contribution is greater than 40% of the total radiated noise power. The experiments show that both the blade passing frequency (BPF) component and the broadband component are important, and the directivity pattern is not symmetric with respect to the plane between the upstream and downstream flow. A noise source model of the fan radiator combination for predicting both the directivity and the frequency spectrum is developed from the experimental measurements. In this paper, a physical interpretation is made for modeling the types, shapes, and locations of the simple sources depending on the fan noise generating mechanisms. The minimum number of measurements for the cross‐spectral approach are also considered based on the tolerable accuracy and the physically interpretable simplicity.

ISSN:0001-4966    

DOI:10.1121/1.419234

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This study examines the ways in which jazz musicians use rhythmic alterations of standard ballad melodies for expressive purposes. Without altering a single pitch, these performers take melodies which are so well‐known as to be cliched and make them sound fresh. The primary technique is that of extensive rubato, where the melody is played at a varying tempo against the steady rhythmic background of a rhythm section (typically drums, bass, and piano). This results in a displacement of the melodic note relative to its nominal position against the background. Types and degrees of rhythmic variation are shown to depend on motivic structure (parallel motives are performed in similar ways, preserving the originals proportional rhythmic structures), metric emphasis (downbeats are treated differently than other locations), and harmonic structure (melodic tones belonging to the underlying harmonies are rhythmically displaced in a more extreme manner than tones outside the harmonies). Consideration is given to the way in which very familiar tunes are treated compared with less‐familiar melodies (for example, ‘‘My Funny Valentine’’ compared with ‘‘Goodbye, Pork‐Pie Hat’’), leading to discussion of the role of long‐term memory in expressive performance. Analyses are given of several melodies, taken from commercial recordings by master improvisors.

ISSN:0001-4966    

DOI:10.1121/1.419239

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

It is well known that cochlear pathology produces alterations in neurons of the cochlear nucleus, but there are few data that directly relate such neuronal changes to the physiology of auditory nerve fibers and structure of auditory nerve endings. These relationships are addressed in a naturally occurring model of deafness, the congenitally deaf white cat. Cats with no intact organ of Corti exhibited no measurable hearing thresholds and had severely reduced spontaneous activity in single auditory nerve fibers. Furthermore, there was structural atrophy of endbulbs of Held in the anteroventral cochlear nucleus, and hypertrophied synapses. These hypertrophied synapses were characterized by large postsynaptic densities with highly thickened membrane specializations and markedly diminished numbers of synaptic vesicles. Cats with elevated but measurable hearing thresholds had normal‐appearing organs of Corti with abundant spontaneous single unit activity but infrequent sound‐evoked activity. These partially deaf cats displayed modest atrophy of their endbulbs and exhibited more normal appearing synapses. The data reveal that early onset partial or total deafness that is uncorrected for 6 months or more produces unambiguous alterations in synaptic structure. These observations are relevant to selection strategies for human cochlear implant candidates. [Work supported by NIH/NIDCD.]

ISSN:0001-4966    

DOI:10.1121/1.419240

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The past quarter century of research on the development of speech perception has documented a number of important capacities that infants possess for recovering information from the speech signal. Such capacities provide the infant with the means to initially categorize speech input. Yet, because individual languages differ greatly in their sound structures, any language‐general capacities for processing speech must eventually give way to ones that are more appropriately tailored to the specific language spoken in the infant’s native environment. Recent studies suggest that during the second half of their first year, infants are learning about the specific properties and organization of native language sound patterns. Moreover, there is evidence that infants begin to adapt and adjust their capacities to the structure of their language input. In addition to outlining some of the landmarks observed in the development of speech perception capacities during the first year, some consideration will be given to how these capacities interact with the input to facilitate the acquisition of a native language.

ISSN:0001-4966    

DOI:10.1121/1.419245

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP