摘要:

COVERT is a computer model which predicts the effects of stochastic volume scattering on the vertical spatial coherence of an acoustic signal along a single path (as determined by a mean sound‐speed field). The propagation path is termed a macro‐ray path and is described by a sequence of straight line segments. The program algorithm calculates the effects of scattering over each segment in turn as if it were in a homogeneous medium. The scattering term [J. J. McCoy and M. J. Beran, J. Acoust. Soc. Am.66, 1468–1481 (1979)] takes into account the angle of the path segment relative to the anisotropic scattering mechanism, and the scattering strength may vary with the depth of the segment. Major advantages of COVERT are that it is easy to use and relatively fast. Its current principal limitations are that it treats a single macro‐ray path and does not incorporate coupling between the volume scattering and subsequent effects of refraction and diffraction. Thus, COVERT's main utility is for single‐path, high‐frequency (above 100 Hz) problems, and as a basis of comparison for more complex models.

ISSN:0001-4966    

DOI:10.1121/1.2020235

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The canonical deep ocean sound channel is parametrized by the sound‐speed minimum, buoyancy frequency scale, and adiabatic fractional sound‐speed gradient. Variations in the sound‐speed parameters depend on spatial and temporal variations in temperature and salinity. These variations are described by probability density functions. The sound channel near the sound‐speed minimum is approximated by a Hirsch sound‐speed profile, and the low‐frequency acoustic field is derived. Analytic field density functions for received frequency, intensity, wave vector, and arrival time are derived by the application of the transformation of random variables, assuming the sound‐speed parameters are uncorrelated Gaussian random variables. The resulting field density functions are non Gaussian. Estimates for the sound‐speed parameters and associated variances are used to evaluate the field density functions.

ISSN:0001-4966    

DOI:10.1121/1.2020241

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Two techniques for studying wave propagation in random media, the quasi‐Rytov method and the smoothing method, have been applied to a problem involving radiation by a point source in a one‐dimensional random medium. Each method was used to calculate the mean field, and the results have been compared. It is found that the results are in substantial agreement outside a small region surrounding the source, but that inside this region they disagree. Some technical difficulties which are associated with the smoothing method, and which are related to this discrepancy, are discussed. It is concluded that, although the two methods yield essentially equivalent results (apart from the discrepancy in the near‐source region, which is generally not of practical importance), the quasi‐Rytov method is easier to apply than is the smoothing method. [Work supported by NORDA.]

ISSN:0001-4966    

DOI:10.1121/1.2020243

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

This study examined whether hearing‐impaired listeners could identify place of articulation for initial stop consonants, using information occurring early in the stop release; for normal‐hearing listeners this release information has been implicated as a highly important cue for place perception of initial stops. Place perception for the release bursts of initial stop consonants was tested for impaired‐(N= 23) and for normal‐hearing (N= 6) listeners. The burst were obtained from ten utterances each of the syllables /pad/, /kad/, /tad/, /bad/, /gad/, /dad/. Three test conditions were used: (1) the entire syllable; (2) the initial release transients alone; and (3) the aspirations following the transients of the voiceless consonants. For the normal‐hearing group, consonant perception was 100% when tested for the entire syllable; consonant place perception for the transients only, was 87%, and for the voiceless aspirations 67%. Among the hearing‐impaired listeners, mean stop place perception was about 60% when tested for the entire syllable. Tests of the burst segments only, yielded chance performance for the impaired listeners.

ISSN:0001-4966    

DOI:10.1121/1.2020248

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

In the acoustic speech signal it is well known that at the segmental level constant perception may result from widely differing acoustic signals. The existence of analogous phenomena in lipreading, however, has not been demonstrated. In principle, that demonstration would take the form of determining whether constant visual perception of a viseme (such as the /i,I/ high front vowel) is associated with significantly varying physical properties of the lipshapes in different phonetic contexts. In this study we present an analysis of physical and perceptual data derived from vowel lipreading stimuli aimed at defining appropriate physical measures of vowel lipshapes, examining a database of vowel lipshapes and associated lipreading confusions, and deriving criteria for perceptual invariance and physical cue variance in visible stimuli. Preliminary results based on five vowels in 11 CVC contexts indicate that perceptual constancy analogous to that in auditory perception may exist in lipreading, although further refinement of the physical cues and the definition of viseme are clearly needed. [Work supported by Department of Clinical Investigation, WRAMC.]

ISSN:0001-4966    

DOI:10.1121/1.2020254

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Few studies have documented the temporal characteristics of the speech of individuals with repaired palatal clefts. Acoustically defined segment durations were compared in the speech of three groups of subjects; cleft talkers with high intelligibility, cleft talkers with low intelligibility, and noncleft talkers. CVC nonsense syllables were produced within a carrier phrase at habitual and rapid speaking rates. The acoustic waveforms of the speech samples were interactively displayed, segmented, and measured employing digital signal processing techniques. The results indicated: (1) cleft speakers produced longer segments than did noncleft speakers; (2) some segment types were more prolonged than others in cleft speech; (3) vowel environment had stronger effects (particularly carryover effects) on duration of adjacent segments for cleft speakers than for noncleft speakers; and (4) when speaking rate was increased, noncleft and high‐intelligibility cleft groups reduced vocalic intervals more than consonant intervals. In contrast the low‐intelligibility cleft group showed the greatest reduction in consonant intervals. Temporal abnormalities thus exist in the speech of some individuals with cleft palate, and appear to be associated with reduced intelligibility.

ISSN:0001-4966    

DOI:10.1121/1.2020259

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Recently, several products have become available to consumers who wish to enhance the spatial qualities of their hi‐fidelity audio systems in homes and automobiles. These products capitalize on the use of electronic circuitry and speaker design which allow for the extraction and synthesis of interaural temporal and intensitive disparities present in records and in listening rooms. While some of these products appear to widen greatly the “sound‐stage” produced by two normally placed loudspeakers, others utilize four or more separate loudspeakers to increase the apparent front‐to‐back size of the listening room. In addition to presenting the theoretical basis for such devices, we will discuss both approaches and evaluate them in terms of their entertainment value and their ability to produce more “realistic” sound images.

ISSN:0001-4966    

DOI:10.1121/1.2020264

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Mammals that use echolocation, or biological sonar, to perceive the locations of objects in their environments have unusually acute capacities for localizing sound (echo) sources and for perceiving reverberation since they rely upon echolocation for spatial perception. Both bats in air and porpoises in water determine the distance to targets from the arrival time of echoes. The sonar signals which they emit are directional, with the possibility that porpoises may transmit different waveforms as well as different signal amplitudes in different directions. The hearing of echolocating animals is also directional, with reception through the external‐ear system in bats and through the lower jawbone in porpoises. Directional hearing provides binaural cues for horizontal localization of targets. In bats, the external ear has directional amplitude and phase responses which the bat uses for vertical localization of targets. In porpoises it is possible that the transmitted waveform contributes to vertical localization by analogous means. The acuity of localization in echolocating animals is adequate to provide acoustic images of orientation in three dimensions.

ISSN:0001-4966    

DOI:10.1121/1.2020267

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

All harmonic distortion analyzer instruments may not give the same reading when measuring a particular audio range signal. A design method is presented that can supply a signal with a precisely determined amount of harmonic distortion. Digital signal generation technique is used. Factors which determine the accuracy of the signal are examined. An instrument was designed using the outlined principles. With it, the operator can control the frequency of operation, the harmonic, and the percentage of distortion. Results from the sample constructed are presented. Performance of a number of distortion analyzers are shown.

ISSN:0001-4966    

DOI:10.1121/1.2020269

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Analytic propeller noise prediction involves a sequence of computations culminating in the application of acoustic equations. This paper describes the prediction sequence currently used by NASA in its ANOPP (Aircraft Noise Prediction) program. No attempt is made here to review the state of the art of noise prediction. Some elements of this sequence represent classic results while other represent the most recent publications. The elements of the sequence are called program modules. The first group of modules analyze the propeller geometry, the aerodynamics, including both potential and boundary layer flow, the propeller performance, and the surface loading distribution. This group of modules is based entirely on aerodynamic strip theory. The next group of modules deals with the actual noise prediction, based on data from the first group. Deterministic predictions of periodic thickness and loading noise are made using Farassat's time‐domain methods. Broadband noise is predicted by the semi‐emipirical Schlinker‐Amiet method. Nearfield predictions of fuselage surface pressures include the effects of boundary layer refraction and (for a cylinder) scattering. Farfield predictions include atmospheric and ground effects. Comparisons are made to experimental data from subsonic and transonic propellers and NASA's future directions in propeller noise technology development are indicated.

ISSN:0001-4966    

DOI:10.1121/1.2020275

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP