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May 1989

Volume 85, Issue 5, pp. 1819-2249

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Phase aberration correction in medical ultrasound using speckle brightness as a quality factor

Levin Nock, Gregg E. Trahey, and Stephen W. Smith

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1819-1833 (1989); (15 pages) | Cited 21 times

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Medical ultrasonic images are degraded by tissues with inhomogeneous acoustic velocities. The resulting phase aberration raises the off‐peak response of the imaging system’s point spread function (PSF), decreasing dynamic range. In extreme cases, multiple images of a single target are displayed. Phase aberration may become a limiting factor to image quality as ultrasonic frequency and aperture size are increased in order to improve spatial resolution. A method is proposed to correct for unknown phase aberration, which uses speckle brightness as a quality factor. The phase delays of a phasd array transducer are modified, element by element, to maximize mean speckle brightness in a region of interest. The technique proposed is analogous to the Muller–Buffington correction technique, [R. A. Muller and A. Buffington, J. Opt. Soc. Am. 64 (9), 1200–1209 (1974)] used to adaptively focus incoherent optical telescopes. The method is demonstrated using a computer model with several different simulated aberration profiles. With this model, mean speckle brightness is calculated using the two‐dimensional PSF. Experiments have also been conducted in which speckle brightness is shown to increase as the phase delays of an ultrasonic scanner are modified in order to compensate for a rippled aberrating layer made of silicone rubber. The characteristics of the proposed method, and the possibility of employing it clinically to correct for unknown inhomogeneities in acoustic velocity, are discussed.
Show PACS
43.20.Fn Scattering of acoustic waves
43.80.Qf Medical diagnosis with acoustics
87.63.D- Ultrasonography

A slender‐body approximation in scattering theory

Michel Tran Van Nhieu

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1834-1840 (1989); (7 pages) | Cited 1 time

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The problem of the scattering of a plane wave by a general slender body of arbitrary cross section with a constant surface impedance is treated by a formalism based on the matched asymptotic method. It is shown that the scattered pressure may be determined by the resolution of a set of 2‐D problems when using the slender‐body approximation that is valid for wavelengths of the order of magnitude or greater than the characteristic cross‐section length of the body. In the case of axisymmetric bodies, an expression for the scattered field can be obtained explicitly. The contributions of the insonified end section are analyzed from a paraboloidal geometry model, and simplified solutions are proposed for rigid and soft conditions. Some theoretical and numerical results for the farfield directivity are presented to illustrate the theory.
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43.20.Fn Scattering of acoustic waves

Acoustic eigenfrequencies of cavities with an internal obstacle: A modified perturbation theory

James B. Mehl and Robert Nyden Hill

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1841-1851 (1989); (11 pages) | Cited 2 times

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The eigenfrequencies of a hard‐walled acoustic cavity resonator are found by solving the Helmholtz equation with Neumann boundary conditions. A cavity C with an internal hard obstacle B of vanishing size was considered. Because the perturbation is never small in the neighborhood of the obstacle even in the limit of a small obstacle, a modified perturbation theory is required. A formalism was developed and tested by calculating the eigenfrequencies of a cylinder with an internal sphere on the axis. The results are compared with theoretical and experimental values determined by other investigators. The difference between the perturbation calculations and the numerical results varies from less than 10−5 to about 0.02 of the unperturbed eigenfrequency, depending on the size of the perturbing sphere and the mode.
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43.20.Ks Standing waves, resonance, normal modes

Green’s functions for propagation of sound in a simply moving fluid

Akhlesh Lakhtakia, Vijay K. Varadan, and Vasundara V. Varadan

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1852-1856 (1989); (5 pages)

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Two approaches involving the spatial and temporal Fourier transforms have been used to derive time‐ and space‐dependent Green’s functions pertinent to the propagation of sound waves in a fluid that is moving with a constant velocity v. The two approaches give rise to differing interpretations of the observations made by a stationary observer vis‐math‐vis those made by an observer moving with the fluid. The properties of the causal and the noncausal Green’s functions are analyzed, and are shown to be equivalent.
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43.20.Rz Steady-state radiation from sources, impedance, radiation patterns, boundary element methods
43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect

Finite amplitude effects in a dual frequency acoustic beam

M. A. Foda and J. H. Ginsberg

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1857-1871 (1989); (15 pages)

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When an axisymmetric, bifrequency transducer mounted in a rigid baffle is excited at acoustic Mach numbers that are a relatively large fraction, the result is a dual frequency sound beam that exhibits harmonic and intermodulation distortion. The present analysis of this problem develops a perturbation solution based on a wave equation that consistently accounts for nonlinearity and diffraction. The linearized problem is described by a King integral for the sound beam at each primary frequency. Asymptotic analysis using Laplace’s method of integration is used to find the second‐order potential. The method of renormalization then leads to a uniformly accurate expression for the acoustic pressure. A technique for improvement in computational efficiency is developed by interfacing the King integral predictions to a farfield model for quasispherical waves. Propagation curves for parametric arrays obtained from the model compare favorably with experimental observations.
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43.25.Lj Parametric arrays, interaction of sound with sound, virtual sources

Complex ray methods for acoustic interaction at a fluid–fluid interface

Evan K. Westwood

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1872-1884 (1989); (13 pages) | Cited 6 times

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The development of a systematic method for finding the reflected and transmitted fields due to a point source in the presence of a plane, penetrable interface is presented. This approach is based on the classical method of steepest descent, but the plane‐wave reflection and transmission coefficients are allowed to influence the location of the saddle points and their steepest descent paths. As a consequence, saddle points are, in general, complex, and complicated processes such as the reflected lateral wave field and the evanescent field in the bottom are incorporated in the saddle point formulation. The geometric interpretation of the saddle point criterion is derived in terms of eigenrays and their characteristics, from which expressions are obtained for the ray displacement upon reflection and transmission. Summaries of the eigenray structure of the reflected and transmitted fields are given for low‐frequency situations.
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43.30.Cq Ray propagation of sound in water
43.20.Dk Ray acoustics

Ray methods for flat and sloping shallow‐water waveguides

Evan K. Westwood

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1885-1894 (1989); (10 pages) | Cited 4 times

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A ray method for finding the acoustic field due to a point source in the presence of a plane, penetrable interface is extended to two simple models for shallow‐water ocean environments: the flat, isovelocity waveguide (the Pekeris model) and the sloping, isovelocity waveguide (the penetrable wedge). In both cases, the sound speed in the bottom is assumed larger than that in the water. The total field is expressed as a sum of ray fields, each of which takes the form of a plane wave integral. The integrals are solved using the method of steepest descent, where the plane wave reflection and transmission coefficients are allowed to influence the location of the saddle points. For a Pekeris waveguide in which three modes are trapped, agreement between the ray model and the SAFARI model is nearly perfect at all ranges, while the discrete normal mode solution is in error when a mode is near cutoff. The ray model is accurate even when the water depth is half of the acoustic wavelength. For the penetrable wedge problem, the plane wave integral for the ray field is developed, and the origin of the multiple lateral wave fields is examined. Excellent agreement between the ray model and a two‐way coupled mode model is demonstrated. Examples of the eigenray structure in both the flat and sloping waveguides are given.
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43.30.Cq Ray propagation of sound in water
43.20.Mv Waveguides, wave propagation in tubes and ducts
43.20.Dk Ray acoustics

A parabolic equation model for scattering in the ocean

Michael D. Collins and Michael F. Werby

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1895-1902 (1989); (8 pages) | Cited 6 times

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See Also: Erratum

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The small‐angle‐of‐propagation limit and the method of matched asymptotics are applied to derive an efficient model for solving realistic underwater acoustics problems involving both propagation and scattering from a submerged object. The propagation and scattering aspects of the waveguide scattering problem are decoupled by approximating the waveguide Green’s function on the surface of the scatterer. For low frequencies, the small‐angle limit also allows one to approximate the incident field with a horizontally propagating plane wave and the scattered field with an azimuthally specular point‐source field. With these approximations, scattering calculations can be performed efficiently in the time domain. Calculations involving the three‐dimensional parabolic equation and the time‐domain parabolic equation are presented.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Bp Normal mode propagation of sound in water
43.30.Dr Hybrid and asymptotic propagation theories, related experiments

A bistatic, high‐frequency, under‐ice, acoustic scattering model. I: Theory

Garner C. Bishop

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1903-1911 (1989); (9 pages)

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A model is described that has been developed to evaluate the scatter produced by a high‐frequency acoustic pulse that originates from a stationary and arbitrarily located source; is incident on and scattered from an under‐ice surface characteristic of pack ice regions of the interior Arctic; and is detected by a stationary and arbitrarily located receiver. Measured, two‐dimensional, under‐ice, acoustic profile data and several empirical results that relate various geometric parameters of large‐scale under‐ice relief features (e.g., ice keels) are used to construct a three‐dimensional, under‐ice surface model consisting of first‐year ice keels and sloping, flat ice regions. A first‐year ice keel is modeled as an ensemble of randomly oriented ice blocks on a planar surface inclined at some slope angle with respect to a horizontal plane at sea level. Ice blocks are modeled as layered, viscoelastic solids with smooth rectangular faces or facets. A region of flat ice is modeled as a smooth planar surface whose slope angle is less than some arbitrary minimum slope angle. The Helmholtz–Kirchhoff integral equation and the Kirchhoff approximation are used to evaluate the scattered pressure field. Time is partitioned into bins and, beginning at the bin corresponding to the time the scattered pressure field of each scattering facet arrives at the receiver, the scattered pressure field is added coherently in all bins spanning the temporal duration of the incident pulse.
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43.30.Hw Rough interface scattering
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics

A bistatic, high‐frequency, under‐ice, acoustic scattering model. II: Applications

Garner C. Bishop

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1912-1924 (1989); (13 pages)

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A model that has been developed to calculate the scatter produced by a high‐frequency acoustic pulse that originates from a stationary and arbitrarily located source; is incident on and scattered from an under‐ice surface characteristic of pack ice regions of the interior Arctic; and is detected by a stationary and arbitrarily located receiver is used to calculate a variety of acoustic data. Scattering from small‐scale surface roughness produced by ice blocks as well as from large‐scale surface roughness produced by ice keels is calculated and discussed. The effects of physical parameters on the scatter of a high‐frequency plane wave from an ice block whose physical and geometric parameters are characteristic of those found in the Arctic are calculated and discussed briefly. The ice block scattering model is modified to calculate the near field target strength of a circular ice piston as a function of incidence angle. When measured and modeled facet target strength data are compared, it is shown that, near normal incidence, measured and modeled target strength levels agree reasonably well, but that the details of their dependence on incidence angle are different and that differences between the measured and modeled target strength data increase as frequency increases. Monostatic reverberation time series, facet target strength distributions, and correlation data are calculated and compared with measured data for three Arctic sites, and it is shown that measured and modeled data agree reasonably well. For the Chukchi Sea site, a variety of monostatic and bistatic time series, distribution, and correlation data are calculated and discussed for ice keels and flat ice features as well as for the entire under‐ice surface. It is shown that the predominant large‐scale, under‐ice scatterers are ice keels; that ice keels increase reverberation levels; that the facets of ice blocks from which ice keels are composed produce high‐level echos or glints; that generally the spatial location of these high‐level echoes depends on source–receiver location, although some may persist from one location to the next; and that high‐level echos from ice facets are one of the principal means by which ice keels increase high‐frequency, under‐ice reverberation.
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43.30.Hw Rough interface scattering
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics

Viscous attenuation of acoustic waves in suspensions

Richard L. Gibson, Jr. and M. Nafi Toksöz

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1925-1934 (1989); (10 pages) | Cited 7 times

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A model for attenuation of acoustic waves in suspensions is proposed that includes an energy loss due to viscous fluid flow around spherical particles. The expression for the complex wavenumber is developed by considering the partial pressures acting on the solid and fluid phases of the suspension. This is shown to be equivalent to the results of the Biot theory for porous media in the limiting case where the frame moduli vanish. Unlike earlier applications of the limiting case Biot theory, however, a value for the attenuation coefficient is developed from the Stokes flow drag force on a sphere instead of attempting to apply a permeability value to a suspension. Accurate modeling of observed phase velocities from suspensions of spherical polystyrene particles in water and oil and successful inversion for kaolinite properties using attenuation and velocity data from kaolinite suspensions at 100 kHz show that this viscous dissipation model is a good representation of the effects controlling the propagation of acoustic waves in these suspensions. The viscous effects are shown to be significant for only a limited range of solid concentration and frequency by the reduced accuracy of the model for attenuation in a kaolinite suspension at 1 MHz.
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43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics

The generation of infrasonic ambient noise in the ocean by nonlinear interactions of ocean surface waves

A. C. Kibblewhite and C. Y. Wu

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1935-1945 (1989); (11 pages) | Cited 3 times

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Expressions for the spectra of infrasonic ocean noise and microseisms induced by nonlinear wave interaction are derived theoretically for an ocean environment modeled as a water layer overlying a solid half‐space. A series of spectral transfer functions relating the source pressure field induced by the wave action to the underwater acoustic noise field and that of the microseisms this generates in the seabed are defined and calculated. The effect of bottom reflections on the transfer function is examined and an estimate of the contribution of the Rayleigh wave component of the microseism signal received onshore is also made.
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43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.30.Nb Noise in water; generation mechanisms and characteristics of the field

A reexamination of the role of wave–wave interactions in ocean noise generation

A. C. Kibblewhite and C. Y. Wu

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1946-1957 (1989); (12 pages) | Cited 1 time

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A close connection between microseisms and ocean‐wave activity has been recognized for many years. Various mechanisms have been proposed to explain the interaction, most favored being nonlinear interactions between ocean surface waves. Interest in these processes has increased in recent years as underwater acousticians have extended their investigations to infrasonic frequencies. This contribution builds on a study recently reported that confirmed the role of nonlinear wave–wave interactions at infrasonic frequencies but left some questions unresolved.
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43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.30.Nb Noise in water; generation mechanisms and characteristics of the field

The 1984 bottom‐mounted Gulf Stream tomographic experiment

Yehuda Agnon, Paola Malanotte‐Rizzoli, Bruce D. Cornuelle, John L. Spiesberger, and Robert L. Spindel

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1958-1966 (1989); (9 pages)

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In this paper, data from a Gulf Stream tomographic experiment carried out in October 1984 are analyzed. The experiment used acoustic sources and receivers bottom mounted beneath the stream to measure Gulf Stream dynamics. However, due to an unfortunate electronic malfunction of the source, only 2 days of acoustically measured travel time data are available. Nevertheless, some new and positive results are obtained. Bottom reflected acoustic rays having up to two bottom bounces are unambiguously identified by solving the direct problem of tracing rays both in a reference climatological profile and in actual range‐dependent sound‐speed sections from a hydrographic survey carried out during the experiment. It is also shown that these rays do not appear to be affected by important nonlinearities so that they can be used to provide consistent results in inverse solutions.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography

Thermodiffusive surface waves in semiconductors

Bogdan Maruszewski

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1967-1977 (1989); (11 pages) | Cited 2 times

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This article deals with the considerations of the propagation of the coupled surface waves along the surface that bounds the semiconducting half‐space with relaxation of heat and charge carrier fields. Based on the general equations, two particular cases are investigated: the elastodiffusive surface waves of the electron field and the thermodiffusive surface waves of the electron fields.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.35.Pt Surface waves in solids and liquids

Neuronal responses to amplitude‐modulated and pure‐tone stimuli in the guinea pig inferior colliculus, and their modification by broadband noise

Adrian Rees and Alan R. Palmer

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1978-1994 (1989); (17 pages) | Cited 13 times

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Neuronal responses were recorded to pure and to sinusoidally amplitude‐modulated (AM) tones at the characteristic frequency (CF) in the central nucleus of the inferior colliculus of anesthetized guinea pigs. Temporal (synchronized) and mean‐rate measures were derived from period histograms locked to the stimulus modulation waveform to characterize the modulation response. For stimuli presented in quiet, the modulation gain at low frequencies of modulation (approx <50 Hz) was inversely proportional to the neuron’s mean firing rate in response to both the modulated stimulus and to a pure tone at an equivalent level. In 43% of units the mean discharge rates in response to the AM stimuli were greatest for those modulation frequencies that generated the largest temporal responses. These discharge‐rate maxima occurred at signal intensities corresponding to the steeply sloping part of the neuron’s pure‐tone rate‐intensity function (RIF). The change in mean‐rate response to modulated stimuli, as a function of intensity, was qualitatively similar to the pure‐tone RIF. Adding broadband noise to the modulated stimulus increased the neuron’s temporal response to low modulation frequencies. This increase in modulation gain was correlated with mean firing rate in response to the modulation but did not bear a simple relationship to the noise‐induced shift in the RIF measured for a pure tone.
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43.64.Qh Electrophysiology of the auditory central nervous system
43.80.Lb Sound reception by animals: anatomy, physiology, auditory capacities, processing

Synchronized discharge rate representation of voice‐onset time in the chinchilla auditory nerve

Donal G. Sinex and Lynn P. McDonald

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 1995-2004 (1989); (10 pages) | Cited 4 times

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Responses of chinchilla auditory nerve fibers to synthesized stop consonant syllables differing in voice‐onset time (VOT) were obtained. The syllables, heard as /ga/–/ka/ or /da/–/ta/, were similar to those previously used by others in psychophysical experiments with human and chinchilla subjects. Synchronized discharge rates of neurons tuned to frequencies near the first formant increased at the onset of voicing for VOTs longer than 20 ms. Stimulus components near the formant or the neuron’s characteristic frequency accounted for the increase. In these neurons, synchronized response changes were closely related to the same neuron’s average discharge rates [D. G. Sinex and L. P. McDonald, J. Acoust. Soc. Am. 83, 1817–1827 (1988)]. Neurons tuned to frequency regions near the second and third formants usually responded to components near the second formant prior to the onset of voicing. These neurons’ synchronized discharges could be captured by the first formant at the onset of voicing or with a latency of 50–60 ms, whichever was later. Since these neurons’ average rate responses were unaffected by the onset of voicing, the latency of the synchronized response did provide an additional neural cue to VOT. Overall, however, discharge synchrony did not provide as much information about VOT as was provided by the best average rate responses. The results are compared to other measurements of the peripheral encoding of speech sounds and to aspects of VOT perception.
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43.64.Sj Neural responses to speech
43.64.Pg Electrophysiology of the auditory nerve
43.71.Qr Neurophysiology of speech perception
43.71.An Models and theories of speech perception

Reductions in overshoot following intense sound exposures

Craig A. Champlin and Dennis McFadden

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 2005-2011 (1989); (7 pages) | Cited 13 times

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Overshoot refers to the poorer detectability of brief signals presented soon after the onset of a masking noise compared to those presented after longer delays. In the present experiment, brief tonal signals were presented 2 or 190 ms following the onset of a broadband masker that was 200 ms in duration. These two conditions of signal delay were tested before and after a series of exposures to a tone intense enough to induce temporary threshold shift (TTS). The magnitude of the overshoot was reduced after the exposure when a TTS of at least 10 dB was induced, but not when smaller amounts of TTS were induced. The reduction in overshoot was due to a decrease in the masked thresholds with the 2‐ms delay; masked thresholds with the 190‐ms delay were not different pre‐ and post‐exposure. The implication is that the mechanisms responsible for the normal overshoot effect are temporarily inactivated by the same stimulus manipulations that produce a mild exposure‐induced hearing loss. Thus the result is the paradox that exposure to intense sounds can produce a loss of signal detectability in certain stimulus conditions and a simultaneous improvement in detectability in other stimulus conditions.
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43.66.Cb Loudness, absolute threshold
43.66.Dc Masking
43.66.Ed Auditory fatigue, temporary threshold shift
43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music

The measurement of loudness in individual children and adults by absolute magnitude estimation and cross‐modality matching

Amy A. Collins and George A. Gescheider

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 2012-2021 (1989); (10 pages) | Cited 4 times

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Twelve adults and 11 children (age range 4–7 years) performed absolute magnitude estimation of the apparent lengths of lines and the loudnesses of 1000‐Hz tones as well as cross‐modality matching between loudness and apparent line length. Consistent with the notion that children and adults have similar impressions of loudness, there were no major differences between the absolute magnitude estimation (AME) and cross‐modality matching (CMM) data of the adults and children. A direct comparison between the exponents for loudness by AME and CMM was made when a correction factor was employed to eliminate the effects of idiosyncratic use of numbers from the AME exponents. The results support the hypothesis that, with proper instructions, both children and adults can judge stimuli on an absolute scale. Specifically, for 9 out of 12 adults and 9 out of 11 children, lines and tones assigned the same number in absolute magnitude estimation were judged to be subjectively equal in cross‐modality matching.
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43.66.Cb Loudness, absolute threshold

Detection and discrimination of frequency modulation of complex signals

J. Wiebe Horst

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 2022-2030 (1989); (9 pages) | Cited 1 time

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Detection and discrimination of frequency modulation were studied for harmonic signals with triangular spectral envelopes. The center frequency of the stimuli was near 2 kHz; the fundamental frequency was near 100 Hz. To prevent the possibility that the discrimination was based on differences of initial or final frequencies, these frequencies were equal within and across modulations in each individual experiment. Differences between modulations consisted of differences in the trajectories between the initial and final frequencies. Performance worsened as the slopes of the spectral envelopes decreased. Addition of noise also impaired modulation discrimination. The dependence on the signal‐to‐noise ratio was similar to what is found for stationary stimuli: Discrimination of frequency modulation deteriorated more rapidly with decreasing signal‐to‐noise ratio when stimuli had shallow spectral slopes than when they had steep spectral slopes. In spite of the precautions taken (i.e., initial and final frequency the same), the discrimination of these stimuli was more likely based on quasistationary frequency discrimination than on discrimination of modulation rate. This conclusion is consistent with previous findings for pure tones presented in quiet that frequency discrimination is more acute than modulation‐rate discrimination.
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43.66.Dc Masking
43.66.Fe Discrimination: intensity and frequency
43.66.Ba Models and theories of auditory processes

On the minimum audible angle—A decision theory approach

William Morris Hartmann and Brad Rakerd

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 2031-2041 (1989); (11 pages) | Cited 14 times

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The minimum audible angle (MAA) technique is a well‐known psychoacoustical paradigm often used in the study of localization of sound. A difficulty with this paradigm, however, is that, in terms of decision theory, it is subject to two quite different interpretations. Although it is normally regarded as involving a discrimination task, this work suggests that it is more likely to be an absolute identification task. Because of this difference in interpretation, it is found that previous work has overestimated the ability of listeners to localize sources of sound.
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43.66.Qp Localization of sound sources
43.66.Ba Models and theories of auditory processes
43.66.Pn Binaural hearing

Resolution of steady‐state sounds in simulated auditory space

Pierre L. Divenyi and Susan K. Oliver

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 2042-2052 (1989); (11 pages) | Cited 5 times

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A simulated acoustic equivalent of a sound source placed on the perimeter of a circle having a 4‐m radius in the frontal horizontal half‐plane was generated by obtaining, in 5‐deg azimuthal steps, head‐related transfer functions measured in both ears of an artificial head placed in an anechoic room. When an arbitrary sound is passed through the digital filter defined by the left and right transfer functions corresponding to a given angle, the sound will acquire a subjective azimuth comparable to the one at which the transfer function was measured [J. Blauert and P. Laws, Acustica 29, 273–277 (1973)]. This technique was used to measure the resolution of pairs of simultaneous steady‐state stimuli (frequency‐modulated, amplitude‐modulated, and pure sinusoids or noises). The listener was required to identify the relative location of the two sounds. In one set of experiments, the position of the two sounds was fixed, and thresholds were obtained for the modulation‐ or carrier‐frequency difference at which the two sounds could be localized. In other sets of experiments, the pair of sounds was constant and the minimum audible angular separation was determined at which association of each sound with a particular locus was possible. Generally, both kinds of resolution were better around the midline than at the sides. Angular resolution at the sides approached the threshold for the minimum audible angle between serially presented sounds only for pairs of complex (FM‐) stimuli having little spectral overlap; for most other sound pairs, spatial resolution was possible only at angular separations of 60° and larger. Results suggest that segregation of the two component sounds and their correct localization constitute competing processes. Spatial resolution will be inefficient when either the localizability of the sounds is poor (e.g., for long‐duration sine waves) or when the spectra of the two components display large regions of overlap (e.g., for two noise signals). The relationship between the data and the ‘‘cocktail‐party effect’’ is discussed.
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43.66.Rq Dichotic listening
43.66.Pn Binaural hearing
43.66.Yw Instruments and methods related to hearing and its measurement
43.55.Ka Computer simulation of acoustics in enclosures, modeling

Comparison of target monitoring and two‐ear monitoring dichotic listening procedures

Charles Speaks, Nancy Niccum, Ruth Leathers, and Jun Katsuki‐Nakamura

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 2053-2058 (1989); (6 pages)

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The dichotic listening performance of 40 listeners was assessed for consonant–vowel (CV) nonsense syllables with two procedures. One was a conventional two‐ear monitoring task in which listeners attended to both ears and provided two responses for each pair of syllables. The ear advantage was described by %RE−%LE. The second was target monitoring, a yes/no task in which listeners attended to only one ear and listened for the presence of a target syllable. That procedure provided both hit and false alarm rates for each ear, and the ear advantage was described by P(C)maxRE−P(C)maxLE, which is insensitive to decision variables. Although both procedures yielded mean right‐ear advantages (REA), the mean REA of +7.5% with two‐ear monitoring was significantly different from the mean REA of +2.6% with target monitoring. In addition, although 62% of the listeners had a significant REA with the conventional procedure, only 40% had a significant REA with target monitoring. Decision variables, which are not controlled with conventional dichotic testing methods, may contribute to the ear advantage as it is described frequently in the literature.
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43.66.Rq Dichotic listening

Vibrotactile masking: Effects of stimulus onset asynchrony and stimulus frequency

G. A. Gescheider, S. J. Bolanowski, Jr., and R. T. Verrillo

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 2059-2064 (1989); (6 pages) | Cited 3 times

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Vibrotactile thresholds for the detection of a 50‐ms vibratory stimulus on the thenar eminence of the hand were measured in the presence of and in the absence of a 700‐ms suprathreshold vibratory masking stimulus. When thresholds were measured in the presence of the masking stimulus, stimulus onset asynchrony (SOA) was varied so that backward, simultaneous, and forward masking could be measured. The amount of masking, expressed as threshold shift, was greatest when the test stimulus was presented near the onset or offset of the masking stimulus. For both backward and forward masking, the amount of masking decreased as a function of increasing stimulus onset asynchrony. Comparisons were made of the amounts of masking measured when the test and masking stimuli were both sinusoids, and when the test stimulus was a sinusoid and the masking stimulus was noise. In all conditions, the masked threshold decreased approximately 4.0 dB when SOA was increased from 100 to 650 ms with reference to the onset of the 700‐ms masking stimulus. More simultaneous masking was observed when sinusoidal test stimuli were detected in the presence of noise than when they were detected in the presence of sinusoidal maskers of the same frequency. The functions were essentially identical for detection of a low‐frequency (20 Hz) test stimulus mediated by a non‐Pacinian channel and detection of a high‐frequency (250 Hz) test stimulus mediated by the Pacinian channel.
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43.66.Wv Vibration and tactile senses
43.66.Dc Masking

A model of spatiotemporal tactile sensitivity linking psychophysics to tissue mechanics

Clayton L. Van Doren

J. Acoust. Soc. Am. Volume 85, Issue 5, pp. 2065-2080 (1989); (16 pages) | Cited 1 time

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Sensitivities were measured for tangible spatiotemporal sinusoids applied to the index fingertip. The sinusoids had temporal frequencies of 8 and 128 Hz, in order to selectively activate the non‐Pacinian I (NP I) and Pacinian (P) cutaneous mechanoreceptor systems, respectively, and had spatial frequencies from 0.00–1.03 cycles/mm. The sensitivity of the NP I system increased as the spatial frequency increased, whereas the sensitivity of the P system generally decreased as the spatial frequency increased. A mechanical model of the fingertip was used to calculate the normal and shear strains in the tissue, and a psychophysical linking hypothesis was introduced to predict tactile sensitivities based on the calculated strains. Specifically, the fingertip was modeled as a slab of a linear, isotropic, homogeneous, viscoelastic material. The boundary conditions were imposed by the spatiotemporal sinusoid at the top of the slab and the rigidly attached bone at the bottom of the slab. It was then assumed that the detection threshold was equal to the stimulus amplitude, which produced a constant, criterion strain at the location of the receptor. For both the P and NP I responses, the agreement between the predicted and measured sensitivities was best for calculations based on the normal strain, and for spatial frequencies below 0.5 cycles/mm. At higher spatial frequencies, the measured sensitivities were higher than predicted. The model also predicted the location of the P and NP I receptors in the tissue, the thickness of the tissue, and the value of the threshold strain for both receptor types. The predicted values agreed reasonably well with independent anatomical and physiological measurements.
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43.66.Wv Vibration and tactile senses
43.64.Vm Physiology of the somatosensory system
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