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Top 20 Most Read Articles
December 2006
The 20 articles with the most full-text downloads during the month, in descending order.
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Acoustic response from adherent targeted contrast agents J. Acoust. Soc. Am. Volume 120, Issue 6, pp. EL63-EL69 (2006); (7 pages) Online Publication Date: 23 Oct 2006
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In ultrasonic molecular imaging, encapsulated micron-sized gas bubbles are tethered to a blood vessel wall by targeting ligands. A challenging problem is to detect the echoes from adherent microbubbles and distinguish them from echoes from nonadherent agents and tissue. Echoes from adherent contrast agents are observed to include a high amplitude at the fundamental frequency, and significantly different spectral shape compared with free agents (p<0.0003). Mechanisms for the observed acoustical difference and potential techniques to utilize these differences for molecular imaging are proposed.
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J. Acoust. Soc. Am. Volume 121, Issue 1, pp. EL1-EL7 (2006); (7 pages) Online Publication Date: 08 Dec 2006
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The use of conventional metrics to quantify the perception of nonlinearly propagated noise has been studied. Gaussian noise waveforms have been numerically propagated both linearly and nonlinearly, and from the resulting waveforms, several metrics are calculated. These metrics are overall, A-, C-, and D-weighted sound pressure levels, perceived noise level, Stevens Mark VII perceived loudness, Zwicker loudness, and sharpness. Informal listening demonstrations indicate that perceived differences in annoyance between linearly and nonlinearly propagated waveforms are substantial. Because the metrics studied seem inadequate in representing the perceived differences, rigorous subjective testing is encouraged to properly quantify and understand these differences.
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J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 4100-4114 (2006); (15 pages)
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In 2002 and 2003, tagged sperm whales (Physeter macrocephalus) were experimentally exposed to airgun pulses in the Gulf of Mexico, with the tags providing acoustic recordings at measured ranges and depths. Ray trace and parabolic equation (PE) models provided information about sound propagation paths and accurately predicted time of arrival differences between multipath arrivals. With adequate environmental information, a broadband acoustic PE model predicted the relative levels of multipath arrivals recorded on the tagged whales. However, lack of array source signature data limited modeling of absolute received levels. Airguns produce energy primarily below 250 Hz, with spectrum levels about 20–40 dB lower at 1 kHz. Some arrivals recorded near the surface in 2002 had energy predominantly above 500 Hz; a surface duct in the 2002 sound speed profile helps explain this effect, and the beampattern of the source array also indicates an increased proportion of high-frequency sound at near-horizontal launch angles. These findings indicate that airguns sometimes expose animals to measurable sound energy above 250 Hz, and demonstrate the influences of source and environmental parameters on characteristics of received airgun pulses. The study also illustrates that on-axis source levels and simple geometric spreading inadequately describe airgun pulse propagation and the extent of exposure zones.
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Synthesis of audio spectra using a diffraction model J. Acoust. Soc. Am. Volume 120, Issue 6, pp. EL70-EL77 (2006); (8 pages) Online Publication Date: 03 Nov 2006
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It is shown that the intensity variations of an audio signal in the frequency domain can be obtained by using a mathematical function containing a series of weighted complex Bessel functions. With proper choice of values for two parameters, this function can transform an input spectrum of discrete frequencies of unit intensity into the known spectra of different musical instruments. Specific examples of musical instruments are considered for evaluating the performance of this method. It is found that this function yields musical spectra with a good degree of accuracy.
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J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3493-3502 (2006); (10 pages)
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Acoustic waves in tissues and weakly attenuative fluids often have an attenuation parameter, α(ω), satisfying α(ω) = α0ωy in which α0 is a constant, ω is the frequency, and y is between 1 and 2. This power law attenuation is not predicted by the classical thermoviscous wave equation and researchers have proposed different modified viscous wave equations in which the loss term is a convolution operator or a fractional spatial or temporal derivative. In this paper, acoustic waves undergoing power law attenuation are modeled by a modification to the thermoviscous wave equation in which the time derivative of the viscous term is replaced by a fractional time derivative. An explicit time domain, finite element formulation leads to a stable algorithm capable of simulating axisymmetric, broadband acoustic pulses propagating through attenuative and dispersive media. The algorithm does not depend on the Born approximation, long wavelength limit, or plane wave assumptions. The algorithm is validated for planar and focused transducers and results include radiation patterns from a viscous scatterer in a lossless background and signals reflected from a viscous layer. The program can be used to determine scattering parameters for large, strong, possibly viscous scatterers, in either a lossless or viscous background, for which analytic results are scarce.
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Vowel recognition via cochlear implants and noise vocoders: Effects of formant movement and duration J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3998-4006 (2006); (9 pages)
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Previous work has demonstrated that normal-hearing individuals use fine-grained phonetic variation, such as formant movement and duration, when recognizing English vowels. The present study investigated whether these cues are used by adult postlingually deafened cochlear implant users, and normal-hearing individuals listening to noise-vocoder simulations of cochlear implant processing. In Experiment 1, subjects gave forced-choice identification judgments for recordings of vowels that were signal processed to remove formant movement and/or equate vowel duration. In Experiment 2, a goodness-optimization procedure was used to create perceptual vowel space maps (i.e., best exemplars within a vowel quadrilateral) that included F1, F2, formant movement, and duration. The results demonstrated that both cochlear implant users and normal-hearing individuals use formant movement and duration cues when recognizing English vowels. Moreover, both listener groups used these cues to the same extent, suggesting that postlingually deafened cochlear implant users have category representations for vowels that are similar to those of normal-hearing individuals.
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The acoustic signature of bubbles fragmenting in sheared flow J. Acoust. Soc. Am. Volume 120, Issue 6, pp. EL84-EL89 (2006); (6 pages) Online Publication Date: 09 Nov 2006
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Measurements of the sound of bubbles fragmenting in fluid shear are presented and analyzed. The frequency, amplitude, and decay rate of the acoustic emissions from 1.8-mm-radius bubbles fragmenting between opposed fluid jets have been determined. A broad band of frequencies (1.8 to 30 kHz) is observed with peak pressure amplitudes in the range of 0.03 to 2 Pa. While the peak pressure amplitudes show no significant scaling with frequency, the frequency dependence of the decay rates is consistent with the sum of thermal and acoustic radiation losses.
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Virtual pitch in a computational physiological model J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3861-3869 (2006); (9 pages)
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A computational model of nervous activity in the auditory nerve, cochlear nucleus, and inferior colliculus is presented and evaluated in terms of its ability to simulate psychophysically-measured pitch perception. The model has a similar architecture to previous autocorrelation models except that the mathematical operations of autocorrelation are replaced by the combined action of thousands of physiologically plausible neuronal components. The evaluation employs pitch stimuli including complex tones with a missing fundamental frequency, tones with alternating phase, inharmonic tones with equally spaced frequencies and iterated rippled noise. Particular attention is paid to differences in response to resolved and unresolved component harmonics. The results indicate that the model is able to simulate qualitatively the related pitch-perceptions. This physiological model is similar in many respects to autocorrelation models of pitch and the success of the evaluations suggests that autocorrelation models may, after all, be physiologically plausible.
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The relationship between frequency selectivity and pitch discrimination: Effects of stimulus level J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3916-3928 (2006); (13 pages)
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Three experiments tested the hypothesis that fundamental frequency (f0) discrimination depends on the resolvability of harmonics within a tone complex. Fundamental frequency difference limens (f0 DLs) were measured for random-phase harmonic complexes with eight f0’s between 75 and 400 Hz, bandpass filtered between 1.5 and 3.5 kHz, and presented at 12.5‐dB∕component average sensation level in threshold equalizing noise with levels of 10, 40, and 65 dB SPL per equivalent rectangular auditory filter bandwidth. With increasing level, the transition from large (poor) to small (good) f0 DLs shifted to a higher f0. This shift corresponded to a decrease in harmonic resolvability, as estimated in the same listeners with excitation patterns derived from measures of auditory filter shape and with a more direct measure that involved hearing out individual harmonics. The results are consistent with the idea that resolved harmonics are necessary for good f0 discrimination. Additionally, f0 DLs for high f0’s increased with stimulus level in the same way as pure-tone frequency DLs, suggesting that for this frequency range, the frequencies of harmonics are more poorly encoded at higher levels, even when harmonics are well resolved.
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Constructing and disrupting listeners’ models of auditory space J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3957-3965 (2006); (9 pages)
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A major problem for an auditory system exposed to sound in a reverberant environment is to distinguish reflections from true sound sources. Previous research indicates that the process of recognizing reflections is malleable from moment to moment. Three experiments report how ongoing input can prevent or disrupt the fusion of the delayed sound with the direct sound, a necessary component of the precedence effect. The buildup of fusion can be disrupted by presenting stimuli in alternation that simulate different reflecting surfaces. If buildup of fusion is accomplished first and then followed by an aberrant configuration, breakdown of the precedence effect occurs but it depends on the duration of the new sound configuration. The
Djelani and Blauert (2001)
finding that a brief disruption has no effect on fusion was confirmed; however, it was found that a more lengthy disruption produces breakdown.
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The relationship between frequency selectivity and pitch discrimination: Sensorineural hearing loss J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3929-3945 (2006); (17 pages)
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This study tested the relationship between frequency selectivity and the minimum spacing between harmonics necessary for accurate f0 discrimination. Fundamental frequency difference limens (f0 DLs) were measured for ten listeners with moderate sensorineural hearing loss (SNHL) and three normal-hearing listeners for sine- and random-phase harmonic complexes, bandpass filtered between 1500 and 3500 Hz, with f0’s ranging from 75 to 500 Hz (or higher). All listeners showed a transition between small (good) f0 DLs at high f0’s and large (poor) f0 DLs at low f0’s, although the f0 at which this transition occurred (f0,tr) varied across listeners. Three measures thought to reflect frequency selectivity were significantly correlated to both the f0,tr and the minimum f0 DL achieved at high f0’s: (1) the maximum f0 for which f0 DLs were phase dependent, (2) the maximum modulation frequency for which amplitude modulation and quasi-frequency modulation were discriminable, and (3) the equivalent rectangular bandwidth of the auditory filter, estimated using the notched-noise method. These results provide evidence of a relationship between f0 discrimination performance and frequency selectivity in listeners with SNHL, supporting “spectral” and “spectro-temporal” theories of pitch perception that rely on sharp tuning in the auditory periphery to accurately extract f0 information.
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J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3946-3956 (2006); (11 pages)
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This study examined the deleterious effects of a later-arriving sound on the processing of interaural differences of time (IDTs) from a preceding sound. A correlational analysis assessed the relative weight given to IDTs of source and echo clicks for echo delays of 1–64 ms when the echo click was attenuated relative to the source click (0–36 dB). Also measured were proportion correct and the proportion of responses predicted from the weights. The IDTs of source and echo clicks were selected independently from Gaussian distributions (μ = 0 μs, σ = 100 μs). Listeners were instructed to indicate the laterality of the source click. Equal weight was given to the source and echo clicks for echo delays of 64 ms with no echo attenuation. For echo delays of 16–64 ms, attenuating the echo had no substantial effect on source weight or proportion correct until the echo was attenuated by 18–30 dB. At echo delays ≤ 4 ms, source weights and proportions correct remained high regardless of echo attenuation. The proportions of responses predicted from the weights were lower at echo delays ≥ 16 ms. Results were discussed in terms of backward recognition masking and binaural sluggishness and compared to measurements of echo disturbance.
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Acoustic beams with angular momentum J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3475-3478 (2006); (4 pages)
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A family of exact solutions of the Helmholtz equation is used to represent transversely bounded helicoidal sound beams. Simple results are obtained for the energy content per unit length, the momentum content per unit length, and the angular momentum content per unit length. The analysis is restricted to lossless media; scattering and viscous damping are neglected. The energy, momentum, and angular momentum are calculated to second order in the velocity potential. The angular momentum content is always equal to m/ω times the energy content, where m (an integer) is the topological charge and ω is the angular frequency.
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International Meetings Calendar J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3431-3431 (2006); (1 page)
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J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3988-3997 (2006); (10 pages)
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The extension to the speech intelligibility index (SII; ANSI S3.5-1997 (1997)) proposed by Rhebergen and Versfeld [
Rhebergen, K.S., and Versfeld, N.J. (2005). J. Acoust. Soc. Am. 117(4), 2181–2192
] is able to predict for normal-hearing listeners the speech intelligibility in both stationary and fluctuating noise maskers with reasonable accuracy. The extended SII model was validated with speech reception threshold (SRT) data from the literature. However, further validation is required and the present paper describes SRT experiments with nonstationary noise conditions that are critical to the extended model. From these data, it can be concluded that the extended SII model is able to predict the SRTs for the majority of conditions, but that predictions are better when the extended SII model includes a function to account for forward masking.
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Perceptual adaptation by normally hearing listeners to a simulated “hole” in hearing J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 4019-4030 (2006); (12 pages)
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Simulations of cochlear implants have demonstrated that the deleterious effects of a frequency misalignment between analysis bands and characteristic frequencies at basally shifted simulated electrode locations are significantly reduced with training. However, a distortion of frequency-to-place mapping may also arise due to a region of dysfunctional neurons that creates a “hole” in the tonotopic representation. This study simulated a 10 mm hole in the mid-frequency region. Noise-band processors were created with six output bands (three apical and three basal to the hole). The spectral information that would have been represented in the hole was either dropped or reassigned to bands on either side. Such reassignment preserves information but warps the place code, which may in itself impair performance. Normally hearing subjects received three hours of training in two reassignment conditions. Speech recognition improved considerably with training. Scores were much lower in a baseline (untrained) condition where information from the hole region was dropped. A second group of subjects trained in this dropped condition did show some improvement; however, scores after training were significantly lower than in the reassignment conditions. These results are consistent with the view that speech processors should present the most informative frequency range irrespective of frequency misalignment.
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Individual differences in the sensitivity to pitch direction J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3907-3915 (2006); (9 pages)
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It is commonly assumed that one can always assign a direction—upward or downward—to a percept of pitch change. The present study shows that this is true for some, but not all, listeners. Frequency difference limens (FDLs, in cents) for pure tones roved in frequency were measured in two conditions. In one condition, the task was to detect frequency changes; in the other condition, the task was to identify the direction of frequency changes. For three listeners, the identification FDL was about 1.5 times smaller than the detection FDL, as predicted (counterintuitively) by signal detection theory under the assumption that performance in the two conditions was limited by one and the same internal noise. For three other listeners, however, the identification FDL was much larger than the detection FDL. The latter listeners had relatively high detection FDLs. They had no difficulty in identifying the direction of just-detectable changes in intensity, or in the frequency of amplitude modulation. Their difficulty in perceiving the direction of small frequency/pitch changes showed up not only when the task required absolute judgments of direction, but also when the directions of two successive frequency changes had to be judged as identical or different.
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J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 4090-4099 (2006); (10 pages)
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A portable electrophysiological data collection system was used to assess hearing in a captive population of bottlenose dolphins by recording auditory evoked potentials (AEPs). The AEP system used a transducer embedded in a suction cup to deliver amplitude modulated tones to the dolphin through the lower jaw. Evoked potentials were recorded noninvasively using surface electrodes. Adaptive procedures allowed hearing thresholds to be estimated from 10 to 150 kHz in a single ear in about 45 min. Hearing thresholds were measured in 42 bottlenose dolphins (28 male, 14 female), ranging in age from 4 to 47 years. Variations in hearing sensitivity with age and sex followed patterns seen in humans and terrestrial mammals: generally, within the population there was a progressive loss of high frequency hearing with age and an earlier onset of hearing loss in males than in females. Hearing loss generally occurred between the ages of 20 and 30, and all animals over the age of 27 had some degree of hearing loss. Two dolphins with profound hearing loss were found within the population. Aberrant hearing patterns were observed in related dolphins suggesting genetic links to hearing ability may exist.
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Impact of perforation impedance on the transmission loss of reactive and dissipative silencers J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3706-3713 (2006); (8 pages)
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The effect of perforation impedance on the acoustic behavior of reactive and dissipative silencers is investigated using experimental and computational approaches. The boundary element method (BEM) is applied for the prediction of transmission loss of silencers with different perforation geometries. The variations are considered in the porosity (8.4 and 25.7%) and hole diameter (0.249 and 0.498 cm) of perforations for both reactive and dissipative silencers, as well as the fiber filling density (100 and 200 kg/m3) for the latter. The acoustic impedance for a number of perforations in contact with air alone and fibrous material has been incorporated into the predictions, which are then compared with the measured transmission loss using an impedance tube setup. The results demonstrate the significance of the accuracy of the perforation impedance in the predictions for both reactive and dissipative silencers.
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Optimum beamformer in correlated source environments J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3770-3781 (2006); (12 pages)
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A new method for overcoming signal cancellation problems due to correlated interferences which occur in a minimum variance distortionless response beamformer is proposed. Instead of decorrelating the correlated interferences, the coherently combining signal-to-interference plus noise ratio (CC-SINR) beamformer regards them as replicas of the desired signal and coherently combines them with the desired signal. This method uses an eigenvector constraint that suppresses only noise and uncorrelated interferences but retains the desired signal and correlated interferences. The CC-SINR beamformer does not require any preliminary information on correlated interferences. The signal-to-interference plus noise ratio (SINR) of the proposed beamformer output was compared to that of a conventional SINR beamformer when correlated interference, uncorrelated interference, and white noise exist. In addition, various key parameters that affect the performance of the beamformer, such as signal-to-noise ratio, uncorrelated interference-to-noise ratio, angular separation between signals, attenuation factor, phase delay of correlated interference, and the number of sensors were analyzed. All of the experimental results were in good agreement with the analytical results.
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