摘要:

Sensor arrays play a major role in underwater acoustic applications. They allow propagating signals to be spatially separated after beamforming giving direction of arrival information. They are usually designed by sensor spacing selection for a particular frequency but used over a band of frequencies. For wideband applications, arrays designed with nonuniform sensor spacing have been utilized. But many of the existing acoustic arrays are designed with uniform sensor spacing. When these arrays are used beyond their design frequency spatial resolution is reduced at low frequencies and grating lobes appear at higher frequencies based on the second-order spectrum. To define these effects a theorem and a corollary are stated and proven under general conditions. In contrast, the fourth-order cumulant spectrum provides more flexibility in choosing the frequency response at the output of a beamformer so deliterious effects of wide frequency bandwidths can be reduced. To define these improvements another theorem and corollary are stated and proven for the fourth-order cumulant spectrum. To demonstrate these theoretical results returns from an active sonar system have been simulated. The transmitted waveform is a linear frequency modulated signal and the echo is returned from a spherical target that is either stationary or in motion. The returned signal is beamformed using a one-dimensional line array of uniformly spaced sensors designed for a frequency of 4160 Hz. But the transmitted waveform has a bandwidth from 1 Hz to 8 kHz. The results are given for the matched and mismatched filtered beamformer outputs of a ten-element line array in the frequency and range domains. These results are compared with the fourth-order cumulant spectrum of the beamformer output and its range domain representation.

ISSN:0001-4966    

DOI:10.1121/1.420081

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

In this study, a preparation was developed which continuously monitored thef2−f1,2 f1−f2,and3f1−2 f2distortion products in guinea pigs, in response to various contralateral and ipsilateral tones and broadband noise. A consistent decline over time in the quadratic distortion product,f2−f1,with continuous ipsilateral stimulation was observed, confirming previous findings by Kirk and Johnstone [Hearing Res.67, 20–34 (1993)]. Broadband noise applied to the contralateral ear simultaneously increased both the magnitude and the rate of decline. This effect was shown for primaries across all frequencies tested (f2from 2–16 kHz,f2/f1ratio of 1.26), though a maximal effect was found forf2of 4 kHz. Experiments using contralateral tones demonstrated a half-octave shift in the frequency specificity of this effect. A more “immediate” suppression effect on2 f1−f2(0.6 dB) was found in animals awakening from anesthesia, though this was even larger forf2−f1(5.1 dB). Extended ipsilateral stimulation with contralateral noise results in completely diminishingf2−f1to the noise floor. These findings suggest an adaptive process within the cochlea that may be modulated by efferent stimulation. A model relating the various even- and odd-order distortion products to outer hair cell receptor potentials is presented.

ISSN:0001-4966    

DOI:10.1121/1.420082

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Efferent inhibition of low and medium speontaneous-rate fiber activity in response to characteristic frequency (CF) tones is greater at levels between 50 and 75 dB SPL than at lower levels, and even occurs at levels of 100 dB SPL [Guinan and Stankovic, J. Acoust. Soc. Am.100, 1680–1690 (1996)]. The self-mixing effect of a laser diode was used to measure tone-evoked, basilar membrane (BM) displacements in the basal turn of the guinea pig cochlea during electrical stimulation of the medial efferent system. Efferent stimulation suppresses BM displacement and, for frequencies close to the CF of the measurement site and above, attenuation of tone-evoked displacements appears to be greatest for levels between 50 and 75 dB SPL, and efferent attenuation could exceed 10 dB SPL for tones at 90 dB SPL. By comparison, the attenuation of the BM vibrations caused by tones at frequencies below CF was found to be greatest for low sound pressure levels. The findings of Guinan and Stankovic therefore have their origin in events present in the vibrations of the BM.

ISSN:0001-4966    

DOI:10.1121/1.420083

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The precedence effect refers to an auditory phenomenon which occurs when two similar sounds are presented from different locations with a brief delay, and only one sound is heard whose perceived location is dominated by the first source. Although the lagging source is not localized as an independent event, under some conditions, adults are able to extract its directional cues. Developmental studies suggest that this ability changes during development. However, those studies have used stimulus configurations which minimize the measurement of that ability. In the present study adults were first tested under several conditions, and the one which produced optimal performance was chosen for testing children. Using the minimum audible angle (MAA) task in the azimuthal plane, performance was compared for a single-source condition and two precedence conditions: in lag discrimination the lagging source changed location while the lead remained at midline, and in lead discrimination the reverse occurred. Subjects were 18 months old, 5 years old, and adult. Significant improvements in MAA occurred with an increase in age, especially in the precedence conditions. Within each group, performance was significantly better in single-source condition, followed by the lead and the lag discrimination.

ISSN:0001-4966    

DOI:10.1121/1.420106

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Masked threshold levels for signal detection or pitch ranking (low, middle, or high) were determined in conditions where one of three possible signal frequencies could be presented on a given trial of a three-interval forced-choice task. Thresholds were determined under conditions investigating binaural masking release (the masking-level difference, or MLD) and monaural masking release in modulated noise. It was assumed that part of the masking release in modulated noise was due to a within-channel analysis of information in the masker dips, and that part of the masking release was due to across-frequency analysis of temporal envelope information (comodulation masking release, or CMR). In the MLD experiment, the masker was a low-pass noise with a frequency cutoff of 1000 Hz. In the modulated-noise experiment, a broadband noise was square-wave amplitude modulated at a rate of 25 Hz and then digitally low-passed filtered at 1000 Hz. The results indicated that when the frequency separation between frequency components was relatively wide, masking release for pitch ranking was similar to that for signal detection. However, as the frequency separation between components narrowed, masking release for pitch ranking decreased. The results are consistent with an interpretation that information regarding signal frequency is relatively coarse under the conditions of masking release examined here.

ISSN:0001-4966    

DOI:10.1121/1.420084

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Just-noticeable differences (jnd’s) in the center frequency of bandlimited harmonic complexes were measured for normal-hearing subjects. A triangular and a rounded spectral envelope were used. The center frequency ranged from 500 to 600 Hz in a region representing the first formant of vowels, and from 2000 to 2100 Hz in a second formant region. The slope of the spectral envelope was either 50 or 100 dB/oct for the first formant region and 100 or 200 dB/oct for the second formant region. For the fundamental frequency of the complexes 100 and 200 Hz were used. The jnd’s were determined for various phase relations between the individual components of the complexes. For comparison we also determined jnd’s for a Gaussian white noise that was filtered with the same spectral envelopes as the harmonic complexes. A three-interval, three-alternative forced-choice task was used. All measurements were performed with roving stimulus level. The jnd’s found for center frequencies that were halfway between two harmonics were smaller than those found for center frequencies that coincided with a harmonic. The jnd’s for the noise bands were mostly between those of the two aforementioned groups. Except for a small group of stimuli, the phase relations had little effect on the jnd’s. The majority of the results for both the harmonic and the noise band stimuli can be described by a model using a spectral profile comparison. Most of the remaining data can be explained in the temporal domain from changes in the temporal envelope of the stimuli.

ISSN:0001-4966    

DOI:10.1121/1.420085

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This study examined the perceptual grouping of rapid tone sequences for listeners with normal hearing and listeners with unilateral and bilateral cochlear hearing loss. The sequence ABA–ABA–was used, where A and B represent sinusoidal tones bursts (10-ms rise/fall, 80-ms steady state, 20-ms interval between tones) and – represents a silent interval of 120 ms. Tone A was fixed in frequency at 250, 500, 1000, or 2000 Hz. Tone B started with a frequency well above or below that of tone A, and its frequency was swept towards that of tone A so that the frequency separation between them decreased in an exponential manner. Listeners were required to indicate when they could no longer hear the tones A and B as two separate streams, but heard only a single stream with a “gallop” rhythm. This is called the fission boundary. For the normally hearing listeners, the separation between tones A and B at the fission boundary was roughly independent of the frequency of tone A when expressed as the difference in number of ERBs (delta E) between A and B, which is consistent with a recent model of stream segregation [M. W. Beauvois and R. Meddis, J. Acoust. Soc. Am.99, 2270–2280 (1996)]. For the unilaterally hearing-impaired listeners, there was no consistent difference in the delta E magnitudes across ears, even though the auditory filters were broader in the impaired ears. This is not consistent with the theory of Beauvois and Meddis. The bilaterally hearing-impaired listeners sometimes showed delta E magnitudes within the normal range, and sometimes showed larger than normal delta E magnitudes. The results are discussed in terms of the factors that might influence perceptual stream formation in hearing-impaired listeners.

ISSN:0001-4966    

DOI:10.1121/1.420108

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Measuring thresholds for the detection of brief decrements in the level of a sinusoid is an established method of estimating auditory temporal resolution. Generally, a background noise is added to the stimulus to avoid the detection of the “spectral splatter” introduced by the decrement. Results are often described in terms of a temporal-window model, comprising a band-pass filter, a compressive nonlinearity, a sliding temporal integrator, and a decision device. In this study, thresholds for increments, as well as decrements, in the level of a 55 dB SPL, 4-kHz sinusoidal pedestal were measured as function of increment and decrement duration in the presence of a broadband background noise ranging in spectrum level from−20to+20 dBSPL. Thresholds were also measured using a 55-dB, 8-kHz pedestal in the absence of background noise. Thresholds for decrements, in terms of the dB change in level(ΔL),were found to be more dependent on duration than those for increments. Also, performance was found to be dependent on background-noise level over most levels tested. Neither finding is consistent with the predictions of the temporal-window model or other similar models of temporal resolution. The difference between increment and decrement detection was more successfully simulated by using a decision criterion based on the maximumslopeof the temporal-window output.

ISSN:0001-4966    

DOI:10.1121/1.420086

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The ability to detect amplitude modulation (AM) of a tonal probe can be disrupted by the presence of modulated masking tones. Two experiments examined whether a disparity in the interaural parameters of the probe and masker can reduce the amount of interference. In the first experiment, the effects of interaural time and intensity differences were studied in separate sets of conditions. With low-frequency carriers, the detection of 10-Hz probe modulation in the presence of 10-Hz masker modulation was not significantly affected by interaural time differences. With higher-frequency carriers, dichotic stimuli were generated through combinations of diotic, dichotic, or monotic probe and masker presentations in which the probe and masker did not share a common interaural intensity difference. In these conditions, the amount of interference was affected by the interaural configuration. However, monotic level differences between the probe and masker may have contributed to the effect of interaural configuration. In the second experiment, the probe and masker were presented through separate speakers in an enclosed listening environment. Spatial separation between the sources for the probe and masker led to a small reduction in the amount of interference. When the masker modulation rate was varied with the probe AM rate fixed at 10 Hz, the extent of tuning in the modulation domain in the sound-field conditions was similar to that obtained with diotic stimulus presentation over headphones.

ISSN:0001-4966    

DOI:10.1121/1.420087

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Temporal resolution was examined as a function of frequency region and listening region. The first experiment demonstrated that amplitude- and frequency-modulated tones are not appropriate stimuli to study temporal resolution as a function of frequency region, due to the availability of other cues in addition to temporal ones. In the other experiments, thresholds for detection of sinusoidal amplitude modulation of a noise band were measured as a function of frequency region, bandwidth, and level of surrounding notched noise masker. Temporal modulation transfer functions (TMTFs) measured in low- and high-frequency regions did not differ in sensitivity or in cutoff frequency, suggesting that initial “critical band” filtering did not affect temporal resolution. When the upper cutoff frequency of the noise was held constant, TMTF sensitivity increased with noise bandwidth, while the cutoff frequency of the TMTF did not show measurable change. These results are consistent with the predictions of an envelope detector model if peripheral filtering in the lower-frequency range is assumed to be approximately twice as wide as that estimated by measuring thresholds for a tone in notched noise. Restricting the listening region with notched noise increased thresholds for low modulation frequencies but not for high. This is consistent with other data showing that upward spread of excitation may increase the effective modulation depth, but only for low modulation frequencies.

ISSN:0001-4966    

DOI:10.1121/1.419617

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP