摘要:

Two experiments were performed to determine the effects of random intensity fluctuation on NoSo and NoSπ performance. Noise was used as both signal and masker, and stimuli were bands of noise from either 0–2.0 or 2.0–4.0 kHz. Signal and masker were either coherent (from the same source) or noncoherent (from independent sources). In the first experiment, noise fluctuation was achieved by modulating a wide band of noise. In the second experiment, fluctuation was achieved by narrowing the noise bandwidth. Results from both experiments indicated that NoSo performance was adversely affected by fluctuation and by noncoherent relation between signal and masker. NoSπ detection was not adversely affected by fluctuation at low frequency, and was affected less adversely than was NoSo detection at high frequency. This difference between NoSo and NoSπ performance is an important consideration when making inferences about monaural and binaural processing when the stimuli are fluctuating rather than temporally steady.

ISSN:0001-4966    

DOI:10.1121/1.390023

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A series of experiments investigated the effects of continuous broadband noise (ipsilateral) on wave V of the click‐evoked brainstem auditory evoked response (BAER). In general, a broadband noise masker increases the latency and decreases the amplitude of wave V. Varying both click and noise intensity, it was found that noise levels above about 40 dB SPL increase the latency and decrease the amplitude of wave V, regardless of click intensity. The effects of noise on wave V amplitude appear constant across click intensity, whereas the effects of a constant noise level on wave V latency decrease at higher click intensities. Both masking and adaptation increase wave V latency, but their combined effects are occlusive: rate‐induced wave V latency shift decreases in the presence of continuous broadband noise. The clinical and theoretical implications of these findings are discussed.

ISSN:0001-4966    

DOI:10.1121/1.390024

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A series of experiments evaluated the effects of broadband noise (ipsilateral) on wave V of the brainstem auditory evoked response (BAER) elicited by tone bursts or clicks in the presence of high‐pass masking noise. Experiment 1 used 1000‐ and 4000‐Hz, 60‐dB nHL tone bursts in the presence of broadband noise. With increasing noise level, wave V latency shift was greater for the 1000‐Hz tone bursts, while amplitude decrements were similar for both tone‐burst frequencies. Experiment 2 varied high‐pass masker cutoff frequency and the level of subtotal masking in the presence of 50‐dB nHL clicks. The effects of subtotal masking on wave V (increase in latency and decrease in amplitude) increased with increasing derived‐band frequency. Experiment 3 covaried high‐pass masker cutoff frequency and subtotal masking level for 1000‐ and 4000‐Hz tone‐burst stimuli. The effect of subtotal masking on wave V latency was reduced for both tone‐burst frequencies when the response‐generating region of the cochlear partition was limited by high‐pass maskers. The results of these three experiments suggest that most of the wave V latency shift associated with increasing levels of broadband noise is mediated by a place mechanism when the stimulus is a moderate intensity (60 dB nHL), low‐frequency (1000 Hz) tone burst. However, the interpretation of the latency shifts produced by broadband noise for 4000‐Hz tone‐burst stimuli is made more complex by multiple technical factors discussed herein.

ISSN:0001-4966    

DOI:10.1121/1.390025

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Interpreting plane‐wave beamforming as wave‐vector spectrum estimation, we show that the maximum entropy spectrum estimation technique (MEM) indicates the presence of false targets when used to beamform a spatially oversampled array. The source of this instability lies in the presence in the MEM technique of the prior estimate that the wave‐vector spectrum is white over the Nyquist band. By the use of a new, adaptive, high‐resolution spectral estimator, which includes MEM as a special case, the instability is removed. The results are of particular significance for short arrays operated at low frequencies.

ISSN:0001-4966    

DOI:10.1121/1.390026

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The usual maximum entropy power estimate is obtained under the assumption that the uniform line array has sensors half a wavelength apart. In this paper an approximate method is presented for computing the maximum entropy power estimate when the sensor spacing is less than half a wavelength. Results of a simulation illustrate the effectiveness of the method.

ISSN:0001-4966    

DOI:10.1121/1.390027

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Ultrasonic computed tomography (UCT) suffers significant quantitative errors when attempting attenuation imaging of refractive objects. The problem can be aggravated when a relatively large, single element, phase sensitive receiver is used. A method designed to maintain high resolution while minimizing refractive error is introduced. This technique (referred to as profile migration) utilizes an array receiver and an algorithm based on aperture diffraction theory. The profile migration and a more conventional migration approach are applied to data obtained from single projection scans of an object under refractive and nonrefractive conditions. Attenuation projections generated using two single‐element receiver methods are also compared. Reconstructed images using projections produced via the profile migration method are compared to those using a technique currently employed on our UCT breast scanner. For both minimal and realistic refractive situations the profile migration method displays advantages over the other methods based on a qualitative assessment of the projections and images.

ISSN:0001-4966    

DOI:10.1121/1.390028

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Following earlier work by Chessell [J. Acoust. Soc. Am.62, 825–834 (1977)] it is shown that his single‐parameter theory can be used to predict the measured transmission spectra between a source and receiver located above ground surfaces having a wide range of acoustic impedance—or effective flow resistivity. Surfaces behaving essentially as locally reacting range from new‐fallen snow, effective flow resistivity σ=10–30 cgs rayls, through grass‐covered ground, σ=150–300 rayls, to mature asphalt, σ=30 000 rayls. The thermal‐conduction and viscous boundary layer of the surface limits the effective flow resistivity of even the hardest and most impervious surface to the range 105–106rayls, depending on frequency: this value is appropriate to evaluate the complex reflection coefficient of the paint‐sealed surface of a thick slab of reinforced concrete.

ISSN:0001-4966    

DOI:10.1121/1.390029

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Fluctuations of sound propagating vertically through the ocean are considered. Expressions are derived for the acoustic phase structure function and spectra. As an application, the errors incurred by systems based on travel time measurements (depth sounding and tracking systems) are estimated. They are found to be generally negligible.

ISSN:0001-4966    

DOI:10.1121/1.390030

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Surface‐reflected/bottom‐reflected transmissions over a slowly sloping bottom in a deep ocean are investigated using ray theory. For convenience, sound speed is taken to be bilinear with depth in the water, while the bottom structure is assumed to be uniform. The sound source and receiver are located on or near the ocean boundaries, and the effects of bottom slope on ray geometry, per‐ray travel time, and incoherent total‐field amplitude are examined. For transmission ranges of tens of km, and for typical deep‐ocean slope inclinations, travel time may change by about 1.0 s, relative to its value for a horizontal bottom. The time difference between ray arrivals decreases (increases) in the divergent (convergent) channel. Principal effects of bottom slope on total‐field amplitude arise primarily through bottom‐loss modifications, rather than through spreading loss. An inclination‐angle magnitude of only 0.25° is shown to cause a decrease of about 8 dB (increase of about 4 dB) in the incoherent amplitude in the convergent (divergent) channel, for ranges of about 100 km. Changing bottomed receiver location in a convergent channel, in order to improve source detectability, is shown in an example to be much more effective than in the case of a horizontal bottom. In a corresponding divergent channel, however, the strength of the acoustic reception is much less sensitive to variations in receiver location.

ISSN:0001-4966    

DOI:10.1121/1.390031

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

An expression for the spatial coherence in a semicircular noise field is obtained which involves the functionJ0(x) and a single integral that can be evaluated by Gaussian quadrature. The semicircular noise model is motivated by an idealized representation of ship traffic plying a well‐defined sea lane. The model coherence is characterized by a marked dependence on sensor orientation relative to the sea lane. An extension of the single lane model to the case of multiple parallel lanes yields a coherence expression which involves the weighted sum ofJ0(x) and the previously derived coherence function for a single lane. For this general situation the weighting factors depend on the relative power distribution among the lanes. Model calculations show good qualitative agreement with coherence estimates ford/λ≤1 which were computed from acoustic data for sensor orientations both parallel and perpendicular to known shipping lanes.

ISSN:0001-4966    

DOI:10.1121/1.390032

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP