摘要:

An ultrasound system has been developed to generate microbubblesin vivofor use as ultrasound contrast agent. Possible applications include diagnosis of reflux in the urinary tract. In experiments designed to elucidate the contrast microbubble generation process, acoustic bursts (at 1.8 MHz, 125 ms) were propagated through a latex rubber balloon, modeled after a rabbit urinary bladder, containing fluids of various air and carbon dioxide saturations and concentrations of cavitation nuclei (0.198-μm-diam polystyrene particles). The peak rarefactional pressure threshold for contrast microbubble generation, as visualized with a diagnostic ultrasound system, decreased approximately a factor of 2 for increasing particle concentration from108to1010 particles/cc,with the lowest threshold of 5.24 MPa. For samples with gas saturations below 50% and1010 particles/cc,the average thresholds were at least twice as high as those of more saturated fluids (with mean threshold for saturated fluids of 6.45 MPa), and samples containingCO2had considerably lower thresholds than respective under-saturations in air. At a fixed pressure amplitude, echogenicity tended to increase with both increasing particle concentration and gas saturation; this was more favorable for samples containingCO2.Even in a restricted-nuclei environment such as the urinary bladder, generation of vaporous cavitation should be possible; however, subsequently, abundant gas is needed to grow vaporous bubbles to persistent and imageable sizes, to assist in the diagnosis of urinary reflux.

ISSN:0001-4966    

DOI:10.1121/1.423070

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Finite-amplitude stationary acoustic waves in a closed resonator subjected to simultaneous excitation at fundamental frequency and its second harmonic are analyzed in the frame of the second-order nonlinear theory. The conditions when the dual-frequency excitation leads to a quality factorQhigher than in the case of a single-frequency excitation are derived. TheQenhancement can be explained in terms of active suppression of cascade processes of the generation of higher harmonics. The second harmonic generated in the resonator is compensated by the second harmonic radiated by the piston. Suppression of higher harmonics also can be explained as being due to effective down conversion of energy from the second harmonic to the fundamental wave in the nonlinear parametric process.

ISSN:0001-4966    

DOI:10.1121/1.423090

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Matched-field source localization methods can be sensitive to environmental parameter mismatch. A statistically optimal approach to source localization in the presence of environmental uncertainty is the maximuma posterioriprobability (MAP) estimator. Unfortunately, practical implementation of the MAP estimator results in a computationally intensive processor. In this paper, a localization technique is presented that is a computationally efficient approximation to the MAP estimator. A two-step search procedure is used to estimate source position. The first step utilizes an approximation to the MAP estimator which allows much of the computation to be computed efficiently off-line. This step also includes a computationally efficient range-depth smoothing which provides robustness to grid density. In step two, ambiguities arising from step one are resolved using a fine-grid search procedure over the source location parameters. Simulations using the NRL Workshop benchmark environment, which has seven uncertain environmental parameters, show the performance of the technique to be comparable to that of the MAP estimator while requiring only a fraction of the computations.

ISSN:0001-4966    

DOI:10.1121/1.423091

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

In this Letter a method is described for computing the response of a linear three-dimensional classical model of the cochlea. The method can be applied when the geometry of the model lends itself to separation of variables, for instance, when the model has the shape of a homogeneous rectangular block or box. The method is an improvement over previously published computation methods, because it handles forward and retrograde traveling waves symmetrically, and it can be used for forward as well as inverse solutions. Furthermore, the method can be extended to a time-domain solution.

ISSN:0001-4966    

DOI:10.1121/1.423071

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This comment specifies reservations about the generalizability of de Boer’s letter [J. Acoust. Soc. Am.103, 3725 (1998)] in which he claims that the real (nonlinear) cochlea can be analyzed using a straightforward extension of a common linear analysis. Two points are elaborated. Basically they regard physical aspects of boundary conditions at the cochlear partition and at the helicotrema.

ISSN:0001-4966    

DOI:10.1121/1.423092

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Auditory saltation is a mislocalization phenomenon in which click trains presented successively at two discrete spatial locations appear to originate from a smoothly varying series of source locations spanning the true source locations. The temporal parameters of this illusion are investigated with click stimuli delivered via headphones with the goal of establishing a portable test of the illusion. A simple rating scale was developed which required human participants to indicate the degree to which the perceived sound locations were heard as continuous (evenly distributed across space) versus discontinuous. Control conditions were devised to provide a baseline against which to compare the illusion. Auditory saltation is discussed in terms of perceptual grouping.

ISSN:0001-4966    

DOI:10.1121/1.423093

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Woolsey Hall, Yale University, which houses the Newberry Memorial Organ is not climate controlled and some listeners have the impression that the tone of the organ varies with the seasons. The present work measured the overall SPL, and the SPL’s of the harmonics of the C’s of a number of flue and reed stops of the organ in August and March, 1996 to determine possible seasonal differences. The C’s of flue and reed stops were recorded in the hall using a calibrated microphone and digital tape recorder system. The tapes were played into a computer, using Spectraplus 4 software, and the overall SPL levels of each C together with the SPL levels of the harmonics were extracted. The overall levels of 67 out of the 76 observations of the C’s were 0.5–14 dB higher in August than in March, confirming the impression that the organ sounds louder in the summer. In 55 observations higher numbered harmonics that were present in August were absent in March. These findings are compatible with the impression that the organ sounds brighter in summer.

ISSN:0001-4966    

DOI:10.1121/1.423094

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP