摘要:

This paper examines the Department of the Army's (DA) audiometric follow‐up program subsequent to identifying standard threshold shift (STS). The Occupational Safety and Health Administration's current policy has provisions for one optional follow‐up audiometric test to be completed within 30 days of identifying an STS. Current DA policy requires two follow‐up audiometric tests to be conducted within 60 days of identifying the STS. Successes and failures of the DA policy are presented and discussed using 387 000 annual and follow‐up audiograms obtained on noise‐exposed civilians and soldiers and entered into the Hearing Evaluations Automated Registry System (HEARS) audiometric database. Program participation rates, elapsed time for completion of the follow‐up audiograms, and noted changes in STS identified in the follow‐up will be presented. The results have implications for hearing conservation policy decisions on follow‐up audiometric requirements.

ISSN:0001-4966    

DOI:10.1121/1.2028543

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Breaking waves produce large numbers of gas bubbles immediately below the ocean surface. The effect of these bubbles on the sound speed is important in studies of sound propagation in the upper ocean. Recent studies have also indicated that the bubbles can play a major role in gas exchange through the air‐sea interface. The bubbles are also being used as passive tracers to study the flow field in the upper ocean. For most studies involving the bubbles, it is of importance to determine the actual size distribution of the bubbles. Here, some results from the SWAPP and Critical Sea Test experiments, where a six‐frequency acoustic backscatter technique was used, will be presented. Variability in the bubble size distribution will be related to the “age” of the bubble clouds as well as to the presence of coherent flows in the ocean, such as Langmuir circulation.

ISSN:0001-4966    

DOI:10.1121/1.2028588

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In the last few years, it has been realized that the near‐surface clouds of bubbles produced by breaking waves may play an important role in the generation of oceanic ambient noise. In addition, the clouds are potentially very strong scatterers of sound and may therefore be responsible for the anomalous backscattering data reported by several investigators. In the first part of this paper, some theoretical results are reviewed concerning the active and passive acoustic behavior of bubble clouds and are compared with experiment. In the second part of the paper, the results obtained with a simple model of the evolution of the clouds in space and time are described. The model is based on two‐fluid‐averaged equations and describes the motion of the bubbles under the action of buoyancy and drag.

ISSN:0001-4966    

DOI:10.1121/1.2028627

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The results of laboratory experiments designed to characterize the underwater sound field radiated by a breaking wave are presented. Two general types of experiments were performed. First, progressive gravity waves were produced in a flume tank (2×2×12 ft; 7×7×6 ft) by a plunger wavemaker (wedge‐shaped) at one end of the flume. Near the other end was placed an airfoil with a given angle of attack forcing gravity waves to break in the tank region. The second series of experiments involved the introduction of a jet of water, simulating a plunging breaker, onto the plane water surface of the tank. The water jet was produced by releasing a fixed volume of water, held in a cylindrical container, onto the surface. The jet characteristics were varied by changing the container's volume and height. The hydrodynamic characteristics of the bubble plumes generated by these two mechanisms were measured by using a high‐speed video camera. The underwater acoustic emissions from these air entrainment processes were recorded simultaneously with the high‐speed video camera and a digital oscilloscope that possessed a large memory. Concurrent video image, power spectrum, and observed time evolution of the bubble plumes generated by these two mechanisms reveal a dominant acoustic signal occurring at frequencies lower than 200 Hz. [Work supported by ONT.]

ISSN:0001-4966    

DOI:10.1121/1.2028668

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

It is not difficult to predict how sound scatters from a fluid‐loaded elastic shell based on exact elastodynamic theory, provided the shell is a sphere or an infinite cylinder. Problems arise for more general shapes, however, and although some success has been obtained for spheroids and cylinders with hemispherical end caps, the results are rather tedious, if not disappointing, when one wishes to extend the frequency range or aspect ratio of the target. Some progress has been made for results predicted from thin‐shell theories either utilizing finite‐element methods in two and three dimensions orTmatrices based on thin‐shell theories. In this study, some common thin‐shell theories that are employed for spherical elastic shells with exact normal mode theory are examined, with the goal of extending the results to elongated targets. Limitations of the various thin‐shell theories are explored for both the frequency range and thickness.

ISSN:0001-4966    

DOI:10.1121/1.2028703

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

At a previous meeting of the Society [R. Lim and R. H. Hackman, J. Acoust. Soc. Am. Suppl. 187, S40 (1990)], an improved transition matrix formulation for multiple scattering calculations was suggested, capable of exact numerical results at low to moderateka. Exact results for acoustic scattering from finite linear arrays of up to ten spherical shells were presented. In the far field, comparisons with the field due to truncated infinite arrays showed remarkable agreement. Hence it is possible to approximate the acoustic far field due to moderate to large finite groups of scatterers with fast calculations involving infinite arrays. In this presentation, results for the scattering from finite and infinite linear arrays imbedded in elastic hosts will be given. Here the strong coupling, possibly due to the host's shear degrees of the freedom, causes end effects to be more important. Energy cross sections for both compressional and shear wave incidence are presented and discussed.

ISSN:0001-4966    

DOI:10.1121/1.2028746

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Limit‐cycle‐oscillator models of spontaneous otoacoustic emissions have been used to provide qualitative descriptions of the interactions of spontaneous emissions with external tones and their response to aspirin administration. The parameters of several classes of such models are estimated from data on the relaxation dynamics of spontaneous emissions, associated with the onset of, and recovery from, suppression by external tones. Aspirin administration is used to control the emission levels, with the relaxation time being observed to increase with decreasing emission level. The models considered include a modified Van der Pol oscillator, with the possibility of linear passive damping in the limit of large displacements, and an oscillator with nonlinear time‐delayed elasticity. [Work supported by NIDCD.]

ISSN:0001-4966    

DOI:10.1121/1.2028802

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The absolute sensitivity of 3‐month‐old human infants was estimated using ABR and a behavioral measure. The stimuli were tone pips with 2‐cycle rise, no steady state, and 2‐cycle fall. Frequencies of 1, 4, and 8 kHz were used. For the ABR, thresholds were determined by reducing the level of the tone pip from 20 dB nHL until the response was no longer observed, and then increasing the level until a response was again seen. Threshold was defined as the lowest level at which the ABR could be observed. Behavioral thresholds were determined using an adaptive procedure. Threshold was defined as the average of the last eight reversals obtained. A trial was considered “correct” if an observer said “yes” when a signal was presented to the infant, or ”no” when no signal was presented. The observer had no prior knowledge of the type of trial being presented, and none of the adults in contact with the infant could hear the tone pips. The correlation between ABR and behavioral thresholds depended on the frequency of the tone pip. This suggests that different factors contribute to between‐subject variability in the two measures, but that under certain circumstances, infant behavioral threshold can be predicted from the ABR threshold with some degree of accuracy. [Work supported by NIH DC000520.]

ISSN:0001-4966    

DOI:10.1121/1.2028851

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

ABRs were studied in cats for up to 45 days after kainic acid injections into the superior olivary complex (SOC) or cochlear nucleus (CN), which destroyed neurons while sparing fibers of passage and the terminals of axons of extrinsic origin. With destruction of the CN neurons, there was a loss of components P2 (III), and P3 (IV) and only a modest attenuation and prolongation of the latency of components P4 (V) and P5 (VI). Unexpectedly, to stimulation of the ear contralateral to the injection side, waves P2 (III), P3 (IV), and P4 (V) were also attenuated and delayed in latency. The sustained potential shift from which the components arose was not affected. Following bilateral SOC destruction, there was a moderate attenuation of wave P2 (III), and a marked attenuation of up to 80% of P3 (IV), P4 (V), P5 (VI), and the sustained potential shift. No component of the ABR was totally abolished. The results are compatible with multiple brain regions contributing to the generation of the components of the ABR beginning with P2 (III) and that components P3 (IV), P4 (V), and P5 (VI) and the sustained potential shift depend particularly on the integrity of the neurons of the SOC bilaterally.

ISSN:0001-4966    

DOI:10.1121/1.2028892

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The modes of vibration of four tennis rackets are compared: two of the new “wide‐body” rackets, a midsize racket with a graphite composite frame, and an inexpensive midsize racket with an aluminum frame. In all four rackets, the strings vibrate in modes that resemble those of a membrane, while the frames show a variety of bending and torsional modes. The coupling between the string modes and the frame modes is weaker than expected. The enormous popularity of wide‐body rackets is attributed to their greater “power,” resulting from greater frame stiffness. This is especially evident in the higher frequencies of the torsional modes of the frame, although bending modes occur at higher frequencies in the wide‐body rackets as well. The wide‐body rackets in this study were furnished by Pro‐Kennex.

ISSN:0001-4966    

DOI:10.1121/1.2028902

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP