摘要:

The rate of damping of bubble oscillations provides us with a clue as to their precise origin and mode of generation. Data on rates of damping of acoustical pulses from various sources will be examined, particularly those from low sea states. At a given frequency, the observed rates vary by a factor of order 10; the minimum damping rate corresponds closely to thermal damping. Theoretical reasons for the higher damping rates will be discussed, including the transfer of some energy to shape oscillations.

ISSN:0001-4966    

DOI:10.1121/1.2028897

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The physics of sound generation from light and heavy rain will be discussed in the context of the measurement of rainfall rate by monitoring underwater ambient sound levels. The physics of sound generation by small drops/light rain at normal and oblique incident will be reviewed. These results will be contrasted with preliminary field measurements of the sound produced by heavy rain during the Acoustic Rain Experiment at the Ocean Test Platform in the Gulf of Mexico. The underwater sound spectra generated by heavy rain will be explained using new laboratory measurements of the sound generated by individual large drops showing acoustic energy from initial impacts (at 10–15 kHz) and from entrained bubbles at lower frequencies.

ISSN:0001-4966    

DOI:10.1121/1.2028946

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Due to recent advances in DSP technology, active control now shows much promise for reducing sound and vibration in a number of difficult applications. In this tutorial the concepts behind the active control approach will be introduced with some historical perspective. The basics of feedforward control will then be reviewed and related to the more traditional state feedback methods. Application of the technique to fundamental problems in the control of sound and vibration will be given with some consideration of the design and choice of the appropriate control transducers and cost functions. Useful analytical methods and mechanisms of control will be considered. Recent successful applications of the technique to realistic systems will be reviewed and future promising research directions discussed.

ISSN:0001-4966    

DOI:10.1121/1.2028990

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

It has been established that some dolphins possess well‐developed acoustic orientation (echolocation) and information gathering abilities, though substantially less is known about the system of sound generation and beam formation. Dolphins use a narrowly focused sound beam that emanates from the forehead and rostrum during echolocation. The primary objectives of this study were to simulate the effects of modeled tissues on beam formation, and to test the viability of various hypothetical sound source locations. 2‐D simulations of sound propagation using parasagittal outlines from reconstructions of CT scans through the forehead tissues of two delphinids were conducted. Finite difference wave propagation programs were run on a Cray supercomputer. Preliminary simulations suggest that beam formation results primarily from reflection off of the skull and air sacs. These results do not depend strongly upon the precise values of velocity and density assumed for the bone. Beam angles closely approximate those measured by experimental methods for a source located in a region of the model corresponding to the monkey lip/dorsal bursae complex (MLDB), and not elsewhere. These results suggest that: (1) the skull and air sacs play the central role in beam formation; (2) the geometry of reflective tissue is more important than the exact acoustical properties assumed; and (3) experimentally observed beam patterns are best reproduced in the simulations when the sound source is placed in the region of the dolphin's head known as the MLDB (Cranford, 1988).

ISSN:0001-4966    

DOI:10.1121/1.2029031

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Low‐frequency vocalizations were recorded from fin whales,Balaenoptera physalus, in the Gulf of California, Mexico (mainly the Sonoran coast) during three cruises in March 1985, and February and August 1987. The predominant sounds were patterned 20‐Hz pulses. In March 1985, they were in sequences characterized by regular interpulse intervals of 9 s, whereas in August 1987, they were mainly in sequences of doublets in alternating 5‐ and 17‐s interpulse intervals. In February 1987, no patterned pulse sequences were detected. The 20‐Hz pulses were about 0.5‐s duration, typically sweeping from 42 to 20 Hz. Other fin whale vocalizations were of similar duration and typically modulated downward in frequency, averaging 82, 56, and 68 Hz, respectively, for the three cruises. Gulf pulses differed from those elsewhere in terms of frequency sweep, sound level, and time interval patterns. This uniqueness points out the possibility that fin whales in the Gulf of California may represent a regional stock based on their sound characteristics.

ISSN:0001-4966    

DOI:10.1121/1.2029083

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The noise induced by the turbulent boundary layer pressure fluctuations on hydrophones embedded in an elastomer layer is usually predicted by assuming that the layer is unbounded in the lateral direction and the hydrophone does not influence the stress field [S. H. Ko and H. Schloemer, J. Acoust. Soc. Am.85, 1469–1477 (1989)]. These predictions neglect the flow‐induced noise associated with the edges of the layer as well as the diffraction, reflection, or shadowing of the stress field by the hydrophones. To avoid these limitations, the finite element code ATILA [B. Hamonicet al., J. Acoust. Soc. Am.86, 1245–1253 (1989)] is used to model hydrophones embedded in viscoelastic layers of finite extent. The transfer function of the layer and/or the noise sensed by the hydrophone are computed for a given excitation of the outer surface or of the edges of the layer for different sizes. More complex multilayered structures are also examined. [Work partially supported by the Direction des Recherches Etudes et Techniques.]

ISSN:0001-4966    

DOI:10.1121/1.2029120

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A hybrid discrete‐Wigner distribution/autoregressive moving average (DWD/ARMA) cyclespectrum estimator is developed and applied to signaturing of nonstationary acoustic returns. The ARMA effects a high resolution mapping from the time‐frequency Wigner plane to the cyclespectrum. The hybrid estimator is used to characterize cyclic correlations hidden in conventional spectral estimators that prove useful to signal source identification and to signal and noise decomposition. The ARMA performs the high resolution needed in the cycle‐frequency domain to characterize the signal source. The autoregressive model provides resolution, inclusion of the moving average into the model reduces noise sensitivity. The results of the hybrid DWD/ARMA cyclespectrum estimator are integrated into the development of a signature identification system and tested on sonar data. [Work supported by a NAVCOASTSYSCEN's Independent Research project sponsored by the ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2029167

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Rayleigh's principle, based on a circular jet, yields the cell length (defined by the first mode) of a square jet only 4% too long [J. Acoust. Soc. Am. Suppl. 187, S80 (1990)]. Yet the cellstructureis fundamentally different. The square one is dominated by expansive plane waves moving in (at propagation velocityc) from each side (lengthl) reflecting repeatedly from the opposite sides, with axial period corresponding to 2l/c, accompanied by expanding compressive waves centered on each corner that bring the plane waves to ambient pressure at the edges. The axial pressure distribution is quite nonperiodic: The periodic cell length, corresponding to (1/√)×2l/c, is not very overt. In the circular case the converging cylindrical wave “reflects” from the center with a rather impulsive compressive wavefront, resulting in almost ambient pressure across the jet at timeD/c; the original overpressure is almost exactly reversed at time 2D/c. The periodic cell length, however, corresponds to(π/2j0.1) × (2D/c) = 0.6531 × 2D/c. [Work partially supported by the Texas Advanced Research Program.]

ISSN:0001-4966    

DOI:10.1121/1.2029207

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The boundary element method (BEM) has been utilized to formulate a three‐dimensional math model for the foam earplug‐earcanal and earmuff‐earcanal systems. The BEM technique consists of transforming the partial differential equation describing the behavior of the unknowns, inside and on the boundary of the domain, into an integral equation relating only boundary values. In representing the viscoelastic properties of the foam earplug, both integer differential operator and fractional operator constitutive equations were utilized. The BEM model was then utilized to study the steady‐state and transient responses (including the prediction of each system's insertion loss) for different earplug‐earcanal and earmuff‐earcanal configurations. The basic BEM model has demonstrated the ability to predict the internal resonances for a production earmuff shell‐earcanal system and the measured insertion loss of a foam earplug‐earcanal system subjected to an impulse. Together with these findings a general discussion of the need for improvements in the ability to model a variety of earplug‐ and earmuff‐earcanal configurations will be presented.

ISSN:0001-4966    

DOI:10.1121/1.2029247

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The application of insert earphones for annual audiometry was evaluated in an on‐going occupational hearing conservation program. For four consecutive years employees were given an extra left‐ear audiogram using a 3A‐type insert earphone immediately after completing their standard annual audiograms using TDH‐49 earphones in supra‐aural cushions. For the final 3 yr an additional left‐ear audiogram using the TDH‐49 earphone was given last, to control for order effects. Sixty employees participated all 4 yr. Analysis of thresholds obtained each year using each earphone indicated that small threshold correction factors for the insert earphone are necessary at each frequency (not just at 6 and 8 kHz) to allow results to be compared directly to supra‐aural earphone thresholds. The manufacturer's suggested 10‐dB correction factor at 8 kHz appeared too small for the particular combination of TDH‐49 and insert earphones used. Standard deviations of thresholds between individuals were similar for both earphones. Year‐to‐year repeatability of thresholds within individuals was equivalent with both earphones. As long as absolute calibration is accounted for, insert earphones are a viable alternative transducer for audiometry in hearing conservation.

ISSN:0001-4966    

DOI:10.1121/1.2028508

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP