摘要:

The effects of a random current on the fluctuations of an underwater sound transmission are considered for a horizontal isospeed channel. The current is assumed to have a constant unperturbed part, plus small depth‐dependent random variations in magnitude and direction. Using ray theory the current influence on ray geometry, travel time, spreading loss, and boundary effects is investigated. Then consistent approximations to total‐field phase and amplitude at a fixed receiver are obtained. Acoustically pertinent statistics, including the mean and variance of total‐field phase, are found as functions of the statistics of the current fluctuations. Conversely, it is shown how statistics of the assumed current structure can be described in terms of the total‐field phase statistics. This initial study provides a basis for future investigations which will utilize more elaborate current and sound‐speed models. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2015409

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The sound from a steady distant sinusoidal source in the sea fluctuates in amplitude because of multipath interferences, surface motion, internal waves, microstructure, and other propagation effects. Such fluctuations are found to follow a Rician or modified‐Rayleigh distribution having as parameter the fraction of random power in the received signal. At the output of a conventional sonar processor—namely, a narrow‐band filter, a squarer, and an integrator—the fluctuation statistics are accordingly determined by the propagation processes occurring in the sea between source and receiver. On the other hand, ambient sea noise is found, from analyses of field recordings, to have fluctuation statistics determined by the processor itself; ambient noise samples at the processor output obey a chi‐square distribution having a number of degrees of freedom equal to twice the BT product of the processor, as would be expected from a Gaussian input. The two distributions—Rician‐power for signals and chi‐square for noise—while formally different, have remarkable similarities in the limits. In short, the output fluctuations statistics of narrow‐band signals and Gaussian ambient noise can apparently be predicted from estimates of the degree of randomness introduced by the prevailing propagation conditions, and from a knowledge of the processor, respectively, provided these statistics remain stationary during the analysis period. [Work supported by the Navy ASW Systems Project Office, Code ASW‐13.]

ISSN:0001-4966    

DOI:10.1121/1.2015419

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

In our original simulation model [J. Acoust. Soc. Am.54, 996 (1973)] we modeled sound absorption in seawater with the coupled system(Mg+2, SO4−2, CO2, H2O)and achieved reasonable fits to the ocean data. The main absorption is due to theMg+2SO4−2ion pairing but the low‐frequency (<10 kHz) excess was attributed to theMg+2‐HCO3−ion‐pairing process. The quantitative fit was very dependent on the parameters of theMg+2‐SO4−2process. We have recently redetermined theMgSO4system with much greater precision. In addition we have examined the sound absorption due toNa+‐SO4−2ion‐pairing and the popular boric acid‐borate process. We present a more elaborate simulation of seawater absorption using all of the above data. The simulation provides a more meaningful fit to the actual ocean data. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2015422

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

An abandoned water line, 1000 ft in length, consisting of 2‐in.‐i.d. steel pipe buried a depth of approximately 3 ft, has been used to investigate the magnitude of earth surface accelerations generated by propagation of plane acoustic waves in the gas contained within the pipe. Measurements have been made of the attenuation of the acoustic waves, of the radial and longitudinal acceleration of the walls of the pipe, and of the acceleration at the earth's surface over a frequency range of 200–2000 Hz at a point 120 ft from the acoustic source. Results of this investigation have indicated that, in general, radiation from the pipe walls is large compared to that calculated from theory for a small hole in the pipe. A general description of the facility and experimental measurements will be presented. [Work supported by the American Gas Association.]

ISSN:0001-4966    

DOI:10.1121/1.2015427

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The method of phase comparison of continuous waves is applied to determine the dispersion relation, phase velocity, and group velocity as a function of frequency in dispersive materials. A combination of the variable frequency method and the variable path‐length method is found necessary to eliminate any uncertainty in the dispersion relation determination. Experiments are performed on specimens of various thicknesses. A constraint equation is derived from the assumption that the dispersion relation is unique and independent of the specimen thickness. This equation provides a procedure for determining the absolute number of wavelengths in the specimen. A transducer self‐compensation procedure is also used to reduce the errors in analyzing the test results among the specimens. Measurements in unidirectional, fiber‐reinforced Boron‐Epoxy specimens show good agreement with results reported previously in the literature. [Supported by AFOSR Grant 76‐299.2.]

ISSN:0001-4966    

DOI:10.1121/1.2015433

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Measurements are reported of the arrival times of broadband ultrasonic pulses scattered by a circular, cylindrical, solid inclusion imbedded in a matrix whose longitudinal wave speed is lower than that of the cylinder. The wavenumber‐inclusion radius product,ka, ranged from approximately 1 to 15. The matrix materials were water, acrylic, and brass and the inclusion materials were copper, quartz, stainless steel, and tungsten. A strong, scattered pulse, corresponding to a circumferential wave on the cylinder is observed forcs>cl, wherecsis the shear wave speed of the inclusion material andclis the longitudinal wave speed of the matrix material. These observations are in agreement with published results. The experiments also show that the pulse corresponding to the circumferential wave is not observed forcs

ISSN:0001-4966    

DOI:10.1121/1.2015454

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Analytical models of a wave interference directional microphone were developed for use on a digital computer. Many microphone design parameters were analyzed in detail, Wave interference directional microphones consist of cylindrical tubes with sound pressure receiving ports spaced along their length. Electroacoustical transducers are placed near one end. Both ends are terminated by acoustical impedance elements. The microphone is classified as an end‐fire receiving array for air use. Variations of this type of microphone have existed for many years; however, a detailed analytical model was never developed and verified. The microphone was modeled using acoustical transmission line theory. Various microphone configurations were evaluated for frequency response and directivity. The most important effects on performance, tests proved, are caused by internal sound wave propagation, and these effects can alter performance drastically from that predicted by geometric line array theory. The models were verified by comparison of measured and predicted frequency response and directivity of actual microphones. Many design parameters of the directional microphone were analyzed, utilizing the theoretical models. This information will be presented.

ISSN:0001-4966    

DOI:10.1121/1.2015461

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Working group chairpersons will present reports of their recent progress on writing and processing letter ballots on various shock and vibration standards. In addition, there will be a report on the interface of S2 activities with those of ISO/TC 108, (The Technical Advisory Group for ISO/TC 108 consists of members of S2, S3, and other persons not necessarily members of those committees.) The TAG meeting will be held jointly with the meeting of Standards Committee S2.

ISSN:0001-4966    

DOI:10.1121/1.2015466

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A review of the hydrodynamic interpretation of the various sound or collective modes which propagate in superfluid helium will be presented. These will be separated into two categories: (a) bulk geometries, (b) helium confined between walls as narrow as 10 Å. As regards the bulk, the thermal propagating waves (second sound) as well as ordinary density waves will be discussed. For the confined geometries third sound (analogous to long gravity waves with the restoring force replaced by the van der Waals attraction to the substrate) and fourth sound (another pressure wave propagated through a modified compressibility) will be discussed. Use of the propagation and attenuation of sound to measure various thermodynamic properties will be explored.

ISSN:0001-4966    

DOI:10.1121/1.2015467

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

High precision measurements of the velocityu, the attenuation α, and dispersionDof first sound have been made along the lambda line in4Hefor frequencies up to 1 MHz and for temperatures |T−Tλ| ⩽1 mK. Our data can be understood in terms of order‐parameter relaxation and order‐parameter fluctuations. We find that the time τ characterizing the relaxation process belowTλand the time τ characterizing the fluctuations both above and belowTλhave the same temperature and pressure dependence; that is, only one time scale is important very near the phase transition. The analysis of the attenuation due to fluctuations aboveTλallows us to determine a simple scaling law for all our data, valid over four decades in ωτ,α(T > Tλ)/α(T = Tλ) = ωτ/(c + ωτ), wherec≃ 0.5. A similar relation for the dispersion atT>Tλwas also determined.

ISSN:0001-4966    

DOI:10.1121/1.2015475

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP