摘要:

A general formula is derived for the acoustic intensity reflected by an object of arbitrary shape embedded in a medium of variable index of refraction. Formulas are applicable in the ray acoustics limit. As specific examples, contours of equal acoustic intensity over a planar receiver surface are calculated for a plane wave of sound incident upon an ellipsoid and upon a sphere embedded in medium in which the velocity is a linear function of depth.

ISSN:0001-4966    

DOI:10.1121/1.1903445

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

In a previous paper [J. Acoust. Soc. Am.53, 719 (1973)] we discussed the calculation of the scattered field for scalar plane‐wave incidence on a type of filled periodic corrugated surface. The surface consisted of parallel half planes, alternate adjacent pairs of which were filled with a material having different density, and supporting waves of different velocity structure from those in the surrounding medium. This paper considers a related problem with plates of finite rather than semi‐infinite depth. Both soft‐ (Dirichlet) and hard‐ (Neumann) boundary conditions are considered. Limiting cases of the problems considered here correspond to the results of Deryugin. The most characteristic effects observed are the Brewster‐angle and resonance anomalies.

ISSN:0001-4966    

DOI:10.1121/1.1903446

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The errors of a ray‐theory solution of simple harmonic acoustic propagation in an inhomogeneous medium are shown to be corrected by radiation from a distribution of simple source whose density has the phase of the incident ray amplitude. In the case of singularities in the ray field, the density involves symbolic functions. The radiation from the distributed sources is obtained by iterative ray techniques based either on a Green's function formulation or an integral equation. An alternative technique uses an iterated artificial wavenumber. In an example, the modified ray‐theoretic solution for lateral waves is given.

ISSN:0001-4966    

DOI:10.1121/1.1903447

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The underlying assumptions of Kirchhoff‐type formulations of the cross sections of cylindrical and spherical scatterers are reviewed, with emphasis on the respective merits of alternative asymptotic techniques. Both the geometric‐ and physical‐acoustics formulations are shown to lead to substantial errors when applied to rigid scatterers partially coated with a finite‐impedance coating. The sources of these errors are discussed in some detail.

ISSN:0001-4966    

DOI:10.1121/1.1903448

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The scattering of time‐harmonic waves by an obstacle in a locally inhomogeneous infinite medium—i.e., a medium which becomes essentially homogeneous at some distance from the obstacle—is treated. An artificial outer boundary is drawn within the homogeneous region and an outer‐boundary integral equation (OBIE) is used to relate the dependent variable and its normal derivative at this boundary. This in turn is used to complete another set of equations obtained by standard techniques—e.g., finite differences—for the inhomogeneous interior region. Numerical examples are given for water wave scattering by a circular island surrounded by various locally variable depth topographies.

ISSN:0001-4966    

DOI:10.1121/1.1903449

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

A reasonably comprehensive technical effort is described dealing with the investigations of acoustically generated seismic waves of Apollo 16 and Apollo 17 origin along the eastern seabord of the United States. This expanded effort is a continuation of earlier, rather successful detections of rocket‐generated seismic disturbances on Skidaway Island, Georgia. The more recent effort has yielded few positive results other than a recording of an early‐arriving seismic wave from Apollo 16 that was detected in Jacksonville. Evaluation of the negative results obtained in the Fort Monmouth area, with earlier studies of infrasound, local weather conditions, and geology, could be advantageous in the process of trying to gain a better insight into the acoustic‐seismic resonance mechanism requiring phase‐velocity matching at the atmosphere‐ground interface. The evaluation of the recording of early‐arriving seismic disturbances in Jacksonville also yielded certain new information about this acoustic‐seismic resonance phenomenon.

ISSN:0001-4966    

DOI:10.1121/1.1903450

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Constant‐frequency infrasound (about 5 to 10 Hz for different sources) has been detected at a number of locations from different directions. In an intensive study of 8.5‐Hz infrasound detected regularly at Lamont‐Doherty Geological Observatory of Columbia University in Palisades, N. Y., positive sound fixes have been obtained at the Tappan Zee Bridge. Geophones placed at several locations on the bridge recorded vibrations of the same frequency. We conclude that the vibrational motions of bridges radiate infrasound into the atmosphere and that the infrasound is most readily detected during times when atmospheric wind and temperature structure favor acoustic channeling near the ground. The acoustic signal occurs in pulses having a duration which, although fairly constant on a given occurrence, may vary from a fraction of a minute to several minutes.

ISSN:0001-4966    

DOI:10.1121/1.1903451

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

A two‐layer fluid which is inviscid and which has no conduction is considered. Once rotation is included in the perturbation equations, cubic equation for the square of the wave speeds of two‐dimensional waves results. Two of the roots can be closely approximated in rather simple analytic form. These roots are generally more unstable than the nonrotating waves. The root which is not degenerate as rotation approaches zero owes its greater instability under the presence of rotation to the fact that the fluid interface is tilted with respect to the gravity vector and the stabilizing role of the stratification is thereby reduced.

ISSN:0001-4966    

DOI:10.1121/1.1903452

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Utilization of acoustic waves for atmospheric studies through echo sounding requires an efficient antenna with high on‐axis gain and suppressed sidelobes. The antenna pattern for a piston source, as it influences the evaluation of the acoustic radar equation, is calculated and the effective gain factor obtained. Existing microwave antennas and optical searchlights have been adapted for acoustic use and their performance measured. A conical horn‐reflector antenna made of fiberglass gives excellent performance in the frequency region 1–5 kHz after being coated with a viscous dampening tar to shorten vibration time. This damping allows reception of weak echoes with minimum delay following the transmission of a high‐power acoustic pulse from the antenna. Septum dampers have been used successfully on parabolic‐dish antennas. Antenna beam patterns have been measured on an outdoor range, with some measurements being made across a canyon to avoid multipath ground‐reflection effects. Mainlobe to 90° sidelobe ratios are 40 to 50 dB. The measured antenna pattern for the horn‐reflector antenna agrees closely with predictions for a perfect piston source. The parabolic‐dish antennas produce a more complex diffraction pattern. Absorbing cylindrical antenna enclosures have been found effective in further reducing sidelobe reception by 10 to 20 dB, in agreement with predictions. Attention to antenna design allows acoustic echo‐sounder operation in a noisy urban environment, and keeps noise pollution from the sounder pulses below the annoyance threshold.

ISSN:0001-4966    

DOI:10.1121/1.1903453

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Infrasound from distant artificial sources has been detected at 2 Hz by a chain of stations in Sweden. A number of infrasound sources were located and identified as hydroelectric power plants, industrial plants, and oil fields. The infrasound from distant artificial sources propagates with reflections in the upper atmosphere: during summer mainly with reflections in the stratospheric sound channel and during winter with reflections in the upper sound channel.

ISSN:0001-4966    

DOI:10.1121/1.1903454

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP