摘要:

Even in isovelocity water the wave or modal nature of the sound propagation causes special effects at four particular depths. Thus, the range‐averaged level shows a zero at the free surface, a maximum at the source depth, and effects of uncertain sign at the bottom and near the source complementary depth. These peaks show further curious behavior, akin to interference, as source depth is varied. The present source depth peaking is a wave effect in isovelocity water, in contrast with the flux‐theory infinities predicted for refracting conditions. For large numbers of modes, the present source depth peak is normally 50% above the average, and this anticipates a similar wave‐theory result for refracting conditions.

ISSN:0001-4966    

DOI:10.1121/1.384591

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The formal energy‐flux solution for range‐averaged level in a varying duct depends on the definition of transmission loss, on the local impedances, and in a complicated way on the vertical angles and cross‐track variations. But for most purposes a very simple low angle version is sufficiently accurate and well established to be virtually independent of the detailed profile. Surprisingly, the convergence factor with circular symmetry is also independent of the detailed profile; this is true for unbounded media as well. Depth transforms are presented for the angular distributions of intensity, initially without specific reference to sources. Angular distributions for discrete sources show some characteristic zeros and infinities. It is shown that only the Gaussian distribution is self‐reproducing with changes in depth. The law of conjugate depths states that intensities will be equal at such depths, and it is expected that this law will be very robust. Work on these and allied energy‐flux methods is reviewed.

ISSN:0001-4966    

DOI:10.1121/1.384592

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Recent attention to the effects of atmospheric turbulence on the interference of sound waves above both hard and soft boundaries has provided a spherical wave theory that allows for partial covariance between direct and ground‐reflected sound. Measurements of the interference spectrum at distances up to 45 m over a flat asphalt surface as well as measurements of jet noise over grass to a distance of 1100 m were shown previously to be in satisfactory agreement with the theory. The covariance between the two sound paths was estimated with one adjustable parameter by considering the transverse autocorrelation of the phase and amplitude fluctuations. The theory has now been further extended by considering explicitly, in addition to the transverse autocorrelation, the longitudinal autocorrelation of the phase and amplitude fluctuations along each path. At shorter distances of propagation it is found that the longitudinal autocorrelation, being close to unity, is necessary to estimate the partial covariance between the direct and reflected sound. At larger distances of propagation the longitudinal autocorrelation of both the phase and amplitude fluctuations goes to zero, contrary to the transverse autocorrelation which remains if the path separation is still of the same order as the correlation length of the medium. The new theory is compared with previous experimental results and yields essentially identical agreement as before but no longer requires an adjustable parameter.

ISSN:0001-4966    

DOI:10.1121/1.384593

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The magnitude of the temperature structure coefficient is computed from echosonde data. The results clearly show that the magnitude ofCT2fluctuates with time. The fluctuation has a positive bias and fits the logarithmic normal distribution. The analysis of the spectra ofCT2shows that the spectrumPf(CT2) usually follows thef−3rule in both stable and unstable stratifications. There is a peak range of energy in the frequency range 10−3to 3×10−3s−1. The analysis of time‐height cross sections ofCT2indicates that the distribution has a horizontally stratified structure in stable conditions, and follows the ruleCT2∝ z−4/3in unstable conditions. In addition, the variation ofCT2during a cold front passing the station and the relation between the weather system and boundary layer structure are analyzed.

ISSN:0001-4966    

DOI:10.1121/1.384596

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Kirchhoff’s time‐dependent solution of the wave equation is transformed into a convolution in the time domain, by means of a simple but effective parametrization of a plane‐wave front. It is shown how this representation can be applied to the diffraction of a plane pulse by thin arbitrarily shaped obstacles. The diffracted pulse is shown to depend almost entirely on a single function which can be easily determined from the specified geometry. The general method is then applied to two classical problems: diffraction by a half‐plane and circular screen. The solutions obtained are expressed in terms of elementary functions and are demonstrated to be uniformly valid across shadow boundaries. The method is ideally suited to numerical computation in the case of complicated geometries.

ISSN:0001-4966    

DOI:10.1121/1.384597

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A finestructure has been observed in the 5° to 30° angular range of forward scattering patterns of single nylon filaments and a pair of parallel filaments in water. The observations agree with the calculations of the total radiation pattern resulting from the interference of the radiation from those parts of the ultrasonic beam remaining after the removal of the radiation incident upon the scattering object by absorption.

ISSN:0001-4966    

DOI:10.1121/1.384598

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A technique is described with which it is possible to simultaneously transmit and receive in different modes (directivity radiation patterns) of an array utilizing correlation detection and the unique properties of statistically independent noise signals. The feasibility of a particular application—phased arrays which scan simultaneously in more than one direction—is demonstrated by using a simple ultrasonic three transducer linear array and a random signal ultrasonic correlation system. Theory and experimental results are presented which evaluate the effects of clutter and variations in the system parameters on the system signal‐to‐noise ratio. Results show that multi‐mode (multiple scan direction, multiple focal point, etc.) operation can improve phased array performance by decreasing the scanning time necessary to cover a desired volume of space even though extra correlation time may be needed in some cases to reduce the small amount of interference noise which exists between modes.

ISSN:0001-4966    

DOI:10.1121/1.384599

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The diffraction of an acoustic wave by a rectangular aperture in a soft plane screen is examined by a method which uses orthogonal functions and Fourier transformations. Numerical results of the plane‐wave transmission coefficients for normal incidence are given forb/a=1∠8 andka=0∠2, where 2aand 2bare the side lengths of the aperture andkis the wavenumber.

ISSN:0001-4966    

DOI:10.1121/1.384600

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The linearized sound field of a baffled piston source (radiusa, wavenumberk) in a dissipative fluid is considered. A simplified parabolic equation is derived forka≫1 and then solved analytically. The solution matches a plane collimated beam in the vicinity of the source and has the Bessel function directivity in the farfield. The nearfield–farfield transition is studied. The range of validity of the parabolic equation is discussed. Its exact solution is shown to be the first term of an expansion in powers of (ka)−2for the solution of the Helmholtz equation. The higher‐order terms are secular at distances of ordera(ka)1/3from the piston.

ISSN:0001-4966    

DOI:10.1121/1.384601

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A square microphone array with 256 elements has been constructed along with interfacing electronics to study low‐frequency (1–5 kHz) sound sources in air using the principles of acoustical holography. The array is used in the nearfield of a radiating object and the sound source structure of that object is reconstructed with an on‐line minicomputer. Reconstruction of the source structure of a point source and an unbaffled, free rectangular plate point excited below its coincidence frequency are presented. The latter shows clear evidence of ’’corner’’ and ’’edge’’ modes in which the respective areas of the plate are the dominant radiating sources. A new imaging process which is not limited in resolution by the wavelength of the radiated sound is introduced.

ISSN:0001-4966    

DOI:10.1121/1.384602

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP