摘要:

A jet flow was used to model roughly a localized region of atmospheric turbulence, simulating a single idealized “eddy.” The jet was arranged in the UTIAS 80‐ft sonic‐boom generator horn so as to blow either against or with the direction of boom propagation. The two cases produced spiked and rounded boom signatures, respectively, qualitatively in accord with theory. The resemblance to signatures resulting from supersonic flight under turbulent atmospheric conditions was especially marked with the spiked “superbooms.”

ISSN:0001-4966    

DOI:10.1121/1.1913411

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The formulation of output autocorrelation function of a digital beamformer into an infinite series leads naturally to the introduction of “quantizer functions” of various orders as a convenient vehicle in the evaluation of beamformer performance corresponding to different input quantizer designs. For arrays operating in a Gaussian ambient noise field, the quantizer function of a general nonuniform quantizer is defined and evaluated in terms of “reduced Hermite polynomials,” which facilitate less costly machine computation via their unique recursion formulae. A rigorous proof of the convergence of the series of quantizer functions is given to ensure confidence in numerical results. Examples of the use of quantizer functions given are (i) determination of optimum step sizes of 2‐, 3‐, and 4‐bit uniform quantizers which yield maximum array gain for a typical array, (ii) calculation of directivity patterns of digital beamformer with input signals of arbitrary bandwidth, and (iii) evaluation of the effect of post filtering on the directivity patterns of digital beamformers. The special case of 1 bit per channel quantization (DIMUS) was especially discussed in detail with respect to both array gain and directivity patterns. The use of quantizer functions can also be extended to the solution of active‐sonar problems involving sine‐wave signals, and a brief example was given to illustrate the partial recovering of clipping loss in DIMUS by post filtering.

ISSN:0001-4966    

DOI:10.1121/1.1913412

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Relative maxima in ultrasonic backscattering were observed at critical angles of incidence that correspond to Lamb modes on brass plates located between air and water. Critical angles were determined utilizing schlieren visualization of beam displacement. Results are compared to backscattering the Rayleigh angle at a brass bulk solid‐water interface.

ISSN:0001-4966    

DOI:10.1121/1.1913413

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The use of optical interferometry for measurement of ultrasonic surface waves has been limited heretofore by the necessity of maintaining the surface under test within a fraction of a wavelength of light of some fixed position. A new technique, using two coherent light beams focused on the specimen surface, avoids this limitation. This technique is useful for directly recording standing‐wave ratio, attenuation, transmission, harmonic content, and other characteristics of surface waves at frequencies ranging from audio to beyond 200 MHz. In addition to excellent sensitivity, the method allows absolute calibration of the acoustic wave amplitude.

ISSN:0001-4966    

DOI:10.1121/1.1913414

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Backscatter measurements were taken from an irregular rubber layer of uniform thickness, 3 mm, at five discrete frequencies (0.33, 0.54, 0.78, 1.0, and 1.5 MHz). Comparisons were made with data taken from a much thicker rubber target (mean thickness 38 mm) of the same surface roughness. It was found that there were no significant differences in the angular behavior of the return signals at the three lower frequencies. However, at higher frequencies, an oscillatory component appeared in the return from the layer. This implies that at least for the case studied (a rough layer with rms roughness of its interfaces comparable to its mean thickness), a layer must be more than about two wavelengths thick before it can be detected. It also follows that a multifrequency sensor is essential for exploring unknown terrains with possible subsurfaces.

ISSN:0001-4966    

DOI:10.1121/1.1913415

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Backscatter measurements were made, at frequencies of 0.5, 1.0, and 1.5 MHz, of waves scattered from three different targets: (1) an inhomogeneous medium with a plane boundary, (2) a homogeneous medium with a rough boundary, and (3) an inhomogeneous medium with a rough boundary. The surface roughness of targets 2 and 3 is the same and so are the inhomogeneous properties of targets 1 and 3. It is found that for the cases studied surface roughness is more sensitive to frequency than the inhomogeneities and that the inhomogeneities contribute more significantly to the backscattered power at large incident angles than surface roughness.

ISSN:0001-4966    

DOI:10.1121/1.1913416

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The radiation resistance of a small sonar transducer at an air‐water surface has been measured in the presence of a hard or soft sphere placed in the water. The diameter of the sphere was nearly a wavelength. The sphere was shown to have substantial effect when placed within two wavelengths of the transducer and might have usefulness as a device to control the radiation impedance of transducers used in sonar arrays.

ISSN:0001-4966    

DOI:10.1121/1.1913417

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP