摘要:

Backscattered measurements of acoustics waves returned from three known surfaces of different roughnesses were made at four different frequencies. Two surfaces are made of mild steel and one of rubber polymer. The reflection coefficients were measured and found to be approximately constant over the scattering angles ranging from 0° to 30° from the vertical. The correlation functions and the standard deviations of the surface heights were calculated from the surface profiles obtained with a depth gauge. The correlation lengths and the standard deviations were found to be 6.00–27.75 mm and 0.74–1.08 mm, respectively. The range of the working wavelengths is from 1 to 3 mm. Thus, all parameters needed in the Kirchhoff's scattering law are known. Comparison with the usual Kirchhoff's theory [P. Beckman and W. K. Klemperer, “Interpretation of the Angular Dependence of Backscattering from the Moon and Venus,” Nat. Bur. Stand. Radio Sci.69D, 1669–1676 (Dec. 1965); T. Hagfors, “Relationship of Geometric Optics and Autocorrelation Approaches to the Analysis of Lunar and Planetary Radar.” J. Geol. Res.71, 379–383 (1966)] shows serious disagreement. It is found that this is due to an overapproximation in evaluating the Kirchhoff's integral. When the integral is properly evaluated, very good agreement between theory and experimental results is obtained. This appears to be the first theory that is capable of explaining both the angular and frequency behaviors of the return signal. We restrict our backscatter angles to be within 30° so that we are sure that the Kirchhoff's theory applies, and no polarization, shadowing, or multiple scattering effects are significant. Thus, the results obtained apply equally well to electromagnetic scattering. The distributions of the surface slopes are also calculated from the surface profiles so that geometric‐optics theories of wave scattering (which may be considered to be a special case of the Kirchhoff's theory can also be checked.

ISSN:0001-4966    

DOI:10.1121/1.1911821

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Analytical examination is carried out of the interaction of pressure fields with boundaries possessing surface‐impedance nonuniformities. The analysis is limited to surface impedances that are point reacting. The analysis is performed in spectral space, and the spectral surface impedances are defined in terms of convolution operators. These operators are shown to generate a spectral component from a spectrum of spectral components in the blocked pressure field on the boundary; the generation is associated with induced changes in the wave vectors of the original components. The nonuniformities that give rise to the convolution operators are thus termed wave‐vector convertors. Application of the analysis is limited to boundaries possessing periodic surface‐impedance nonuniformities. It is shown that boundaries possessing such surface‐impedance nonuniformities act so as to effectively increase the spectral density of the pressure fields on the boundary and consequently increase the radiative efficiency of the boundaries as compared with those of equivalent boundaries possessing uniform surface impedances. The equivalence is expressed in terms of equal effective spectral surface impedances. Calculation of the pressure field on a boundary possessing surface‐impedance nonuniformities and subject to the pressure field of a turbulent boundary layer is exemplified.

ISSN:0001-4966    

DOI:10.1121/1.1911822

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Several examples of acoustic resonance in the frequency range from 0.001 to 1.0 Hz have been observed in limestone caverns. In some cases, the cavern geometry is simple enough for direct application of the Helmholtz resonator theory, and good agreement is found. Three experiments are described. Spectral analysis indicates that subacoustic resonance is responsible for numerous reports of periodically fluctuating or reversing cavern winds, and that appropriate wind measurements can provide new information about chambers not accessible to exploration.

ISSN:0001-4966    

DOI:10.1121/1.1911823

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Design and performance of a simple first‐order pressure‐gradient microphone based on the electrostatic principle using foil electrets are discussed. Such a microphone discriminates against air‐borne noise as well as against solid‐borne vibrations transmitted through the microphone case. For air‐borne sound originating from a spherical sound source positioned close to the microphone, the frequency response is flat within ±3 dB in the frequency range 0.1 to 3 kHz with a sensitivity of about −50 dBV/μbar in the direction of the microphone axis. In a plane sound field, the microphone exhibits the typical first‐order gradient behavior, i.e., its sensitivity rises proportional to frequency over most of the mentioned frequency range. At 0.1 and 1 kHz, the plane‐wave sensitivity is 30 and 10 dB, respectively lower than the sensitivity to sound originating from a nearby source, making this microphone a noise‐discriminating device for close‐talking applications. Rejection of solid‐borne vibrations is due to the lightness of the foil‐electret diaphragm. Compared with electrodynamic and electromagnetic transducers, the foil‐electret microphone responds 30 to 40 dB less to solid‐borne vibrations, referred to equal output for air‐borne sound.

ISSN:0001-4966    

DOI:10.1121/1.1911824

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

The analysis of thickness‐twist or SH waves in an infinite piezoelectric plate by Bleustein [J. Acoust. Soc. Amer.45, 614–620 (1969)] is supplemented by a few figures that show effects of the electromechanical coupling factor, the mass of electrodes, and finite dielectric constants on dispersion curves.

ISSN:0001-4966    

DOI:10.1121/1.1911827

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

A recent proposal to use digital Fourier transformation in forming acoustic beams is discussed tutorially. Relations to other procedures for beamforming, and for spectral analysis, are given. Need is shown for a lower limit on the data batch size.

ISSN:0001-4966    

DOI:10.1121/1.1911828

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP