摘要:

A general review is presented of most areas of sound‐propagation outdoors that are of interest for the control of community noise. These areas are geometrical spreading, atmospheric absorption, ground effect, (near horizontal propagation in a homogenous atmosphere close to flat ground), refraction, the effect of atmospheric turbulence, and the effect of topography (elevation, hillsides, foilage, etc.) The current state of knowledge in each area is presented and suggestions made concerning research activities, applications of existing research, and practical problems which arise in the prediction of noise levels.

ISSN:0001-4966    

DOI:10.1121/1.381455

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

This paper will describe measurements of sound pressures on the surface of a prolate spheroid whose length‐to‐diameter ratio is 6:9. The sound source was positioned a distance from the illuminated pole equal to the length of the body; irradiation was axisymmetric. Data at surface locations along the axis of the body was collected for acoustic wavelengths λ ranging from 1/150 to 1/38 of the interfocal distanced. A condenser microphone equipped with a long probe tube was used to avoid extraneous scattering from the sensor. Sound pressure levels, expressed relative to those measured on the illuminated pole, showed the existence of expected creeping‐wave interference patterns near the antipode and an illuminated spot on the pole. The measured levels on that pole are shown to decrease roughly as the reciprocal of frequency; they are 13 dB less than those at the illuminated pole ford/λ=38. In the shadow zone the measurements are shown to be well approximated by Keller’s geometric theory of diffraction. Deviations from the theory on the order of 3 dB are observed at high frequencies and at coordinates off the antipole.

ISSN:0001-4966    

DOI:10.1121/1.381456

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The condition for the maximum directivity from a spherical radiator is presented and it is shown that this directivity index is 20 log N, whereNis the number of consecutive modes excited. For superdirectivity, the relationship among the expansion coefficients is particularly simple and a closed‐form expression for the beam‐pattern function results. Using this method, a high directivity may be obtained no matter how small the sphere is, provided that the prescribed velocity distribution is maintained. Examples of the resulting beam patterns are given.

ISSN:0001-4966    

DOI:10.1121/1.381457

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The method of matched asymptotic expansions has been used to obtain a low‐frequency solution for the diffraction of a plane compressional or shear wave by an elastic ellipsoidal inclusion. Attention has been focused on the scattered farfield, angular dependences of which are illustrated graphically for ellipsoidal cavities of different axes ratios.

ISSN:0001-4966    

DOI:10.1121/1.381458

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The radiation of sound through the open end of a cylindrical or conical pipe of any cross section, or through a hole in a plane wall, is analyzed theoretically. The scattering of a sound wave by the end of a rod or slab is also treated. Only the case in which the wavelength λ is large compared with a typical radial dimensionaof the opening or of the end, is considered. The method of matched asymptotic expansions is employed. Results on end corrections and reflection coefficients previously obtained by Helmholtz, Rayleigh, and Bazer and Karp using intuitive arguments are recovered and verified. Agreement is found with the exact results of Levine and Schwinger and Vainstein, and with the smalla/λ results of Lesser and Lewis, in the cases they treated. In addition various new results are obtained.

ISSN:0001-4966    

DOI:10.1121/1.381459

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The expression obtained by Hasegawa and Yosioka [J. Acoust. Soc. Am. 46, 1139–1143 (1969)] for the acoustic radiation pressure on a solid elastic sphere in a plane progressive sound field is reformed into a simpler form, in which the acoustic‐radiation‐pressure function is represented only in terms of scattering coefficients and the size parameterka. On the other hand, the extension of the Maidanik calculation [J. Acoust. Soc. Am. 29, 936–940 (1957)] for a fluid sphere to solid elastic spheres is attempted. The result is compared with the former. It is demonstrated that the computational differences are commensurate with the same result.

ISSN:0001-4966    

DOI:10.1121/1.381460

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Three different models are described in an attempt to account for the frequency dependence of measured impedances of grass‐covered ground. These are (a) a homogeneous ground of constant porosity, (b) a finite layer, also of constant porosity, overlying an infinite impedance, and (c) porosity decreasing with depth. The latter structure provides the best fit to the experimental data.

ISSN:0001-4966    

DOI:10.1121/1.381461

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A technique for obtaining a significant increase, compared to more conventional means, in percentage bandwidth for a projecting transducer has been developed. Multiple sets of elements with each set tuned to a slightly different frequency are used to obtain the increase in bandwidth. Phase problems at midfrequencies normally associated with multifrequency operation are solved to a satisfactory degree. An analytical study concerned with bandwidth, beam pattern, and relative per‐element sensitivity is presented. Experimental data are shown to support the analytical study.

ISSN:0001-4966    

DOI:10.1121/1.381462

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Consider a spatially homogeneous and temporally stationary random pressure field exciting a spatially homogeneous structure. It is well known that when the power exchange between the pressure field and the structure results from a ’’surface interaction’’ rather than from a scattering‐induced ’’edge interaction,’’ it is immaterial whether a finite or infinitely extended model of the structure is used to calculate this power exchange. This equivalence is discussed and used to estimate vibration response of fluid‐loaded structures when excited by a low‐speed turbulent boundary‐layer flow. Connections, established previously for structuresinvacuo, between group velocity, modal density, and point and line conductances, are also generalized to apply to fluid‐loaded structures. Finally, an approximate but simple method is suggested for estimating vibration characteristics of spatially nonhomogeneous structures excited by surface interactions.

ISSN:0001-4966    

DOI:10.1121/1.381463

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The reduction of signal‐to‐noise ratio due to hydrophone and preamplifier noise is a critical problem in the design of acoustic receiving systems. A signal‐to‐noise degradation factor is defined which depends upon figure of merit of the hydrophone, the amplifier noise figure, and the ambient noise spectrum. For piezoelectric hydrophones, the figure of merit is determined by fundamental material parameters and the volume of active material. Both voltage and charge amplifier configurations are considered. The charge amplifier is found to be inherently noisier.

ISSN:0001-4966    

DOI:10.1121/1.381464

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP