摘要:

A new high‐frequency asymptotic theory of propagation in ducts with continuously varying refractive index is presented. The theory is based on local wave fields with complex phase and constitutes a special application of the evanescent wave tracking theory developed by the authors. It is shown, for analytic profiles and for refractive indexes that vary only transversely to the duct direction, how the coefficients in the asymptotic expansion are evaluated explicitly. When the method is applied to parabolic and hyperbolic secant profiles for which exact solutions of the wave equation are available, the asymptotic expansions so generated agree term by term with the asymptotically expanded exact results. The method is then applied to a class of polynomial profiles for which exact results in terms of known functions are not available.

ISSN:0001-4966    

DOI:10.1121/1.381791

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

An analytical and experimental study is presented of sound propagation through a circular duct in the presence of an expansion chamber. Results are presented for various sound frequencies, expansion ratios, and chamber lengths. It is found that the analytical results are in good agreement with the experimental results.

ISSN:0001-4966    

DOI:10.1121/1.381792

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The analytical formulation for the total scattering cross section of a cylindrical cavity in an elastic medium is given for the case of inclusion of compressional and shear wave attenuations. Examples are computed of relatively small attenuations exemplified by the material polymethacrylate and relatively large attenuations exemplified by polyethylene. The inclusion of wave attenuations known to exist in these materials cause the scattering cross section to diminish significantly for wave‐number cylindrical‐cavity‐radius product up to 10.

ISSN:0001-4966    

DOI:10.1121/1.381793

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Sound transmission in a 180° bend containing a curved partition positioned on the axial center line is investigated theoretically and experimentally using equations for sound propagation in straight and curved ducts. Good agreement is obtained and small discrepancies are discussed. The partition is found to significantly alter the sound propagation through the bend and reasons for the different acoustic behavior are given.

ISSN:0001-4966    

DOI:10.1121/1.381794

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The interaction between a low‐level band of noise and a finite‐amplitude tone has been measured in air in a plane progressive wave tube. Noise bands generally 1–3 octaves wide with center frequencies in the range 0.7–1.2 kHz were used. At the source the tone SPL was in the range 140–151 dB (re20 μPa); the frequency was either within the noise band or somewhat above. The data show the following. When the tone is absent, the noise propagates as a small‐signal wave. When the tone is turned on, however, a strong modulation takes place. After a propagation distance of only a few meters, not necessarily beyond the point of shock formation for the tone, the noise spectrum extends many octaves above the original noise band and, moreover, becomes nearly flat. The energy required to extend the noise apparently comes from the tone and its harmonics, not the original noise band. Theoretical predictions have also been obtained. A computer algorithm based on the Earnshaw solution, with corrections for tube wall absorption added, was used. The input waveform of the noise was constructed from the measured source power spectrum, the components of which were assigned random phase. The computed results agree well with the experimental data.

ISSN:0001-4966    

DOI:10.1121/1.381795

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A preliminary version of an echosonde interferometer is described and some of the first results obtained with it are presented. The results demonstrate that the formation of an acoustic interference pattern from a pair of coherent sources extends to the sound field scattered by turbulent fluctuation and that an echosonde interferometer is practicable. Possible applications to the remote sensing of atmospheric temperature profiles are discussed.

ISSN:0001-4966    

DOI:10.1121/1.381796

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Experiments are described in which exponential attenuation factors for surface waves both in the direction of propagation along a complex impedance surface and perpendicular to the surface are measured. The surface impedance calculated from these attenuations is then compared to that measured in an impedance tube.

ISSN:0001-4966    

DOI:10.1121/1.381797

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Underwater acoustic reflection characteristics for a thin stiffened aluminum plate were investigated. The thin stiffening rib extends parallel to the edge across the entire width of the plate. A plain plate without the stiffening rib was used as a control. The targets were sequentially suspended in water during the experiments. The stiffening rib on the experimental target was positioned in the acoustic shadow of the plate to avoid specular returns from its edges. Internally guided longitudinal stress waves were detected in both targets for a critical angle of incidence of 15.8°, at a frequency of 60 kHz and a 30‐μsec pulse width. Based on ray theory and the assumption that these stress waves were reflected at the reinforcing rib, theoretical travel times, and pulse‐reflection periods were calculated. Good agreement between theoretical and experimental results was achieved. Pulse decay rates in the form of logarithmic decrement of internally guided longitudinal stress waves are shown to be a significant clue to the presence and location of a reinforcing rib in thin plates.

ISSN:0001-4966    

DOI:10.1121/1.381798

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A study is made of sound propagation in surface channels with rough boundaries. A generalized phase integral approach is formulated for calculating the complex eigenvalues due to leakage and surface roughness. Calculations of propagation loss versus range are given for the bilinear profile and compared with experimental data.

ISSN:0001-4966    

DOI:10.1121/1.381799

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

We investigate how the force and moment impedance of a line discontinuity on a plate influences the sound scattered by impingement of plate flexural waves on the line discontinuity. Effects of fluid loading are accounted for. For a specific high‐frequency situation, we numerically evaluate three line‐discontinuity models: a simple support, a clamped support, and a support whose force and moment susceptances cancel out the corresponding susceptances of the plate driving‐point admittances. Calculated are total acoustic powers radiated and farfield pressure patterns. We also calculate, for two angular locations, contours of constant radiated pressure in the line‐discontinuity force‐admittance–moment‐admittance plane. It is shown that under certain circumstances slight changes in the line‐discontinuity admittance can cause relatively large changes in the radiated pressure.

ISSN:0001-4966    

DOI:10.1121/1.381800

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP