摘要:

Various studies of sound transmission in urban areas that involve multipath propagation or multiple reflections are reviewed. Studies of propagation in a single street or corridors display important descrepancies between theoretical analyses and field experiments. Model studies suggest that scattering may play an important, but as yet unexplained role. Sources above the street show good correspondence between field and laboratory data and theory. Intrusion of aircraft noise into an urban area shows that sound may be slightly amplified above an open‐terrain condition due to channeling effects. The general urban background noise appears to be accounted for by surface transportation sources distributed over the city, aside from a shielding factor of 10–15 dB of as yet undetermined origin.

ISSN:0001-4966    

DOI:10.1121/1.1914527

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

A literature review has been prepared to summarize the present state of knowledge concerning the attenuation by barriers. Frequently used prediction methods for the performance of screen‐type barriers are analyzed with respect to errors resulting from particular source and receiver positions, from ground reflections, from absorptive properties of the barrier and the ground, and from the influence of wind. Recently developed formulas describing the diffraction by wedge‐shaped and wide barriers are reduced to simplified methods for the prediction of the shielding achievable in comparison to screen‐type barriers. Unsolved problems apparent from discrepancies of the field performance of barriers and the results of theoretical and model studies are summarized to outline areas requiring further research.

ISSN:0001-4966    

DOI:10.1121/1.1914528

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The propagation of coherent elastic shear waves in a medium containing a dilute and uniform distribution of cylindrical inclusions is discussed. The waves are assumed to be polarized parallel to the axes of the cylinders while the cylinders are assumed to be thin compared to the wavelength. The complex wavenumber is calculated to a second order of approximation, the real part of which gives the propagation speed and the imaginary part gives the damping due to scattering. Numerical results are obtained for the case of identical inclusions with the properties of the inclusions varying between hollow (void) and rigid.

ISSN:0001-4966    

DOI:10.1121/1.1914529

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Nonlinear acoustic theory is used to show that the cutoff frequencies and the resonant frequencies of modes in acoustic waveguides of finite length depend upon the mode amplitude. The amplitude εn, of moden>0, produced by a periodic piston motion of amplitude δn, is also determined. It is found that at and near a resonant frequency, εn≏δn1/3while at and near the cutoff frequency εn≏δn1/2. Away from these frequencies εn≏δn, as linear theory predicts. Thus the infinite amplitude, which linear theory yields at cutoff and resonance, is avoided. The results are obtained simply by using the result for the propagation constant as a function of mode amplitude given by J. B. Keller and M. H. Millman [J. Acoust. Soc. Am.49, 329–333(1971)].

ISSN:0001-4966    

DOI:10.1121/1.1914530

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

We analyze by first‐order perturbation theory the manner in which a low‐frequency acoustic wave and a high‐frequency pump beam interact nonlinearly to produce a sum‐ and a difference‐frequency component. Provided that its acoustic beam is well collimated, the pump source can be of arbitrary shape, with arbitrary acoustic distribution over its surface. For the case where the incident low‐frequency wave is plane, we derive the following result: To a close approximation, the acoustic particle velocity at the difference frequency is proportional to the product of (a) the velocity potential of the pump beam, calculated at the difference frequency, and (b) the response function, at the low frequency, of an endfire array stretching from the pump source to the plane of a receiver. The result also holds for the sum frequency. We also prove that the result holds—with a modified endfire response function—when the low‐frequency source is at a finite distance. For a circular plane piston as pump source, we discuss the achievable directivity in detail; also, we show that focusing the pump beam to obtain a stronger signal can succeed only at the expense of good directivity.

ISSN:0001-4966    

DOI:10.1121/1.1914531

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Results of steady‐state and pulse measurements of upstream and downstream attenuation of sound in turbulent pipe flow are presented and compared with calculated values obtained from a simple model in which the sound is treated as a quasistatic perturbation of the steady flow in the pipe.

ISSN:0001-4966    

DOI:10.1121/1.1914532

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Since Westervelt's original proposal [J. Acoust. Soc. Am.35, 535–537(1963)] that nonlinear acoustic interactions may be used to produce relatively narrow beams of sound at relatively low frequencies, a great deal of effort has gone into the study of parametric transmitting arrays, both experimentally and theoretically. In this paper, the behavior of such arrays in their farfield is studied, neglecting higher‐order interaction effects. Results are presented in the form of normalized curves and simplified equations which can be used for the preliminary design of such devices.

ISSN:0001-4966    

DOI:10.1121/1.1914533

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The Kramers‐Kronig dispersion relationship is used to discuss the interdependence between velocity and attenuation in sediments and sea water. It is shown that if the attenuation is proportional to thenth power of the frequency, there is no velocity dispersion ifn= 1 or 2 but that dispersion exists for nonintegral values ofn. The theoretical relation agrees with the experimental measurements of L. Hampton for a sediment withn= 1.37. The relaxation process of sea water is considered and it is shown that the velocity dispersion is slightly greater than the precision of velocity measurements.

ISSN:0001-4966    

DOI:10.1121/1.1914534

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Measurements made with a fixed‐bottomed source and fixed‐bottomed receivers in water 2400 f deep show that for small ocean wave heights, up to about 3 ft, the fluctuations in amplitude of surface‐reflected sine‐wave signals at frequencies of 400, 750, 1000, and 1500 Hz at grazing angles of 18°, 31°, and 85° are in general agreement with theoretical predictions. In this wave height range, the coefficient of variation increased with increasing frequency, grazing angle, and wave height, reaching a maximum at about 5‐ft wave height for each combination of frequency and grazing angle. As the wave height increased further, up to 30 ft, the coefficient for a given combination of frequency and grazing angle generally decreased. For a given combination of frequency and grazing angle, the average peak level of the reflected pulses does not appear to depend strongly upon wave height.

ISSN:0001-4966    

DOI:10.1121/1.1914535

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Asymptotic expansions are found for both long and short waves propagating in elastic rods of arbitrary cross section. The results are compared with the well known approximate theories for long waves and show to what extent those theories are accurate. They also yield corrections to them.

ISSN:0001-4966    

DOI:10.1121/1.1914563

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP