摘要:

Acoustic transmission measurements, in the frequency range 10–60 kHz, have been carried out in the presence of layered, thermal fine structure under arctic ice. The term “layered fine structure” refers to relatively stable, slowly changing thermal layers in the ocean that have a vertical scale of about a meter to several meters. Their horizontal extent may be 10–100 times larger. In these measurements, acoustic transmission is strongly affected by the presence, near the transmitter, of a layer or layers with a sound speed maximum. In the literature these warm layers are sometimes called “antiwaveguide” or “barrier” layers. Three types of space‐time variability have been found in the received intensity versus depth records: (1) A small random variability observed at relatively steep transmission angles, (2) a frequency‐independent variability which can be related to convergence/divergence of rays in the refraction field of the layer, and (3) a frequency‐dependent variability which seems to arise from the interference between direct rays and those sharply refracted by the layer. A spectral analysis of the interference region confirms that the spatial wavelength in depth is inversely proportional to the acoustic frequency. [Work supported by Arctic Submarine Laboratory of the Naval Ocean Systems Center, San Diego, CA.]

ISSN:0001-4966    

DOI:10.1121/1.2004103

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The difficulties that existing models have in predicting coherent pressure levels for broadband computations is discussed, and an alternative technique is suggested. It is assumed that the acoustic field has a raylike representation that can be determined at a few fundamental frequencies. An interpolation algorithm provides additional coherent levels. Unlike other methods, multipath interference does not degrade accuracy since the interpolation is performed on individual eigenrays, rather than on the total pressure. Two examples illustrate this procedure. First, the autocorrelation of a received signal is determined by taking the inverse Fourier transform of the power spectrum density. Then, a frequency‐time display simulates the received signal of a passing noise source.

ISSN:0001-4966    

DOI:10.1121/1.2004110

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Given the physical parameters of the ocean bottom, a plane wave reflection coefficientRpwcan be defined, e.g., the Rayleigh reflection coefficient.Rpw, or the equivalent bottom loss, is normally used in ray‐theory calculations. A reflection coefficient,per se, is not an explicit parameter in a normal mode formulation. [See H. P. Bucker, J. Acoust. Soc. Am.48, 1187–1194 (1971), for an exception to this.] For an inhomogeneous water column an effective plane wave reflection coefficientRecan be defined which plays the role of reflection coefficient in mode theory.ReandRpware equivalent only for an isovelocity water column. The phase ofReplays a direct role in determining the eigenvalues and contributes indirectly to the mode cycle or skip distancerC. The relationship betweenrCand the cycle distance arising from beam displacementrBD[E. L. Murphy, J. Acoust. Soc. Am.61, 511 (1977)] is shown to be an approximate one. It is further shown thatrBDis a fundamental constituent of mode theory and enters explicitly as a parameter in the mode amplitude. Several simple examples are selected to illustrate the various points. [Work supported by CNR/NORDA.]

ISSN:0001-4966    

DOI:10.1121/1.2004116

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The transmission loss of a normally incident plane wave through dynamically compliant gratings has been extended to multiple layers. The gratings, which consist of various sized simply supported bars that have flexural resonances in the frequency range of interest, are immersed in a fluid layer between two fluid half spaces. By treating the periodic structure as a wave guide, the solution is formulated in terms of series of propagating and evanescent plane waves. For certain cases, generalization to non‐normal incidence is discussed. The transmission loss bands resulting from staggering fundamental resonances of a single element and/or various elements within the gratings are computed. The effects of the properties of the fluid layer, grating interactions, and material constants of the compliant elements are considered. [Work supported by Naval Sea Systems Command.]

ISSN:0001-4966    

DOI:10.1121/1.2004121

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Good coupling of ground‐borne vibration to a building structure is obtained with piled foundations and with a raft and casement construction. The magnitude of the response of the structure depends on how close the natural frequencies of its main components are to the predominant input frequency. With separate foundation blocks or footings dynamic coupling through the soil can arise between adjacent foundations in different modes. Results will be given of computation of the coupling between foundations resting on an infinite elastic half space and a comparison with some experiments will be made. The transmission of vibration through the buildings will depend upon the degree of damping. Some estimates will be given of theQfactors for modern buildings. Where the expected vibration levels in a building due to ground borne vibration are too high, a degree of decoupling can be achieved at the foundations by mounting the structure on rubber bearings and examples will be shown of actual installations.

ISSN:0001-4966    

DOI:10.1121/1.2004126

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Preliminary experience with the modified tapping machine proposed by T. J. Schultz [J. Acoust. Soc. Am.60, 645 (1976)] revealed some problems with lightweight floors covered with soft carpeting. Measurements of impact velocity and internal impedance of human walkers with a variety of footwear are described. A modified tapping machine based on the measurements of foot impedance was produced. The peak levels produced by this modified machine are compared with those produced by live walkers and standard machines.

ISSN:0001-4966    

DOI:10.1121/1.2004131

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

To evaluate the road traffic noise, a simplified traffic model is usually used, that is, a model with vehicles of the same acoustic power and the equal headway distribution. But in an actual road, fast and slow vehicles and noisy and quiet ones are mixed randomly. The resulting increase inLeqdue to variance of PWL and velocity of vehicles has been investigated for the following three cases: (1) when variation coefficient δvof velocity of vehicles is rather small (i.e., δv≪ 1), (2) when the velocity distribution of the vehicles is the normal distribution, or (3) in the case of gamma distribution of the velocities. As the variance of PWL is 2.55–3.27 (according to the results by Y. Watanabeet al.), the estimated increase ofLeqis about 1 dB. And as variation coefficient of velocity of vehicles is 0.13–0.18 (according to the published data on Route 1, Route 16, and Meishin highway), the increase ofLeqis about 0.1–1.0 dB. Summing up these effects, the increment ofLeqis supposed to be 1–2 dB.

ISSN:0001-4966    

DOI:10.1121/1.2004136

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Up to now, many approaches for estimating the statistics of road traffic noise have been carried out by the introduction of several models, e.g., an equally spaced vehicles model, an exponentially distributed vehicle model, and an Erlang distribution type model in a simplified sound propagation environment such as a free sound field. However, in several studies based particularly on the latest Erlang distribution type model (i.e., gamma headway distribution model), only the first‐ and second‐order moments of the sound intensity fluctuation which can be derived from the statistical information on the location of merely one and/or two vehicles (e.g., the distributions for the position of the vehicle and the distance between two arbitrary vehicles flowing in the same direction) are taken individually into consideration. On the other hand, the higher‐order statistical properties of traffic noise are rather important in order to investigate the whole shape of the noise distribution form from which any noise evaluation index can be derived. Thus, in this paper, our main interest is devoted to considering quantitatively the relation between the multidimensional correlation properties of the sound intensity and the higher‐order information on the vehicles flow by use of Stratonovich's stochastic theory for a random points system. Furthermore, the relation between our theoretical result and well‐known previous studies is discussed with experimental confirmation for several lower‐order moments.

ISSN:0001-4966    

DOI:10.1121/1.2004139

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Empirically, 1 s is widely used as the sampling period in the digital measurement of sound level. In order to clarify the theoretical background of this optimum sampling period, we consider the correlation between the data of each sample and discuss the effectiveness of the data from statistical point of view. In the case of general road sound, the optimum of sampling period is the value of Δtfor which the autocorrelation coefficient ρ(Δt) of the wave form is equal to12, where the envelope wave form is rms value obtained by the integration circuit corresponding to the FAST dynamic characteristics, by weighting the sound withAcharacteristics. This value is a function of the distance between the lane and the measuring point on the road and the vehicle speed. We also measured various kinds of road sounds and obtained the autocorrelation of the time series for each kind. For present status of the road condition in Japan, the optimum of sampling period is in the order 0.5–1.0 s.

ISSN:0001-4966    

DOI:10.1121/1.2004144

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Based on experience in enforcing operational noise standards for medium‐ and heavy‐duty trucks it is, in certain instances, of interest to prescribe a standard more stringent than the maximum noise level vehicles of that type are capable of producing. By taking advantage of the fact that trucks usually need not operate at full power and/or maximum engine speeds on a level street within inhabited areas a “level‐street noise standard” would require restrictions on the truck operator in order for the vehicle to meet the requirements of such a standard. A twofold measurement program was undertaken with a view of constructing a truck noise model from which the effects of various possible standards on the truck population could be determined. A small number of trucks was subjected to strictly controlled tests in order to collect data on the relationship between noise levels and such operational parameters as engine speed and engine torque. A large number of trucks was subjected to simpler controlled tests in order to determine the statistical distribution of truck noise levels as a function of truck design parameters and vehicle speed. Both sets of data were combined with data in the literature to formulate the level‐street truck noise model.

ISSN:0001-4966    

DOI:10.1121/1.2004149

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP