摘要:

Here the object is to estimate the effect of scattering by randomly rough seabed and surface waves on attenuation of low‐frequency sound waves in shallow water. The experiment was pursued with CW signals (100 and 230 Hz) in Barents Sea. The transmission loss and interference pattern of a sound field were measured within 100 km of the acoustic sources. Mesoscale relief of the bottom with spatial resolution 10 m and wind parameters were recorded as well. The experimental results (the average sound intensity) were compared with theoretical ones calculated with and without taking into account seabed and surface roughness. The calculation was carried out on the base of modal theory of sound scattering by rough boundaries in a waveguide. It was demonstrated that the random roughness substantially affects the transmission loss for long distances. In this paper the method is suggested for bottom parameter estimation using the statistical attributes of sound field in shallow water with random rough boundaries as well. [The work was supported by Russian Foundation of Basic Research (Grant 96‐02‐17194).]

ISSN:0001-4966    

DOI:10.1121/1.422820

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Acoustical measurements of speech transmission index, reverberation time, early reverberation time, early‐to‐late energy ratios, loudness (or relative strength), articulation index, background noise levels, and signal‐to‐noise ratios were made in a number of elementary school classrooms in one school district to see how many rooms actually had acceptable acoustical conditions. The source and receiver locations for the acoustical measurements were determined through the participatory action research (PAR) described in Classroom Acoustics I. Measurements were made using a TEF analyzer with custom software to compute additional acoustical measurements. Both omni‐directional and directional loudspeakers were used for the source signals. The acoustical measurements were conducted in a variety of source receiver conditions as observed in the PAR studies in the classrooms. General observations are related regarding the relationship between the measurements and the PAR. For example, the STI was always greater than 0.75 for the conditions under which the teachers actually taught. Speaker‐to‐listener distances of 4 m or less were observed for many classroom activities. Likewise the teachers employed many creative ways to control the behavior of the students to reduce background noise levels while they actually spoke.

ISSN:0001-4966    

DOI:10.1121/1.422824

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The brick/block absorbing walls with openings and backing air space with porous materials are often used for absorption in low frequencies. The absorption in low frequencies can be explained by the well‐known theory of Helmholtz resonance. In some measurement results of absorption coefficient, however, high absorption coefficients are seen in high frequencies. Regarding this problem, the authors have been investigating theoretically and experimentally as presented at the last ASA meeting (at Penn. State). Following these studies, further numerical analysis was carried out on simple 2‐D models. The wave phenomena in the cavity and backing space/material were formulated by combining the FEM with the scattering wave theory. Comparison between the calculated result and the result of a 1/10 scale model experiment has indicated the validity of the numerical analysis.

ISSN:0001-4966    

DOI:10.1121/1.422829

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Numerous methods exist for predicting sound fields in enclosed spaces such as rooms, aircraft cabins, and the sea. These are generally energetic approaches which ignore phase and associated wave effects, and which describe sound reflection from surfaces by the energy absorption coefficient. Many, such as ray tracing, require long calculation times. In the work reported here, a new model is presented which is computationally efficient, includes phase, and decribes surface reflection by the angularly varying surface impedance. It is a triangular‐beam‐tracing algorithm, combined with a Biot‐theory transfer‐matrix model for laterally homogeneous, multilayer, poroelastic surfaces. The Biot model has been validated in the case of plates, porous materials, and seabeds. The combined approach is being validated in comparison with predictions by analytic and numerical approaches, and with experiment. The model is being used to study the effect of surface extended reaction on enclosed sound fields.

ISSN:0001-4966    

DOI:10.1121/1.422833

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

In general vibration theories of thin elastic plate, the plates have been assumed to be nonpermeable. However, previous work on permeable membranes revealed that the acoustic properties of a membrane are strongly affected by its permeability [Takahashietal., J. Acoust. Soc. Am.99, 3003–3009 (1996)]. In this study, permeable elastic‐plate absorbers are proposed and their acoustic performance is examined by means of numerical analysis. For the simplest model, a theory for sound transmission through and sound reflection by a single permeable plate absorber with the oblique incidence of steady‐state plane wave is developed. In this theory, permeability of the plate is represented by unit‐depth flow resistivity [MKS‐rayl/m]. Subsequently, for more complex models, structured absorbers composed of the facing of a permeable plate and several layers of air and/or absorptive materials are investigated theoretically. With these theoretical models, parametric surveys are executed to evaluate the acoustic performance of permeable elastic‐plate absorbers. As a result of numerical calculations, it becomes clear that the flow resistance of the permeable elastic plate has a remarkable influence on its acoustic characteristics, especially at high frequencies.

ISSN:0001-4966    

DOI:10.1121/1.422841

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Speech intelligibility degrades when the listener moves from the source. In a highly reverberant and empty church, scores are measured at different distances from a loudspeaker and compared with those obtained from recordings of the speech at the same location. The phonetic material is a set of phonetically balanced lists of triphonemic French words. Two monophonic (omnidirectional, cardiod) and one stereophonic (ORTF) systems are used. Listening of recorded lists is performed with the same loudspeaker(s) as in the church. Results of the experiment indicate smaller variations of scores with the distance. An important improvement of scores is obtained with the stereophonic system. A second experiment consists of using a sound material composed of two voices performing a singing exercise edited in monophony in order to create two distinct sound planes. Listening tests allow the quantiication of the depth localization variations with distance and sound recording system. Results show a good respect of distance and reveal a sudden change in depth localization for stereophony. Acoustical properties of the church, distance factors of the microphones, properties of the stereophony against monophony, and real listening conditions are considered to discuss the experiments.

ISSN:0001-4966    

DOI:10.1121/1.422984

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Acoustic research has shown that the species of some schooling fish can be assigned based solely on measurement parameters from acoustic returns. For single targets, this process is complicated by the high variability ofin‐situtarget strength (TS) measurements. Attempts at speciation based on the patterns of TS variability were confounded by interactions between acoustic frequency and fish directivity. Initial attempts at defining speciation parameters based on multifrequency target strength measures look promising. This paper reports on the results of simultaneously measuring thein‐situtarget strength of fish at 38‐, 120‐, and 420‐kHz acoustic frequencies, and proposes the definition of several speciation parameters.

ISSN:0001-4966    

DOI:10.1121/1.422850

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A pulse compression process was used to analyze echoes from individual animals of three distinct morphological groups of zooplankton. The groups studied were fluidlike animals (Euphausiids, decapod shrimp), elastic‐shelled (Gastropods), and animals containing a gas inclusion (Siphonophores). A broadband chirp signal was used to insonify freshly collected animals that were then tethered in a seawater tank either in the laboratory or on the deck of a ship. The decapod shrimp was tethered to a stepper motor and was rotated in 1‐deg increments. The other experiments had some animals fixed and others moving freely. The pulse compression processing of the echoes from the animals temporally resolved multiple returns from an individual. Using existing scattering models [Stantonetal., J. Acoust. Soc. Am. (to be published)], the temporal and amplitude differences of the returns were used to estimate size and orientation. Estimates of the size of the animal were made for the Euphausiids, Gastropods, and Siphonophores, while orientation estimates were made for the decapod shrimp. These results agreed well with the actual dimensions of the animals.

ISSN:0001-4966    

DOI:10.1121/1.422855

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The parameters of shock waves in liquids as used for lithotripsy have been measured with needle‐, membrane‐, and fiber optic probe hydrophones. The true rise times of planar shock waves are determined by surface detection or by the transient response of piezoelectric crystals. The different measuring techniques are compared with respect to their suitability for the determination of shock wave parameters, such as risetime, pulse width, positive and negative pressure amplitudes, and also with respect to aging, calibration sensitivity lifetime, etc. The fiber optic probe hydrophone appears as an optimum high precision standard for calibrated shock wave measurements in lithotripsy. Finally, aspects of the shock parameters with respect to the stone destruction efficiency are discussed.

ISSN:0001-4966    

DOI:10.1121/1.422858

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

It is well known how the lithotriptors operate and the discussion concerning the mechanisms of target disintegration are continued. Meanwhile, to understand these mechanisms means to obtain a possibility to influence on the fracture process. The spall mechanism, as a result of acting with the shear forces, the crack growth inside a stone, and an erosion mechanism (impacts of cumulative microjets forming at bubble collapse in the cavitation zone near a target wall), were suggested by Sturtevant, Kuwahara, Delius, Takayama, Groenig, Crum, and others to explain the disintegration effects observed. At the same time, all authors do not deny a rise of the cavitation zone in the vicinity of a focus under the action of a negative pressure phase in ‘‘a tail’’ of a shock wave. However, the role of the cavitation cluster is interpreted in different ways assuming that this effect is a secondary one. This paper proposes an approach to the estimation of the disintegration mechanism, which is based on the development and dynamics of the cavity cluster in a focus vicinity. Within the framework of a two‐phase model of cavitating liquid the problem of bubbly cluster development and its collapse on a solid wall are considered. The experimental data on a simulation of cavity cluster pulsation on a target and the idea that hydraulic impacts generated by cluster can determine the mechanics of target disintegration in a focusing zone of shock wave and its rarefaction phase are discussed.

ISSN:0001-4966    

DOI:10.1121/1.422863

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP