摘要:

The published literature describing three real‐ear‐attenuation‐at‐threshold (REAT), nine above‐threshold, and four objective methods of measuring hearing protector attenuation is reviewed and analyzed with regard to the accuracy, practicality, and applicability of the various techniques. The analysis indicates that the REAT method is one of the most accurate available techniques since it assesses all of the sound paths to the occluded ear and, depending upon the experimenter’s intention, can reflect actual in‐use attenuation as well. An artifact in the REAT paradigm is that masking in the occluded ear due to physiological noise can spuriously increase low‐frequency (≤500 Hz) attenuation, although the error never exceeds approximately 5 dB, regardless of the device, except below 125 Hz. Since the preponderance of available data indicates that attenuation is independent of sound level for intentionally linear protectors, the use of above‐threshold procedures to evaluate attenuation is not a necessity. An exception exists in the case of impulsive noises, for which the existing data are not unequivocal with regard to hearing protector response characteristics. Two of the objective methods (acoustical test fixture and microphone in real ear) are considerable time savers. All objective procedures are lacking in their ability to accurately determine the importance of the flanking bone‐conduction paths, although some authors have incorporated this feature as a post‐measurement correction. The microphone in real‐ear approach is suggested to be one of the most promising for future standardization efforts and research purposes, and the acoustical test fixture technique is recommended (with certain reservations) for quality control and buyer acceptance testing.

ISSN:0001-4966    

DOI:10.1121/1.393228

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

This paper is concerned with the diffraction of a Rayleigh surface wave by an edge crack included at an arbitrary angle to the free surface of a half‐space. The corresponding problem for a normal edge crack was studied in [B. Q. Vu and V. K. Kinra, J. Acoust. Soc. Am.77, 1425–1430 (1985)]. The scattered field was measured on the free surface both in the vicinity of the crack and far away from it. The nearfield possesses a number of interesting features which can be used to characterize an inclined crack. The farfield transmission and reflection coefficients,ATandAR, respectively, were measured under both the steady time‐harmonic motion (tone burst) and the transient motion (spectroscopy) of the half‐space. All three regions of interest were studied: wavelengths large, equal, and small compared to the crack lengths. As expected, in the short‐wavelength limit,ARasymptotically reaches its frequency‐independent limit which is the reflection coefficient of an infinite wedge. This limit was found to agree very well with the earlier results by Viktorov for a wedge [I. A. Viktorov,RayleighWavesandLambWaves—PhysicalTheoryandApplication(Plenum, New York, 1964)]. The transmission coefficient exhibits an oscillatory dependence on frequency; this is attributed to a resonance between the crack tip and the free surface. The spacing between the peaks ofATwas found to satisfy the following simple kinematic condition: wavelength equals twice the crack length. Furthermore,ATwas found to be measurably identical for two complementary cracks of inclination θ and π−θ, where θ is measured from the free surface.

ISSN:0001-4966    

DOI:10.1121/1.393229

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Sound waves incident on a thin elastic rod whose radius is smaller than the wavelength of the incident sound induce flexural and uniform compressional oscillations in the rod. These elastic oscillations, in turn, radiate sound waves into the fluid medium and affect the scattered waves. This paper deals with an analytic study on sound scattering by, and acoustoelastic vibrations of, a thin elastic unbound rod in a viscous fluid. The shear viscosity of the fluid is considered in the solutions to boundary value problems concerning the sound scattering and the elastic response of the rod. Results show that the scattered compressional waves consist of the rigid‐rod scattering of compressional waves, monopolar waves due to the uniform pulsating of the rod, and dipolar waves due to the flexural vibration of the rod. The scattered viscous waves consist of the rigid‐rod scattering of viscous waves and dipolar waves due to the flexural vibration of the rod. Acoustic resonances occur when the effective inertia force of the rod balances the stiffness force of the rod. The fluid viscosity and the scattering of sound give rise to damping effects for the rod vibrations and signficantly affect the acoustic resonances.

ISSN:0001-4966    

DOI:10.1121/1.393230

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The Sommerfeld–Watson transformation (SWT) was recently applied to the acoustic backscattering from elastic spheres in water havingka≫1 [K. L. Williams and P. L. Marston, J. Acoust. Soc. Am.78, 1093–1102 (1985)]. Expressions for the scattering due to each class of elastic surface wave (e.g., the Rayleigh wave) were interpreted in terms of contributions from repeated circumnavigations. In the present paper, these expressions are summed in closed form as in the analysis of Fabry–Perot resonators. The form function is synthesized by adding this sum to the specular reflection. The procedure is confirmed by comparison with the exact form functionffor a tungsten carbide sphere in the range 10≤ka≤80. In this case, the interference of the specular and Rayleigh contributions produces the underlying structure in ‖f(ka)‖, while the whispering gallery wave resonances produce a finer superposed structure. Phase shifts and coupling coefficients are identified which affect the signatures in ‖f(ka)‖ of the Rayleigh wave resonances.

ISSN:0001-4966    

DOI:10.1121/1.393231

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Laboratory measurements on the elastic moduli and the acoustic attenuation of fluid‐saturated porous rock cylinders are frequently affected by the boundary conditions of the rock sample, i.e., whether the curved surface of the cylindrical sample is exposed to air or properly sealed. In this paper, the analytical solutions of these problems for the extensional, torsional, and flexural modes are derived, and several numerical examples are computed. It was found that there exists an ‘‘artificial’’ attenuation caused by the open‐pore boundary condition, whose relaxation frequency, in the case of extensional mode, is directly proportional to the permeability of the rock and inversely proportional to the viscosity of the pore fluid and the square of the sample radius. This attenuation exists in both the extensional and the flexural modes but not in the torsional mode.

ISSN:0001-4966    

DOI:10.1121/1.393232

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Using the technique of mean‐field renormalization, it is shown that the general effect of surface roughness on Love waves is to cause not only a splitting of the mode frequency but also an effective attenuation because of multiple scattering from the rough surface. The precise amounts of splitting and attenuation are shown to be dependent on the power spectrum of the surface roughness as well as on the normal mode. The calculation is restricted to the two‐dimensional problem where the roughness varies in one spatial direction only so that coupling of the Love modes toP–SVwaves is strictly forbidden. For surface roughness, of rms amplitude 〈 f 2〉1/2, occurring on a scale that is long when compared to the Love mode wavelength, both the frequency splitting and effective attenuation are of fractional order ω〈 f 2〉1/2(s21−s22)12/2, where ω is the angular frequency ands1ands2are the slowness on either side of the rough boundary. For a surface roughness dominating the evaluation of certain basic integrals occurring in the mean‐field dispersion relation, it is shown that the modes couple nonlinearly to the rough fluctuations so that both resonant scattering of the modes amongst themselves, as well as nonresonant scattering to ‘‘background’’ oscillations, takes place. These analytic calculations have been done to both illustrate the broad range of diverse phenomena that can arise as well as to provide formulas for later comparison against numerical experiments and synthetic seismograms.

ISSN:0001-4966    

DOI:10.1121/1.393233

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A new method for measuring impedance has been used for evaluating the normal surface impedance of three foams in free field. The results have been interpreted using the Biot theory, the two dilatational waves being taken into account. It has been pointed out from the theory that, for high flow resistance, the ratio of the acoustical velocities of the frame and the air is close to 1 at the surface of the foam. This ratio decreases with flow resistance but is never negligible for the studied foams. For foams with high flow resistance, the contributions of the two waves must be taken into account when calculating the impedance, and a description with only one wave would not be realistic. For foams with medium flow resistance, the one‐wave approximation for calculating the surface impedance is a good approximation in the whole range of acoustical frequencies.

ISSN:0001-4966    

DOI:10.1121/1.393234

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

With rigorous ray tracing, caustics and their associated cusps in addition to those of classical ray tracing are encountered. These additional caustics are associated with classical (WKB) vertex points. Rigorous ray tracing also confirms those caustics that exist under a classical ray tracing representation, albeit both displaced from the classical locations into the classical shadow zone and perhaps deformed with additional cusps.

ISSN:0001-4966    

DOI:10.1121/1.393235

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Kirchhoff–Helmholtz theory is used to derive a generalized ray theory, called the multifold path integral (MFPI) method, that enables the calculation of frequency‐dependent ray theoretical amplitudes at caustics and shadows where ordinary ray theory (geometrical acoustics) fails. The method is then applied to some problems of acoustic propagation in an ocean whose sound‐speed profile, bathymetry, and bottom characteristics are all range dependent. Specifically, considered are: (a) rays with multiple bounces between the surface and a bottom that is rough on a scale much larger than a wavelength of the signal; (b) rays in a sound‐speed channel whose axis height varies with range; and (c) rays around an obstacle such as a seamount. Modeling examples will be presented in a subsequent paper but included here is a discussion of various methods for evaluating the multifold integrals.

ISSN:0001-4966    

DOI:10.1121/1.393236

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Current understanding of the acoustic processes occurring in the seafloor is used to develop a detailed ray approach for simulating the received time series of a broadband acoustical signal interacting once with the seafloor. The environment is assumed to be horizontally stratified, and the seafloor is described in terms of a geoacoustic profile. The frequency response is constructed from the superposition of the individual responses of each of the eigenrays. The ray approach includes the effects of reflection from the water–sediment interface, penetration into the sediment, refraction due to a constant compressional sound‐speed gradient in the sediment, absorption within the sediment, phase shifts due to caustics, and multipaths. The one‐bounce time series is constructed from the inverse Fourier transform of the product of the source spectrum and the eigenray frequency response. As an example application, the one‐bounce time series for an explosive source in a deep water environment is calculated and analyzed in terms of acoustical processes. Excellent agreement with measured time series is demonstrated.

ISSN:0001-4966    

DOI:10.1121/1.393237

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP