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  • 1
    facet.materialart.
    Unknown
    Surface acoustic admittance and absorption of highly porous, layered, fibrous materials (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: Some acoustic properties of Kevlar-29 - a fine fibered, layered material is investigated. Kevlar is characterized by very high strength, uniform filaments arranged in a parallel batt where most filaments are random in the x-y plane but ordered as planes in the z direction. For experimental purposes, volume porosity, static flow resistance and mean filament diameter are used to identify the material. To determine the acoustic surface admittance of Kevlar, batts of the material are cut into small pads and placed into a standing wave tube terminated by a rigid brass plug. The attenuation and relative phase shift are recorded at each frequency in the range of 50 to 6000 Hz. Normalized conductance and susceptance are combined to form the acoustic absorption coefficient. The data are compared with theory by plotting the normalized admittance and normal incident absorption coefficient versus cyclic frequency.
    Keywords: ACOUSTICS
    Type: Studies of Acoustical Properties of Bulk Porous Flexible Mater.; 1 p
    Format: text
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  • 2
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    Acoustic structure and propagation in highly porous, layered, fibrous materials (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The acoustic structure and propagation of sound in highly porous, layered, fine fiber materials is examined. Of particular interest is the utilization of the Kozeny number for determining the static flow resistance and the static structure factor based on flow permeability measurements. In this formulation the Kozeny number is a numerical constant independent of volume porosity at high porosities. The other essential parameters are then evaluated employing techniques developed earlier for open cell foams. The attenuation and progressive phase characteristics in bulk samples are measured and compared with predicted values. The agreements on the whole are very satisfactory.
    Keywords: ACOUSTICS
    Type: Studies of Acoustical Properties of Bulk Porous Flexible Mater.; 16 p
    Format: text
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  • 3
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    Absorptive surfaces of extended reaction angle of incidence considerations (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The acoustic properties of a porous absorbing surface are characterized to apply the concepts to practical situations. Propagation constants, impedance, incident pressure wave, sound waves in the air of the pores, and general acoustic behavior are presented mathematically. The design equations are applicable for highly porous bulk materials. Modified equations are appropriate for bulk liners with stiff frames or flexible frames of finite depth.
    Keywords: ACOUSTICS
    Type: Studies of Acoustical Properties of Bulk Porous Flexible Mater.; 5 p
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  • 4
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    Propagation of sound in highly porous open-cell elastic foams (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: This work presents both theoretical predictions and experimental measurements of attenuation and progressive phase constants of sound in open-cell, highly porous, elastic polyurethane foams. The foams are available commercially in graded pore sizes for which information about the static flow resistance, thermal time constant, volume porosity, dynamic structure factor, and speed of sound is known. The analysis is specialized to highly porous foams which can be efficient sound absorbers at audio frequencies. Negligible effect of internal wave coupling on attenuation and phase shift for the frequency range 16-6000 Hz was predicted and no experimentally significant effects were observed in the bulk samples studied. The agreement between predictions and measurements in bulk materials is excellent. The analysis is applicable to both the regular and compressed elastic open-cell foams.
    Keywords: ACOUSTICS
    Type: Acoustical Society of America, Journal (ISSN 0001-4966); 73; April 19
    Format: text
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  • 5
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    Studies of acoustical properties of bulk porous flexible materials (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Acoustic prediction and measurement of bulk porous materials with flexible frames is investigated. The acoustic properties of Kevlar 29 are examined. Various acoustic tests are employed to determine impedance, sound wave propagation, and wave pressure equations for the highly porous fiber composites. The derivation of design equations and future research goals are included.
    Keywords: ACOUSTICS
    Type: NASA-CR-173622 , NAS 1.26:173622
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  • 6
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    Surface acoustic admittance of highly porous open-cell, elastic foams (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: This work presents a comprehensive study of the surface acoustic admittance properties of graded sizes of open-cell foams that are highly porous and elastic. The intrinsic admittance as well as properties of samples of finite depth were predicted and then measured for sound at normal incidence over a frequency range extending from about 35-3500 Hz. The agreement between theory and experiment for a range of mean pore size and volume porosity is excellent. The implications of fibrous structure on the admittance of open-cell foams is quite evident from the results.
    Keywords: ACOUSTICS
    Type: Acoustical Society of America, Journal (ISSN 0001-4966); 73; April 19
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  • 7
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    Acoustic structure and propagation in highly porous, layered, fibrous materials (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The acoustic structure and propagation of sound in highly porous, layered, fine fiber materials is examined. Of particular interest is the utilization of the Kozeny number for determining the static flow resistance and the static structure factor based on flow permeability measurements. In this formulation the Kozeny number is a numerical constant independent of volume porosity at high porosities. The other essential parameters are then evaluated employing techniques developed earlier for open cell foams. The attenuation and progressive phase characteristics in bulk samples are measured and compared with predicted values. The agreements on the whole are very satisfactory.
    Keywords: ACOUSTICS
    Type: Acoustical Society of America, Journal (ISSN 0001-4966); 76; 1231-123
    Format: text
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  • 8
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    The acoustical structure of highly porous open-cell foams (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: This work concerns both the theoretical prediction and measurement of structural parameters in open-cell highly porous polyurethane foams. Of particular interest are the dynamic flow resistance, thermal time constant, and mass structure factor and their dependence on frequency and geometry of the cellular structure. The predictions of cell size parameters, static flow resistance, and heat transfer as accounted for by a Nusselt number are compared with measurement. Since the static flow resistance and inverse thermal time constant are interrelated via the 'mean' pore size parameter of Biot, only two independent measurements such as volume porosity and mean filament diameter are required to make the predictions for a given fluid condition. The agreements between this theory and nonacoustical experiments are excellent.
    Keywords: ACOUSTICS
    Type: Acoustical Society of America; vol. 72
    Format: text
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  • 9
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    Acoustical properties of highly porous fibrous materials (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Highly porous, fibrous bulk sound absorbing materials are studied with a view toward understanding their acoustical properties and performance in a wide variety of applications including liners of flow ducts. The basis and criteria for decoupling of acoustic waves in the pores of the frame and compressional waves in the frame structure are established. The equations of motion are recast in a form that elucidates the coupling mechanisms. The normal incidence surface impedance and absorption coefficient of two types of Kevlar 29 and an open celled foam material are studied. Experimental values and theoretical results are brought into agreement when the structure factor is selected to provide a fit to the experimental data. A parametric procedure for achieving that fit is established. Both a bulk material quality factor and a high frequency impedance level are required to characterize the real and imaginary part of the surface impedance and absorption coefficient. A derivation of the concepts of equivalent density and dynamic resistance is presented.
    Keywords: ACOUSTICS
    Type: NASA-TM-80135
    Format: application/pdf
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