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Acoustical properties of highly porous fibrous materialsHighly 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.
Document ID
19790023882
Acquisition Source
Legacy CDMS
Document Type
Technical Memorandum (TM)
Authors
Lambert, R. F.
(NASA Langley Research Center Hampton, VA, United States)
Date Acquired
September 3, 2013
Publication Date
July 1, 1979
Subject Category
Acoustics
Report/Patent Number
NASA-TM-80135
Accession Number
79N32053
Funding Number(s)
PROJECT: RTOP 505-03-13-18
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.
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