A review of literature demonstrates that there are many ill-understood factors that determine the results of GH provocative (re)testing, so that these results should be interpreted with extreme caution when used for diagnosis or confirmation of diagnosis of GHD. GH provocation tests are probably of no value at all for what has been called ‘partial GHD’. The phenomenon of ‘normalization’ of test results after long-term treatment with GH needs no ‘transient GHD’ hypothesis as it can be largely explained by the very low reproducibility of the tests and by a regression to the mean effect. Moreover, it is possible that ‘normal values’ increase with age. Other determinants of normal peak values may also change from childhood to adulthood and contribute to ‘normalization’.
The acoustic performance of sound absorbing foams is determined by alarge number of processes and parameters. Chemical formulation, blowing agent and production processes influence the formation of the microstructure of foam. The interaction between the microstructure of af oam and an applied static or oscillatory air flowi sa critical factor in understanding the acoustic performance of ap orous sound absorber.T his paper presents the results of astudy of the effect of the 3D geometry of the microstructure of polymeric foams on sound absorption. The relation between oscillatory fluid flowand real physical features of the morphology of foam-likestructures such as closed cell content, the influence of cell size and, to lesser degree, polydispersity and pore size connections is investigated. Amethodology to generate three dimensional discrete geometries by using numerical tools and x-ray computerized micro-tomographyispresented. These structures can be seen as arepresentative volume element of ar eal sound absorbing polymeric foams and can be used in an umerical treatment for investigating their respective thermal, acoustical and mechanical properties. It is shown that is possible to predict in areliable manner the sound absorption for the audible frequencyr ange and other single valued keyp arameters that are commonly used to differentiate the acoustic performance of porous sound absorbers. This is done by numerically generating three dimensional microstructures ideal foams similar to aW eaire-Phelan foam and by numerically reconstructing afully reticulated polyurethane foam from image data of µCT-scan. The sound absorption of these structures is then examined by using advanced computational fluid dynamics. The obtained numerical results are compared to results obtained by the commonly used 'rigid-frame' or 'equivalent fluid' model introduced by Johnson et al.,Champoux et al. and Lafarge et al. PACS no. 43.20.El, 43.20.Gp. 43.55.Ev Boeckx et al.:A bsorption of porous polymeric foam Vol. 96 (2010)
Besides their structural complexity, the acoustic behaviour of polymer-based poro-elastic layers is complicated also due to their frequency dependent elasticity. In this work, we address the frequency and temperature dependence of the elastic behaviour in general, and the shear modulus in particular, of poro-visco-elastic materials. The analysis is based on the monitoring of mechanically excited guided acoustic wave propagation by means of a laser Doppler vibrometer scanning technique. The concept and practical implementation of the experimental method are presented, as well as the signal processing procedure and data analysis. Experimental data are presented for a polyurethane foam. The observed visco-elastic behavior, complemented with dielectric spectroscopy data, is interpreted in the framework of two underlying relaxation processes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.