Impact of the irregular microgeometry of polyurethane foam on the macroscopic acoustic behavior predicted by a unit-cell model.

This paper deals with the prediction of the macroscopic sound absorption behavior of highly porous polyurethane foams using two unit-cell microstructure-based models recently developed by Doutres, Atalla, and Dong [J. Appl. Phys. 110, 064901 (2011); J. Appl. Phys. 113, 054901 (2013)]. In these models, the porous material is idealized as a packing of a tetrakaidecahedra unit-cell representative of the disordered network that constitutes the porous frame. The non-acoustic parameters involved in the classical Johnson-Champoux-Allard model (i.e., porosity, airflow resistivity, tortuosity, etc.) are derived from characteristic properties of the unit-cell and semi-empirical relationships. A global sensitivity analysis is performed on these two models in order to investigate how the variability associated with the measured unit-cell characteristics affects the models outputs. This allows identification of the possible limitations of a unit-cell micro-macro approach due to microstructure irregularity. The sensitivity analysis mainly shows that for moderately and highly reticulated polyurethane foams, the strut length parameter is the key parameter since it greatly impacts three important non-acoustic parameters and causes large uncertainty on the sound absorption coefficient even if its measurement variability is moderate. For foams with a slight inhomogeneity and anisotropy, a micro-macro model associated to cell size measurements should be preferred.

Investigation of the convergence of the mixed displacement-pressure formulation for three-dimensional poroelastic materials using hierarchical elements.

Recently, a mixed pressure displacement [u, P] formulation based on Biot's poroelasticity equations has been presented for porous materials. This model leads to a reduction of the number of degrees of freedom required for the modeling of three-dimensional porous media in comparison to classical displacement-displacement [u, U] formulations. In this paper, an extension of the [u, P] formulation based on hierarchical elements is presented. First, a variant of the weak integral form of the [u, P] formulation is presented and its numerical implementation using hierarchical elements is detailed, together with the application of boundary and loading conditions. Numerical results are presented to show the accuracy and performance of the present approach. In particular, the importance of correctly capturing the coupling effects between the two phases is highlighted.

Pseudostatic corrections for the forced vibroacoustic response of a structure-cavity system

The analysis of the coupled vibroacoustic behavior of a structure-cavity system is often performed using the in vacuo structure modes and the rigid cavity modes. Unfortunately, the use of such a modal basis can result in a poor convergence when the high-frequency modes of one of the two subsystems are coupled to the low-frequency modes of the other subsystem. This problem is made critical by the lack of a reliable criterion for selecting the number of kept modes for each subsystem. This paper shows that the effect of modal truncation is critical for the convergence of the method and that the convergence can be greatly improved using pseudostatic corrections for both the structure and the cavity. The theory behind the proposed technique is presented together with two generic problems exhibiting strong coupling: (i) an elastic cylindrical cavity filled with air; and (ii) an elastic plate coupled to a rectangular cavity filled with water.

The start-stop-start phenomenon in scalp-sphenoidal ictal recordings.

The start-stop-start (SSS) phenomenon is an apparent abortive ictal onset separated from the main seizure discharge. It was previously described in seizures recorded with subdural electrodes. We have observed this phenomenon in scalp-sphenoidal ictal recordings as well. We retrospectively reviewed 435 seizures recorded with scalp-sphenoidal electrodes from 61 patients with temporal lobe epilepsy. We found SSS onset in 15 seizures of 8 patients, representing 26% of these patients' seizures. The first "start" usually had a narrow field, typically in the sphenoidal electrode. The mean duration of the first "start" was 11 sec and that of the stop 8 sec. The restart had a different morphology and frequency in 87% and had a wider field in 67% of seizures. The clinical onset followed the first start and preceded the restart in most of the seizures. In 1 patient, 1 seizure with SSS was correctly localized and lateralized, whereas 5 of 7 without SSS were falsely lateralized. The recognition of the SSS phenomenon may improve the accuracy of seizure localization in scalp-sphenoidal recordings.