Diffuse sound absorption modelling of complex finite absorbers using a hybrid deterministic-statistical energy analysis approach

This contribution presents a numerical approach to quantify the response of an absorber in a diffuse reverberation room. Conventionally, this is done by considering an infinite absorber coupled to an acoustic halfspace. It is, however, well known that the diffuse absorption coefficient for a finite absorber can be quite different due to what is referred to in literature as the edge effect. A finite size correction has been developed previously, but it is only applicable to homogeneous absorbers and is based on a computationally costly quintuple integration. This contribution presents an alternative approach in which a deterministic model, e.g. using the finite element or modal transfer matrix method, is coupled with a statistical model of the room using a hybrid deterministic-statistical energy analysis framework. With this framework, also the theoretical uncertainty on this diffuse sound absorption that is inherent in the diffuse field assumption can be quantified, i.e. the variance of sound absorption results that can be theoretically expected across an ensemble of reverberation rooms of the same volume. The methodology is numerically and experimentally validated for several absorber types.

Fluctuating viscoelasticity based on a finite number of dumbbells

Two alternative routes are taken to derive, on the basis of the dynamics of a finite number of dumbbells, viscoelasticity in terms of a conformation tensor with fluctuations. The first route is a direct approach using stochastic calculus only, and it serves as a benchmark for the second route, which is guided by thermodynamic principles. In the latter, the Helmholtz free energy and a generalized relaxation tensor play a key role. It is shown that the results of the two routes agree only if a finite-size contribution to the Helmholtz free energy of the conformation tensor is taken into account. Using statistical mechanics, this finite-size contribution is derived explicitly in this paper for a large class of models; this contribution is non-zero whenever the number of dumbbells in the volume of observation is finite. It is noted that the generalized relaxation tensor for the conformation tensor does not need any finite-size correction. Graphical abstract

First-principles study on optical spectra for the oxygen vacancy in lead tungstate crystal

Based on the first-principles, we simulated the spectral properties of PbWO4 (PWO) crystals with an oxygen vacancy. As density functional theory (DFT) underestimates the band gap, the band edge is modified by Heyd-Scuseria-Ernzerhof (HSE). Moreover, artificial interactions of the charged defect of oxygen vacancies with three different charges have been corrected by finite-size correction scheme (FNV). Finally, the optical properties are obtained containing electron–phonon coupling. The calculated absorption band peaks of the F and F[Formula: see text] centers at 1.7[Formula: see text]eV and 2.47[Formula: see text]eV agree well with the experimental value, respectively.