## 1 Introduction
Other authors considered alternative models for artificial AE sources, such as penny-shaped cracks in aluminium alloys under tensile and twisting loading, introduced by [PERSON] et al. [19], who obtained the displacement vectors as a function of time and coordinates for arbitrary distances from the centre of the crack. [PERSON] used a pencil lead-break model implemented through Finite Element analysis [20]. The same author [21] found a new AE source model, which takes into account the microscopic elastic properties near the source. A pencil lead-break model was also used by [PERSON] et al. [22] to validate Finite Element Analysis used to evaluate the influence of waveguides on the AE signals. [PERSON] [23], in order to build representative sources, investigated the difference between nodal sources, line sources and cylindrical sources. In the work of [PERSON] [24], sources are described as time-dependent displacement discontinuities which are translated to volume forces, and the transient waves are computed using a finite element model of the specimen's cross-section. There are also wave propagation models based on the analytical or numerical resolution of the elastodynamic equations. Analytical methods are adapted to simple geometries. [PERSON] et al. proposed a ray method in the context of geophysics [25] and for AE [26; 27]. Thereafter, [PERSON] used it to describe the response of an infinite space to the propagation of a crack [28]. [PERSON] calculated the analytical solution of the surface displacement of a half-space due to buried and surface pulses using Green functions [29], and [PERSON] used the reciprocity theorem to combine a quasi-static crack propagation model and Rayleigh waves Green functions in a half-space [30]. [PERSON] [31] represented a microfailure event as a point source and used the ray theory to develop a physical model of wave propagation. To resolve the problem in complex geometries, the Finite Element Method (FEM) is the one of the most commonly used numerical method. It has been applied and experimentally validated on plates [32; 33] and composite materials [34; 35; 36]. [PERSON] and [PERSON] [33] were the first to show that the explicit finite elements are effective and efficient to compute the far-field structure of wave mode propagation. [PERSON] [21] realised such capabilities by using an implicit three-dimensional finite element code to determine the spectral content of signals originating from the most common damage in composite material. Some scientific works interpretedAE signals through modal analysis [37; 38]. However, only few of the studies performed on laboratory samples took into account the influence of reflections on the AE signals. [PERSON] showed that the presence of reflections modifies the frequency content of the signal [39] and [PERSON] concluded that it is necessary to reduce or cut out reflections in laboratory specimens in order to better simulate the test field conditions on larger specimens [40]. A recent study [41] investigated AE by a combination of experimental, analytical and computational methods. Using full field experimental data obtained from crack initiation, it constructed a phenomenological mechanical law and implemented it in a FEM simulation. This work emphasised the influence of the geometry and of the material properties on the wave propagation. [PERSON] et al. [42] also proposed a modular framework for quantitative forward modelling of the complete AE process. To interpret AE signals in terms of their relevance to the material failure, one needs a worthy knowledge of the AE signal propagation from the source to the surface of the sample and of detection. Modal analysis [43; 44] is a judicious way of analysing AE signals, but the modes used to interpret the signals are often plate modes. The aim of this work is to better understand the relationship between the AE sources and the detected signal by unravelling the effect of the different stages of the acquisition chain, namely the propagation medium and the detection system.