###### Abstract  
This work illustrates the development of passive noise control (PNC) improvements of aircraft headtests to enhance the acoustic comfort for passengers. Two PNC improvements were studied with the aim of reducing the noise perceived by passengers during flight. Two headrest configurations, with and without the lateral caps, and two different materials, a traditional foam and an innovative Silica/Polyvinylpyrrolidone (PVP) woven non-woven mat, were considered, and compared in terms of sound pressure level (SPL) perceived by passengers. Boundary element method (BEM) models were built up to evaluate the acoustic performances of different headrest configurations, varying in terms of shape and textile. A spherical distribution of monopole sources surrounding the headrests was considered as acoustic load, in such a way as to recreate a diffuse acoustic field simulating the cabin noise perceived by passengers during cruise conditions. The impact of the two PNC improvements was analyzed to envisage some general guidelines useful to design advanced headtests from the acoustic viewpoint. a 202010 10 5712 10.3390/app10165712 10.3390/app10165712 10.3390/app10165712 10.390/app10165712 10. Several approaches exist for such a kind of analyses, even though the most widely adopted seems to be the finite element method (FEM), thanks to its widespread field of application [2], supported by advanced implementations to improve the computational efficiency [3; 4; 5]. Deterministic methods, e.g., based on multibody simulation [6], boundary element method (BEM) [7], combined FEM-BEM [8; 9], hybrid BEM/empirical approaches [10], and the fast multipole method (FMM)-BEM [11], are widely adopted to tackle vibration and acoustic assessments of complex structures. In the field of linear acoustics, BEM is an important alternative to the more traditional methods, in particular for exterior problems, where the acoustic domain is so large, e.g., the open air or the ocean, that is acceptable to model it to be infinite in extent. Applying domain methods, such as FEM, to such a problem clearly requires some careful thought (an example of the infinite radiation condition application with FEM is available in [12]). Instead, it is more advantageous to use BEM for this kind of applications since only the surface mesh of the bodies is required, being the Sommerfeld radiation condition naturally satisfied at infinity (this is the condition at infinity for exterior problems that ensure that all scattered and radiated waves are outgoing) [13]. In this way, both preprocessing times and runtimes are reduced. As a matter of fact, BEM has become an established computational method for acoustics, widely adopted for noise calculations in the last decades [14]. FEM and BEM methods are usually applied for low/medium frequency ranges, whereas statistical energy analysis (SEA) is a methodology generally suitable for medium/high frequency ranges [15; 16]. In the mid-frequency range, where deterministic calculations are costly and confidence intervals of SEA cannot be generally satisfied, hybrid FEM-SEA models represent a viable alternative [17; 18]. Aircraft manufacturers are interested in decreasing the amount of noise in the fuselage by using new materials. In particular, they are interested in low-weight materials, which may help reducing the fuel consumption together with achieving lower noise levels. PNC (passive noise control) consists of adopting more performant design configurations and materials to reduce noise. These materials have the ability to reduce noise via dampening or lessening sound wave reflections, as sound energy is converted into heat dissipation. Efficient sound absorbents, such as porous materials, have the ability to prevent as many sound reflections as possible while also dissipating any unwanted sound entering the material. Thicker sound absorbers usually provide larger dissipation of air molecules vibrational energy because of the increased surface interaction area.