Speaker
Description
Designing new kind of absorbers to avoid the use of porous or fibrous materials due to the presence of mean flow in a duct is a challenging topic in the field of noise control. Lately, the development of metamaterials has provided new noise control perspectives for this issue. Traditional Acoustic Black Holes (ABHs) have been derived with closed-ended configurations to achieve broadband absorption. These solutions are appropriate as anechoic termination but they cannot be used as silencers as they require a mean flow going through the axis of the acoustic treatment. The use of open ABH, also known as Rainbow-Trapping Silencers (RTS) has been proposed to tackle this issue. RTS are fully-opened in-duct metamaterials, of rectangular or cylindrical cross-sections, traversed by a low-speed flow and whose walls are lined by graded cavity depths. In order to reduce reflection and transmission and to obtain full dissipation of the acoustic disturbance over a broad bandwidth, a strategy is to enhance its slow sound properties. They are obtained when the axial phase speed gets closer to the bulk flow velocity. Parametric and optimization studies from theoretical (transfer matrix method) and numerical (finite element model) approaches show how a suitable choice of converging RTS parameters maximize slow sound effects. Such compact lightweight devices could be used to reduce broadband sound emissions in transport systems.