Fabio Auriemma

Acoustic performance of micro-grooved elements

Abstract The acoustic performance of a new type of fibre-less sound absorber, the micro-grooved element (MGE), is studied in this paper. A MGE is a double layer element that involve inlet/outlet slots and inner micro-channels engraved onto the mating surface of the layers. MGEs are potential substitute of traditional micro-perforated elements (MPEs) in applications where the absorption coefficient, the acoustic behaviour at high levels of sound excitation and the cost of the absorber are of main concern. The transfer impedance and the absorption coefficient of MGEs and MPEs are measured, modelled and compared in this study. The acoustics of the micro-channels is treated in detail, the impedance end corrections are determined and the non-linear effects are accounted for. The linear behaviour of MGEs is described by extending the models for slit-shaped perforated elements. The quasi- and non- linear behaviours are expressed as a function of the Shear number and Strouhal number by curve fitting the experimental results.

A Novel Design for Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements

Regulations stipulating the design of motorcycle silencers are strict, especially when the unit incorporates fibrous absorbing materials. Therefore, innovative designs substituting such materials while still preserving acceptable level of characteristic sound are currently of interest. Micro perforated elements are innovative acoustic solutions, which silencing effect is based on the dissipation of the acoustic wave energy in a pattern of sub-millimeter apertures. Similarly to fibrous materials the micro-perforated materials have been proved to provide effective sound absorption in a wide frequency range. Additionally, the silencer is designed as a two-stage system that provides an optimal solution for a variety of exploitation conditions. In this paper a novel design for a cruiser type motorcycle silencer, based on micro-perforated elements, is presented. It has been demonstrated that the micro-perforated elements can successfully be used to achieve high attenuation of IC-engine noise in strictly limited circumstances. A technical description of the design and manufacturing of the prototype silencer is given and technological issues are discussed. The acoustical and aerodynamical performance of the silencer is characterized by transmission loss and pressure drop data. The influence of the two-stage system valve operation has been analyzed by studying the acoustics data and engine output characteristics. In addition to the experimental investigations, numerical 1-D models were developed for the optimization of the silencer geometry and the results are compared in a number of operating conditions. The studies have resulted in development of a silencer system for a small series cruiser type motorcycle. The first silencer prototypes have been tested on the motorcycle. While maintaining acceptable pressure drop characteristics, it has proven to comply with standard noise criteria without incorporating fibrous materials. The radiated motorcycle sound, as one of the key features of successful design, has been evaluated. The sound design has been recognized as well suitable for the product.

Damping of Acoustic Waves in Straight Ducts and Turbulent Flow Conditions

In this paper the propagation of acoustic plane waves in turbulent, fully developed flow is studied by means of an experimental investigation carried out in a straight, smooth-walled duct.The presence of a coherent perturbation, such as an acoustic wave in a turbulent confined flow, generates the oscillation of the wall shear stress. In this circumstance a shear wave is excited and superimposed on the sound wave. The turbulent shear stress is modulated by the shear wave and the wall shear stress is strongly affected by the turbulence. From the experimental point of view, it results in a measured damping strictly connected to the ratio between the thickness of the acoustic sublayer, which is frequency dependent, and the thickness of the viscous sublayer of the turbulent mean flow, the last one being dependent on the Mach number. By reducing the turbulence, the viscous sublayer thickness increases and the wave propagation is mainly dominated by convective effects. In the present work, the damping and wall impedance have been extracted from the measured complex wavenumber, which represents the most important parameter used to characterize the wave propagation. An experimental approach, referred to as iterative plane wave decomposition, has been used in order to obtain the results. The investigations have been carried out at low Mach number turbulent flows, low Helmholtz numbers and low shear wavenumbers. The aim is to overcome a certain lack of experimental results found by the authors of the most recent models for the plane wave propagation in turbulent flows, such as Knutsson et al. (The effect of turbulence damping on acoustic wave propagation in tubes, Journal of Sound and Vibration, Vol. 329, No. 22, 2010), and Weng et al. (The attenuation of sound by turbulence in internal flows, The Journal of the Acoustical Society of America 133(6), 2013).