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The goal of this paper is to present a novel type of advanced acoustic material - micro grooved element (MGE) - which is designed for noise control in a wide range of applications. MGEs have been proved to offer a respectable alternative for the existing micro-perforated elements (MPEs), while being cost effective and causing low pressure loss. These elements have been found to be suitable for substitution of fibrous materials, typically present in silencer units. Currently, the cost of the MPEs is relatively high due to the technological complexity of manufacturing process. On the other hand, cheaper solutions of MPEs, based on irregularly shaped micro-apertures, potentially cause higher pressure loss due to surface roughness. The key concept of the MGEs is the use of micro-grooves forming acoustic channels, instead of the micro-holes of MPEs, which the sound wave has to pass.

The goal of this paper is to provide a complete characterization of acoustic performance for a novel type of advanced acoustic material - micro grooved element (MGE). In a previous study, the MGEs have been proved to offer a respectable alternative for the existing and increasingly popular micro perforated elements (MPEs). The MGEs are multi-layer elements where the acoustic attenuation effect originates from viscous losses taking place in a number of sub-millimeter grooves forming acoustic micro-paths inside the material. This new configuration allows to replace the laser perforation process, used to manufacture the MPEs, with less time consuming and more cost effective technologies. Moreover, such elements preserve low weight and surface roughness. Experiments have demonstrated that the MGEs can be regarded as suitable solution for noise control in a wide range of applications.

The acoustic impedance exhibited by a new type of element for noise control, the Micro-Grooved Elements (MGEs), has been widely investigated in this paper. The MGEs are typically composed of two overlying layers presenting macroscopic slots and a number of micro-grooves on one of the contact surfaces. The micro-grooves result in micro-channels as the layers are assembled to form the element. Similarly to Micro-Perforated Elements (MPEs), the MGEs have been proved to provide effective dissipation of acoustic energy by the means of viscous losses taking place in the micro-channels. However, in contrast to the MPEs, the MGEs use the grooves, instead of the holes, in which the air is forced to pass through. It results in more cost effective elements, which have been found to represent an adequate alternative for fibrous materials, typically present in silencer units.

Today, catalytic converters are widely used in small engine exhaust systems to reduce pollutants. Besides reducing harmful pollutants, these devices have a significant effect on the acoustical performance and the pressure drop of the engine exhaust system. A catalytic converter is known to have two distinct acoustic effects: the reactive effect originating from the acoustic wave reflections caused by cross-sectional area changes within the unit and the resistive effect which results in the acoustic wave dissipation caused by viscous losses. The pressure drop in the narrow tubes in the catalytic converter element results in frequency dependent resistive effects on the transmitted sound. In this paper the passive acoustic effect which treats the sound attenuation in the catalytic converters has been investigated. An experimental investigation on small engine catalytic converters treated as acoustic two-ports is carried out.

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.

A modern exhaust silencer system designed for an internal combustion engine typically incorporates a number of acoustic elements, which all contribute in the overall acoustic performance of the system and determine the sound radiation into the surroundings. The characteristics of individual elements in acoustic silencers affecting sound propagation are referred to as the passive acoustic effect treated in this paper. An acoustic transmission loss is a parameter often used in engineering to describe the passive acoustic performance of exhaust system elements. However, in order to provide a complete acoustical characterization of silencers and silencer components the acoustic 2-port elements (the scattering matrix or alternatively the transfer matrix) should be additionally analyzed. In this paper the scattering matrixes are studied systematically for several small engine silencer elements in a variety of operating conditions.

For the last couple of decades, catalytic converters (CC) have become a standard part of the internal combustion engine exhaust systems. Besides reducing toxic components in exhaust gases, catalytic converters can have a certain effect on the acoustic performance of the exhaust system. In this paper the sound transmission and attenuation in the catalytic converters has been investigated. A catalytic converter is known to have two distinct acoustic effects: the reactive effect originating from the acoustic wave reflections caused by cross-sectional area changes within the unit and the resistive effect which results in the acoustic wave dissipation caused by visco-thermal losses. The flow resistance in the narrow tubes in the catalytic converter element results in frequency dependent dissipative effects on the transmitted sound. An experimental investigation on engine catalytic converters treated as acoustic two-ports is carried out.

In order to increase the internal combustion engine efficiency turbocharging is today widely used. The trend, in modern engine technology, is towards higher boost pressures while keeping the combustion pressure raise relatively small. The turbocharger surge occurs if the pressure at the outlet of the compressor is greater than it can maintain, i.e., a reverse flow will be induced. In presence of such flow conditions instabilities will occur which can couple to incident acoustic (pressure) waves and amplify them. The main objective of the present work is to propose a novel method for investigation of turbocharger flow instabilities or surge precursors. The method is based on the determination of the acoustic two-port data. The active part of this data describes the sound generation and the passive part the scattering of sound. The scattering data will contain information about flow-acoustic interaction and amplification of sound that could occur close to surge.

A rapid publicity growth has led to an extensive application of micro-perforated (MP) acoustic elements for broadband sound absorption in the exhaust systems of the internal combustion engine. Most typically, the MPs are exposed to grazing flow conditions, studied thoroughly by various authors in the past decades and represented by adequate acoustic models by now. However, in certain exhaust system designs implemented in the fibreless silencers of modern ground vehicles, an alternative layout for the tubular flow duct MP elements - the flow plug condition has been proven to be useful. In this type of MP’s application, the propagating gas flow is entirely guided through the micro-perforated sections upstream and downstream of the rigid plug, typically increasing the flow resistivity and the viscous damping of the sound in duct. Acoustic studies on such type of MP’s operating condition are scarce.

To respond growingly strict environmental regulations the acousticians are challenging to develop novel types of silencing elements. There are different types of flow duct elements designed for silencing the pulsating gas flows into and out of fluid machines. The silencing effect is typically achieved by introducing acoustic reflection and absorption. In order to achieve a good absorption in a wide frequency band, various fibrous materials e.g. wools are typically implemented. However, the physical properties of such materials do not often remain constant during the lifetime of a silencer. As the fibers tend to relocate and can partly be blown out to surroundings, acoustical performance may deteriorate. Therefore, it is in great interest to avoid fibrous materials in the design of the flow duct silencing elements. The present work is focused on the modern type of absorptive acoustic element - a micro-perforated element.

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.

Micro-grooved elements (MGEs) represent a novel technology developed for noise control in automotive, aerospace and room acoustics. The key concept of the MGEs is based on the use of micro-grooved layers forming micro-paths where the energy dissipation of the acoustic waves is primarily originated by viscous friction. Composed of a multi-layer fiber-less material, the MGEs represent a potential alternative to the traditional fibrous material based solutions as well as to the increasingly popular micro-perforated elements (MPEs). MGEs are designed as cost effective elements, found to be suitable for substitution of fibrous materials, typically present in silencer units. In this paper, a design procedure for a fiber-less small engine silencer based on MGEs is presented and experimentally validated. Hereby, the acoustical performance of the MGEs has been modeled by adapting the theoretical models provided by Allard and Maa for rectangular and circular ducts.

Since the introduction of microperforated (MP) sound absorption elements more than 40 years ago many variations of noise control devices from room acoustics to induct applications have been manufactured based on this technology. It has been demonstrated that micro-perforated elements can provide adequate IC-engine gas exchange noise attenuation. Several exhaust and inlet system silencers incorporating micro-perforated elements have been presented during the past 15 years for engine applications, encouraging the replacement of the typical fibrous materials and aiming several advantages including cleaner environment. The acoustical characteristics of the MP elements, have been studied thoroughly by several authors and good analytical models exist to predict the attenuation performance of those elements. However, almost no published information can be found regarding the reliability of the MP elements utilized in harsh engine exhaust system environment.

To control noise emission from internal combustion inlet, designers often choose small chamber type silencers at the inlet. In order to improve the inlet acoustic efficiency, inlet ducts with improved acoustic attenuation can be used. One potentially applicable material is acoustic metamaterial rapidly gaining popularity in different fields of engineering application. Small engine inlet duct, designed by using acoustic metamaterial structure comprising an array of resonators inside the wall of a rigid duct is investigated in this study. Experimental investigation of different designs is performed to characterize the acoustic behavior in terms of transmission loss (TL). By connecting multiple resonators of different size and location it is shown that a broadband TL can be achieved. The resulted attenuation band can be tuned by varying the resonator physical characteristics, showing promising potentials such of the material in the described application.