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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.

As a suitable replacement for prevalent but environmentally hazardous fibrous materials used in exhaust system silencers, innovative micro-perforated (MP) elements have been progressively implemented for internal combustion engine noise control during the past decades. Although MP elements are already massproduced for IC engine noise control, surprisingly few scientific publications can still be found on the endurance of these MP elements. Recently the reliability of MP element was studied by the authors for a small four-stroke petrol engine silencer application. The results clearly demonstrated that the performance of the micro-perforated elements is influenced by the contamination of residual combustion products of the exhaust gas. In this paper the endurance of MP element tested in small industrial diesel engine application of a novel unmanned ground vehicle (UGV) has been treated.

A critically high noise level inside protective helmet is a prevalent concern for motorcyclists. Especially at highway speeds where the noise level, regardless of helmet type can exceed 100 dB(A) and approaches threshold of discomfort, often resulting in temporary hearing loss. Despite of large share of persons exposed to such noise disturbance around the world, the in helmet noise levels have not significantly decreased over the last decades. Only few scientific publications can be found to systematically address this issue. Furthermore, in respect of driving safety even moderate noise levels are reported to impair reaction times and reduce attention of motorcyclists. At higher speeds the dominant helmet noise source is linked to aerodynamic turbulence around the helmet shell. The loudness and spectral contents mainly depend on the driving speed, windscreen configuration, riding position and helmet geometry.

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.

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.

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.

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 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.

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.

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.

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 acoustic performance of silencers has been investigated. The experimental techniques to study the attenuation of a silencer are in focus. A novel test procedure has been described and used to determine the sound transmission through the silencers in hot mean flow conditions. The experiments have been carried out on two motorcycle engine silencers with different design. The results analyzed and discussed clearly present the influence of the induct flow conditions on the sound transmission loss of the silencer.