A Review on the Acoustical Properties of Natural and Synthetic Noise Absorbents
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paper_title: Tool Wear Monitoring in Turning Processes Using Vibratory Analysis

paper_content:

The main objective of this paper is to develop a signal processing strategy using vibratory signals in order to provide an efficient tool wear monitoring system able to increase machining performance. The method is based on the changes in the vibration signatures acquired during the turning operation over the tool life. Several signal processing techniques based on time and frequency domain analysis are proposed in order to extract a large number of indicators of the cutting tool state as variance, kurtosis, skewness, and coherence function. In this work, one of the innovative results is the tracking of tool wear by variance and coherence estimation. All of these indicators are correlated and validated by using white light interferometry measurements.
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paper_title: Possibility of using waste tire composites reinforced with rice straw as construction materials.

paper_content:

Agricultural lignocellulosic fiber (rice straw)-waste tire particle composite boards were manufactured for use as insulation boards in construction, using the same method as that used in the wood-based panel industry. The manufacturing parameters were: a specific gravity of 0.8 and a rice straw content (10/90, 20/80 and 30/70 by wt.% of rice straw/waste tire particle). A commercial polyurethane adhesive for rubber was used as the composite binder. The water proof, water absorption and thickness swelling properties of the composite boards were better than those of wood particleboard. Furthermore, the flexibility and flexural properties of the composite boards were superior to those of other wood-based panel products. The composite boards also demonstrated good acoustical insulation, electrical insulation, anti-caustic and anti-rot properties. These boards can be used to prevent impact damage, are easily modifiable and are inexpensive. They are able to be used as a substitute for insulation boards and other flexural materials in construction.
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paper_title: Noise Control Using Coconut Coir Fiber Sound Absorber with Porous Layer Backing and Perforated Panel

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Problem statement: Noise control was one of the major requirements to improve the living environment. One of the methods to do that is provided by sound absorber. Commonly, multi-layer sound absorbers are applied to absorb broadband noise that was composed of perforated plates, air space and porous material. However, multi-layers sound absorbers effectiveness depends on their construction. This study was conducted to investigate the potential of using coconut coir fiber as sound absorber. The effects of porous layer backing and perforated plate on sound absorption coefficient of sound absorber using coconut coir fiber were studied. Approach: Car boot liners made from woven cotton cloth were used as type of porous layer in the study. This material has been used widely in automotive industry. Perforated plate used was machined with perforation ratio of 0.20, thickness of 1 mm and holed diameter of 2 mm. The samples were tested at the acoustic lab of the Faculty of Engineering and Built Environment, University Kebangsaan Malaysia, according to ASTM E 1050-98 international standards for noise absorption coefficient. Results: The experiment data indicates that porous layer backing can improve noise absorption coefficient at low and high frequencies with significant increasing. 20 mm thick layer coconut coir fiber with porous layer backing exhibit peak value at frequencies between 2750-2825 Hz with maximum value of 0.97. The experimental results also found that the coconut coir fiber with perforated plate gives higher value for lower frequencies range from 600-2400 Hz. The optimum value for coconut coir fiber with perforated panel is around 0.94-0.95 for the frequency range 2600-2700 Hz. Conclusion: Noise absorption coefficient of coconut coir fiber was increased at all frequency when they were backing with Woven Cotton Cloth (WCC). At low frequency, the NAC have significant increasing. This is because WCC have higher flow resistivity than coconut coir fibers, so that sound can be dissipated as it travels through material significantly. The results from the experimental tests show that it has good acoustic properties at low and high frequencies and can used to be an alternative replacement of synthetic based commercial product. By using the porous layer and perforated plate backing to coconut coir fiber, the sound absorber panel shows a good potential to be an environmentally friendly product. This innovative sound absorption panel has a bright future because they are cheaper, lighter and environmentally compare to glass fiber and mineral based synthetic materials.
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paper_title: Performance improvement of sound-absorbing materials using natural bamboo fibers and their application

paper_content:

An acoustic material has been newly developed from the viewpoint of environmental protection. For this purpose, we used a natural resource, bamboo fiber, to manufacture sound-absorbing material. Until now, the sound-absorbing material has been developed using the crushed bamboo fibers. This material, however, is too heavy. To solve this problem, we have tried to develop a material using the explosion method. The normal incidence sound absorption coefficient of the bamboo fiber material was measured to be confirmed the basic properties. In addition, the characteristic impedance and the propagation constant of the material were measured. The contour lines of the sound absorption coefficient were calculated to find the relation of thickness and air space of the material with respect to density. In addition, we considered the influence of random incidence, and evaluated the material by adapting it to actual loudspeakers. The results show that the weight problem was solved, that the bamboo wool material works effectively in an actual product. Consequently, fundamental design criteria have been confirmed using the newly developed bamboo fiber material for sound absorption.
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paper_title: Rice straw-wood particle composite for sound absorbing wooden construction materials.

paper_content:

Abstract In this study, rice straw–wood particle composite boards were manufactured as insulation boards using the method used in the wood-based panel industry. The raw material, rice straw, was chosen because of its availability. The manufacturing parameters were: a specific gravity of 0.4, 0.6, and 0.8, and a rice straw content (10/90, 20/80, and 30/70 weight of rice straw/wood particle) of 10, 20, and 30 wt.%. A commercial urea–formaldehyde adhesive was used as the composite binder, to achieve 140–290 psi of bending modulus of rupture (MOR) with 0.4 specific gravity, 700–900 psi of bending MOR with 0.6 specific gravity, and 1400–2900 psi of bending MOR with a 0.8 specific gravity. All of the composite boards were superior to insulation board in strength. Width and length of the rice straw particle did not affect the bending MOR. The composite boards made from a random cutting of rice straw and wood particles were the best and recommended for manufacturing processes. Sound absorption coefficients of the 0.4 and 0.6 specific gravity boards were higher than the other wood-based materials. The recommended properties of the rice straw–wood particle composite boards are described, to absorb noises, preserve the temperature of indoor living spaces, and to be able to partially or completely substitute for wood particleboard and insulation board in wooden constructions.
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paper_title: Acoustic properties of rubber crumbs

paper_content:

Abstract The purpose of this article is to demonstrate that in the actual state of the art, recycled rubber products can be used to form acoustic absorbent materials, applicable to control noise pollution. Their applicability in noise screens shows excellent performance of broadband acoustic absorption. The article summarises part of a recent research carried out in the `Instituto de Acustica (CSIC)', that has concluded in a Spanish and EU patent about absorbing noise screens [1] .
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paper_title: Metal foams: A survey

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The current state-of-the-art in the development of cellular metal foams is reviewed, with focus on their fabrication, mechanical/thermal/acoustic properties, and potential applications as lightweight panels, energy absorbers, heat exchangers, and acoustic liners. Foam property charts with scaling relations are presented, allowing scoping and selection through the use of material indices.
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paper_title: Acoustic Properties of Organic/Inorganic Composite Aerogels

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Composite aerogels (with varying concentrations of silica and poly-dimethylsiloxane) were developed and their acoustic absorption coefficient as a function of composition and average pores size have been measured. The polydimethylsiloxane modified the ceramic structure of the silica aerogels, decreasing the material’s rigidity while maintaining the high porosity of the aerogel structure. The composite aerogels were found to exhibit different modes ::: of acoustic absorption than that of typical porous absorbers such as fiberglass. At some frequencies, the composite aerogels had 40% higher absorption than that of commercial fiberglass. Physical data show that these materials have a large surface area (> 400 m2/g) and varying pore sizes (d ˜ 5 - 20 nm).
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paper_title: Processing and damping behaviour of porous copper

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Abstract The well distributed open cellular porous copper was fabricated by present powder metallurgy technique based on space holder method, Depending on the volume fraction and size of the space holding particle, the porosity can be varied in the wide range of 30–85% and pore size from micron to millimetre in magnitude of order respectively. The damping behaviour and related relative dynamic modulus of the porous copper were investigated by a multifunction internal friction apparatus as a function of temperature from room temperature to 600°C. The results of investigation disclose that the porous copper can obtain a higher damping capacity than that of bulk one. In addition to this, an internal friction peak was found in the spectra of internal friction against temperature for the porous copper, it was proposed that the viscous sliding of the grain boundaries should be responsible for the appearance of the peak, and the dependence of the peak on porosity can be understood in terms of the anelastic relax...
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paper_title: Metal foams: A survey

paper_content:

The current state-of-the-art in the development of cellular metal foams is reviewed, with focus on their fabrication, mechanical/thermal/acoustic properties, and potential applications as lightweight panels, energy absorbers, heat exchangers, and acoustic liners. Foam property charts with scaling relations are presented, allowing scoping and selection through the use of material indices.
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paper_title: Applications for silica aerogel products

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Abstract Aerogels, nanoporous lightweight materials, were discovered more than 60 years ago. The supercritical manufacturing process and expensive raw materials typically used to produce aerogels prohibited commercialization on an industrial scale. Recently, a commercially attractive ambient pressure production process was developed which will allow broader commercialization of silica aerogel products. Some aerogel products for insulation applications and their preparation is described. Sound insulation properties including a remarkable absorption in the difficult low frequency range was found. In the case of insulation plates performance depends on binder and can be positively influenced by choosing the right binding material.
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paper_title: ALPORAS Aluminum Foam: Production Process, Properties, and Applications

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The production of foamed aluminum has long been considered difficult to achieve because of problems such as the low foamability of the molten metal, the varying size of the cellular structures, and solidification shrinkage. Researchers have gradually solved these problems and some manufacturers are now producing foamed aluminum by their own methods. Shinko Wire has been manufacturing foamed aluminum under the registered trade name ALPORAS since 1986, using a batch casting process.
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paper_title: Sound absorption in metallic foams

paper_content:

The sound absorption capacity of one type of aluminum alloy foams—trade name Alporas—is studied experimentally. The foam in its as-received cast form contains closed porosities, and hence does not absorb sound well. To make the foam more transparent to air motion, techniques based on either rolling or hole drilling are used. Under rolling, the faces of some of the cells break to form small sharp-edged cracks as observed from a scanning electronic microscope. These cracks become passage ways for the in-and-out movement of air particles, resulting in sound absorption improvement. The best performance is nevertheless achieved via hole drilling where nearly all of the sound can be absorbed at selected frequencies. Combining rolling with hole drilling does not appear to lend additional benefits for sound absorption. Image analysis is carried out to characterize the changes in cell morphologies due to rolling/compression, and the drop in elastic modulus due to the formation of cracks is recorded. The effects of...
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paper_title: Manufacture, characterisation and application of cellular metals and metal foams

paper_content:

The possibilities for manufacturing metal foams or other porous metallic structures are reviewed. The various manufacturing processes are classified according to the state of matter in which the metal is processed — solid, liquid, gaseous or ionised. Liquid metal can be foamed directlybyinjecting gas or gas-releasing blowing agents, or byproducing supersaturated metal–gas solutions. Indirect methods include investment casting, the use of spaceholding filler materials or melting of powder compacts which contain a blowing agent. If inert gas is entrapped in powder compacts, a subsequent heat treatment can produce cellular metals even in the solid state. The same holds for various sintering methods, metal powder slurry foaming, or extrusion and sintering of polymer/powder mixtures. Finally, electro-deposition or metal vapour deposition also allow for the production of highlyporous metallic structures. The various ways for characterising the properties of cellular metals are reviewed in second section of this paper. Non-destructive as well as destructive methods are described. Finally, the various application fields for cellular metals are discussed. Theyare divided into structural and functional applications and are treated according to their relevance for the different industrial sectors. # 2001 Elsevier Science Ltd. All rights reserved.
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paper_title: Measurements of Tyre/Road Noise and of Acoustical Properties of Porous Road Surfaces

paper_content:

The three main noise sources in modern cars and trucks are tyre/road interaction, wind turbulence, and the power plant. The power unit noise is composed of contributions from the fan, engine, exhaust, and transmission. In addition, the engine noise contains contributions from the injection system, intake, and cylinder block. Noise is of course a problem both inside the passenger compartments (interior noise) of vehicles and in residential areas near to the highway on which the vehicles are traveling (exterior noise). In most modern automobiles, the engine and exhaust noise have been sufficiently suppressed so that they are no longer the dominant interior or exterior noise sources. On heavy trucks, the engine is normally a diesel, and the suppression of its noise is more difficult. With vehicle noise, regulations are mostly concerned with exterior noise of vehicles, and this paper will consider mostly the exterior noise. Of course, if the exterior noise is reduced, the interior noise is normally reduced as well, although the relationship may not be quite linear. With modern well designed automobiles, the exterior noise is dominated by tyre noise at almost all speeds, even under accelerating conditions, except for acceleration in the first and second gears. With trucks, however, at speeds up to about 70 km/h, engine and exhaust noise is dominant under accelerating conditions. But above this speed, tyre noise becomes equally as important as engine, exhaust, and other power unit related noise sources, particularly as far as the exterior noise is concerned. This fact has been observed experimentally, and it has been found that similar trends exist for the exterior noise of a wide variety of manufactured vehicles and that the exterior noise of automobiles is significantly less than that of heavy trucks by almost 15 dB at low speed and by almost 10 dB at medium to high speed. One can conclude from the previous discussion that much of the exterior noise emitted from modern cars and trucks, particularly at medium to high speeds, is caused by tyre/road interaction noise. Of course, the frequency content of noise as well as its magnitude is very important. It has been observed that for automobiles travelling at medium to high speeds, the A-weighted noise peaks at about 1000 Hz, while for light and heavy trucks, the annoying noise peaks at a frequency closer to 1200 to 1600 Hz. In addition, it has been shown that the A-weighted exterior sound levels emitted by passenger cars and light and heavy trucks has not changed very much over time. This is probably because the power of automobiles has increased in many countries, particularly in the USA, and it is has been
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paper_title: Hybrid passive/active absorbers for flow ducts

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A new type of acoustic liner developed for broadband noise reduction in flow ducts is considered in this paper. It combines passive absorbent properties of a porous layer and active control at its rear face. The complete design procedure of this hybrid passive/active liner is developed here. The passive part is first considered with the determination of a suitable porous material and the cut-off frequency separating the active low frequency regime from the passive high frequency one. The control system is then presented: a digital adaptive feedback control is performed independently cell by cell, allowing an easy subsequent increase of the liner surface. The entire optimization process has been successfully applied to a laboratory flow duct: both predictions and measurements show the interest of the hybrid liner to reduce the noise radiation.
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paper_title: SURFACE IMPEDANCE CONTROL FOR SOUND ABSORPTION: DIRECT AND HYBRID PASSIVE/ACTIVE STRATEGIES

paper_content:

Some active impedance control experiments in an anechoic chamber are reported. Further active methods are developed for the design of locally controlled absorption liners in air, whose basic principles were described by Olson and May and subsequently investigated practically by Guicking and his colleagues at the beginning of the 80's. Two methods are described and tested. In the first, processing of the acoustic pressure and velocity information from close to the membrane of a control loudspeaker is used to produce a desired impedance. In the second active and passive means are combined: the impedance of the rear face of a porous layer is actively controlled so as to make the front face normal impedance take on a prescribed value. The impedance matching performance of both systems subject to an incident acoustic field including a single secondary loudspeaker is studied for both normal and oblique incidence. The combination of active and passive methods is a pragmatic approach, the aim of which is to simplify the control system for impedance control over extensive areas of wall. Indeed, the association of active control with a porous material allows the active system to be reduced in complexity to a simple active pressure release. Even though somewhat sub-optimal for sound absorption, the hybrid passive/active systems support feedback methods and lead to highly absorptive coatings.
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paper_title: Three dimensional finite element modeling of smart foam.

paper_content:

The "smart foam" concept and design originate from the combination of the passive dissipation capability of foam in the medium and high frequency ranges and the active absorption ability of piezoelectric actuator (generally polyvinylidene fluoride) in the low frequency range. This results into a passive/active absorption control device that can efficiently operate over a broad range of frequencies. In this paper, a full three dimensional finite element model of smart foam is presented including its experimental validation. The modeling tool uses quadratic poroelastic elements, as well as elastic, fluid, and piezoelectric elements. The weak integral formulation of the different media involved is presented with the associated coupling conditions. A simplified orthotropic model of poroelastic media is presented. To validate the developed model, a prototype of a smart foam has been realized and its passive absorption and radiation measured in an impedance tube and compared to predictions. The experimental validation demonstrates the validity of the model. This modeling tool constitutes a general platform to simulate and optimize various configurations of smart foams.
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paper_title: Effects of Total Surface Area and Fabric Density on the Acoustical Behavior of Needlepunched Nonwoven Fabrics

paper_content:

Nonwoven fabrics are ideal materials for use as acoustical insulation products because they have high total surface. The surface area of the fabric is directly related to the denier and cross-sectional shape of the fibers in the fabric. Smaller deniers yield more fibers per unit weight of the material, higher total fiber surface area and greater possibilities for a sound wave to interact with the fibers in the structure. Another important parameter is the packing density of the fibers in the nonwoven material. More fibers per unit volume at the same fabric thickness yield greater possibilities for sound waves to interact with the fibers. Fabric density also affects the geometry and the volume of the voids in the fabric structure. Acoustical properties of fabric materials are measured by one of two methods: the impedance tube method (ASTM C 384-98) and the acoustical chamber method. The impedance tube method uses very small test samples. Large reverberation rooms and large test samples are used for the aco...
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paper_title: Effects of compression on the sound absorption of fibrous materials

paper_content:

Abstract During the compression of a fibrous mat, it is well known that the absorption properties are decreasing. In order to predict this change, some heuristic formulae are proposed which take into account the modifications of the physical parameters (porosity, resistivity, tortuosity and shape factors) which enter the standard “equivalent fluid” model. Numerical predictions are then discussed and compared to experimental data obtained on a fibrous material (uncompressed and then compressed) used in the automotive industry.
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paper_title: Factors Influencing Acoustic Performance of Sound Absorptive Materials

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Today much importance is given to the acoustical environment. Noise control and its principles play an important role in creating an acoustically pleasing environment. This can be achieved when the intensity of sound is brought down to a level that is not harmful to human ears. Achieving a pleasing environment can be obtained by using various techniques that employ different materials. One such technique is by absorbing the sound. Fibrous, porous and other kinds of materials have been widely accepted as sound absorptive materials. This paper review and describes how the physical prosperities of materials like fiber type, fiber size, material thickness, density, airflow resistance and porosity can change the absorption behavior. The effect of surface impedance, placement / position of sound absorptive and compression, on sound absorption behavior of materials was also considered. The sound absorption of different fibrous materials was experimentally tested. The results shows the relationship between the sound absorption and airflow resistance, material thickness, air gap and attachment film. Higher airflow resistance always gives better sound absorption values but for airflow resistance higher than 1000 the sound absorption have less values because difficulty movements of sound wave through the materials. The creation of air gap, 5mm, 10 mm behind the absorptive material increases sound absorption coefficient values in mid and higher frequencies. There is not much difference seen between 5 mm air gap sample and 10 mm air gap sample. Moreover, maxima peak for different air gap is different (higher the air gap distance, maxima peak shift towards lower frequency
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paper_title: Effects of Fiber Denier, Fiber Cross-Sectional Shape and Fabric Density on Acoustical Behavior of Vertically Lapped Nonwoven Fabrics

paper_content:

Acoustical insulation and absorption properties of nonwoven fabrics depend on fiber geometry and fiber arrangement within the fabric structure. The different structures of the fibers result in different total surface areas of nonwoven fabrics. Nonwoven fabrics such as vertically lapped fabrics are ideal materials for use as acoustical insulation products, because they have high total surface. Vertically lapped nonwoven technology consists of carding, perpendicular layering of the carded webs, and through-air bonding using synthetic binder fibers. The surface area of the fabric is directly related to the denier and cross-sectional shape of the fibers in the fabric. Smaller deniers yield more fibers per unit weight of the material, higher total fiber surface area, and greater possibilities for a sound wave to interact with the fibers in the fabric structure. The research in the literature uses two methods for measuring acoustical properties of fabric materials: the impedance tube and reverberation room method. Small test samples are in the impedance tube method and sound absorption coefficient is determined at each frequency. Large reverberation rooms and large test samples are used for the reverberation room method. A direct comparative acoustical properties measurement device that was designed and fabricated at Clemson University School of Materials Science & Engineering was used to measure acoustical insulation in this research. This paper provides a description of the measurement devices and acoustical measurement data for vertically lapped nonwoven fabrics made from three different polyester fiber shape and two denier levels.
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paper_title: Performance improvement of sound-absorbing materials using natural bamboo fibers and their application

paper_content:

An acoustic material has been newly developed from the viewpoint of environmental protection. For this purpose, we used a natural resource, bamboo fiber, to manufacture sound-absorbing material. Until now, the sound-absorbing material has been developed using the crushed bamboo fibers. This material, however, is too heavy. To solve this problem, we have tried to develop a material using the explosion method. The normal incidence sound absorption coefficient of the bamboo fiber material was measured to be confirmed the basic properties. In addition, the characteristic impedance and the propagation constant of the material were measured. The contour lines of the sound absorption coefficient were calculated to find the relation of thickness and air space of the material with respect to density. In addition, we considered the influence of random incidence, and evaluated the material by adapting it to actual loudspeakers. The results show that the weight problem was solved, that the bamboo wool material works effectively in an actual product. Consequently, fundamental design criteria have been confirmed using the newly developed bamboo fiber material for sound absorption.
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paper_title: The master handbook of acoustics

paper_content:

1. Fundamentals of Sound 2. Sound Levels and the Decibel 3. The Ear and the Perception of Sound 4. Sound Waves in the Free Field 5. Speech, Music, and Noise 6. Analog and Digital Signal Processing 7. Reverberation 8. Control of Interfering Noise 9. Absorption of Sound 10. Reflection of Sound 11. Diffraction of Sound 12. Refraction of Sound 13. Diffusion of Sound 14. The Shroeder Diffusor 15. Modal Resonances in Enclosed Spaces 16. Reflections in Enclosed Spaces 17. Comb-filter Effects 18. Quiet Air for the Studio 19. Acoustics of the Listening Room 20. Acoustics of the Small Recording Studio 21. Acoustics of the Control Room 22. Acoustics for Multi-Track Recording 23. Audio/Video Tech Room for Voice-over Recording 24. Adjustable Acoustics 25. Acoustical Measurements and Calculations Appendices
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paper_title: Free field surface impedance measurements of sound-absorbing materials with surface coatings

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Abstract A method has been previously worked out by one of the authors, to measure the surface impedance of glasswools and plastic foams in normal and oblique incidence, in free field, with two microphones. Using an improved version of this method, surface impedance measurements of two sound-absorbing materials with surface coating are presented and compared with measurements performed in a reverberant room. It appears that measurements with our method are possible, in normal and oblique incidence, for these materials.
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Title: A Review on the Acoustical Properties of Natural and Synthetic Noise Absorbents

Section 1: Introduction
Description 1: This section provides a background on acoustics, the necessity for noise isolation, and the evolution of noise absorbent materials over the past 50 years due to technological advances and environmental concerns.

Section 2: Methods
Description 2: This section details the databases and keywords used for the literature search and the number of reviewed papers.

Section 3: Results
Description 3: This section describes the recent research methods for producing and optimizing sound absorbers using natural and recycled materials.

Section 4: Porous Absorbents
Description 4: This section explains the types of porous noise absorbers, including cellular, fibrous, and granular absorbents, and their acoustic properties.

Section 5: Fibrous Porous Material
Description 5: This section delves into natural and artificial fibrous materials, discussing their sources, production processes, and applications in noise absorption.

Section 6: Metal, Ceramic, and Jelly Foam Absorbents
Description 6: This section explores various foam materials, including metal, ceramic, and jelly foams, their properties, and applications in sound absorption.

Section 7: Porous Asphalt
Description 7: This section examines the use of porous asphalt in reducing noise from road traffic, detailing its composition and noise absorption characteristics.

Section 8: Smart Sound Absorbers
Description 8: This section discusses the development of hybrid sound absorbers that combine active and passive noise control technologies for broader frequency range effectiveness.

Section 9: Discussion
Description 9: This section analyzes factors influencing the choice of noise absorbent materials, including fiber size, air flow resistance, porosity, and other related characteristics.

Section 10: Conclusions
Description 10: This section summarizes the development and future prospects of noise absorbent materials, emphasizing the importance of environmental considerations and effective noise absorption factors.