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Acoustical Treatment of Room and Recent Trends in Selection of Acoustic Materials
Abstract and Figures
Every room has its unique sound effect and acoustical properties. The acoustic performance of a room depends on various factors such as reverberation time, its sound absorbing quality, the material used in different surfaces and interiors of room etc. By using different materials or by proper treatment of room as per acoustic performance point of view, the acoustic efficiency of room can be improved. In this paper two major problems are discussed with their possible solutions. First, by adopting different methods of acoustic treatment if the material can't be replaced in room. And second, by placing different materials (including absorbers) at different locations or replacing the surfaces of room by a new acoustic material. These all methods helps in improving the acoustic efficiency of a room.
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There is currently considerable interest in developing sustainable absorbers, either from biomass materials or recycled materials, and it is the former that is the subject of this paper. A number of potential candidate materials are available from the biomass in the form of organic fibres. Non-fibrous materials, such as configurations of whole straw or reed, can also act as sound absorbers. A combination of impedance tube and reverberation chamber measurements have been carried out for a number of biomass materials and the effectiveness of current models for the prediction of the absorptive properties of natural fibres has been investigated. Examination of the acoustical characteristics of a range of natural fibres has confirmed their effectiveness as porous sound absorbers and also the limitations of current models for predicting their performance. Examination of the acoustical performance of materials consisting of different configurations of whole reeds and straws has revealed that these also possess considerable potential for application as broadband sound absorbers with particularly good low frequency absorption characteristics. The combination of natural fibres and whole reeds offer the possibility of developing a range of sustainable absorbers which act very effectively across the complete audio frequency range.
The sound absorption of an industrial waste, developed during the processing of tea leaves has been investigated. Three different layers of tea-leaf-fibre waste materials with and without backing provided by a single layer of woven textile cloth were tested for their sound absorption properties. The experimental data indicate that a 1 cm thick tea-leaf-fibre waste material with backing, provides sound absorption which is almost equivalent to that provided by six layers of woven textile cloth. Twenty millimeters thick layers of rigidly backed tea-leaf-fibres and non-woven fibre materials exhibit almost equivalent sound absorption in the frequency range between 500 and 3200 Hz.
As for the sound absorbing system using an MPP (microperforated panel), a double-leaf MPP sound absorber has been studied so far. However, this structure uses two MPPs, which are still expensive, and is disadvantageous when its cost is concerned. Therefore, it is considered that it can be advantageous if one of the leaves can be replaced with a less expensive material keeping high sound absorption performance. In this study, the possibility of producing a useful sound absorbing structure with an MPP and a permeable membrane as an alternative less expensive material is examined. The acoustic properties of this MPP and permeable membrane combination absorber are analysed theoretically with a Helmholtz integral formulation. The absorption performance and mechanism are discussed through the numerical examples. Also, the effect of a honeycomb in the air cavity, which is to be used for reinforcing the structure, is also discussed through a theoretical analysis.
Waste tires cause both health and environmental problems and this has forced governments to develop laws for recycling. There are several different recycling processes for end-of-life tires in which steel is recovered and rubber is reused. However, the reuse of other parts of the waste tires is currently not possible. These other parts mainly consist of textiles that are separated from the tire during the recycling process – this material is usually known as ‘fluff’. In this study a procedure for the design of materials with superior acoustic properties using this material was developed. The main component of the acoustic material is textile waste from mechanically fragmented tires obtained through recycling activity. The design process is based on several parameters: acoustic absorption coefficient measurement by the impedance tube method, acoustic modelling, mechanical characterization of the material and, finally, finite element modelling of the end acoustic product. In this work it was also established that the mechanical and acoustic properties need to be opposite to obtain the desired characteristics in the materials under investigation. In order to produce a competitive material formed completely from textile waste from recycled tires, a solution formed by two layers is proposed. This approach was used to provide a self-supporting material with a high acoustic absorption coefficient for use in acoustic ceiling tiles.
A paper has been published by Eyring in which he gives as a modification of Sabine’s well known reverberation formula, namely T = 0.05V/αS. The following formula T = 0.05V/ S ln (1-α) where α=Σ α1/S1/S, ft-sec units being used throughout.
This formula is designed to deal with the case of a heavily damped room where α → 1, for which Sabine’s formula obviously breaks down as it makes T approximate to 0.05V/S instead of zero. Sabine’s formula is seen to be an approximation to Eyring’s formula for small values of α.
In some recent work on reverberation measurements on the absorption coefficients of highly absorbing materials anomalous results have been obtained, Eyring’s formula giving coefficients greater than unity for materials for which values of about 0.7 have been obtained by other workers. This suggested that the conditions used the formula is not accurate as the anomalies were too large to be attributed to experimental error. The theory of reverberation has therefore been developed afresh from first principles, and it appears that under certain conditions the formula takes a modified form.
Energy consumption for thermal comfort in buildings reached 20-40% of total energy consumption in the developed countries. This study evaluates the performance of a composite material with enhanced thermal inertia formulated with a solid waste to be used in buildings. The feasibility of incorporating electric arc furnace dust (EAFD) was evaluated. EAFD is a special waste used as filler in a polymer matrix. Paraffin wax is added with two functions: on one side as lubricant agent to promote a correct mixing between the inorganic filler and the polymeric matrix. Moreover, paraffin acts as phase change material (PCM) due to their high thermal energy storage (TES) capacity as latent heat from the phase change. In order to evaluate the performance as part of building systems of new material developed in this paper, several composite formulations were prepared and tensile strength test were performed, the thermal properties were analyzed by differential scanning calorimetry (DSC) and airborne noise acoustic properties were tested using an experimental cabin based on the UNE-EN-ISO140. The results were compared with a commercial material for acoustic insulation in constructive solutions. The material developed was a 3 mm dense sheet able to be used in combination with other materials as constructive systems.
Multi-layer structures have issues with sound insulation at low and mid-frequencies due to mass-air-mass resonance. The purpose of this study is to investigate improvements to the sound insulation performance of multi-layer structures using a microperforated panel (MPP), which can absorb well over a wide frequency range. Although MPPs have been investigated over the last several decades, almost all studies have been conducted in terms of sound absorption. Herein the sound transmission loss of multi-layer structures with flexible MPPs of infinite extent is theoretically investigated. The calculation is based on the wave equation and the equation of panel vibration including the effect of perforation of the panel. Additionally to consider a more realistic sound insulation performance, the effect of the directional distribution of the incident energy in a reverberation chamber is taken into account. Experiments are conducted using an acoustic tube to validate the calculated results and the reverberation chamber method to verify the actual sound insulation characteristics. Both experiments agree well with the theoretically calculated perforation effects. Consequently, MMPs are confirmed to improve the deterioration of sound insulation performance due to mass-air-mass resonance of multi-layer structures.
The predicted values for acoustic insulation of single and double panel walls, using analytical models previously developed by the authors, are compared with experimental findings. The analytical method used fully takes into account the coupling between the air and the solid panels, and there is no restriction on their thickness, as the Kirchhoff or Mindlin approaches require. The laboratory experiments involved placing test specimens between standard chambers. Results are presented for panels made of glass, concrete and steel. From the results we can conclude that the predictive analytical solutions are in good agreement with the experimental results, except when the area of the panels is very small and the frequencies are very low. At low frequencies, the experimental results appear to be significantly affected by the resonance effects associated with the creation of stationary waves within the acoustic chambers, and the vibration modes introduced into the dynamic system by the restriction on the movement of the panel along its boundary.
An analysis based on the assumption that image sources may replace
the walls of a room in calculating the rate of decay of sound intensity
after the sound source is cut off gives the more general reverberation
time equation
[dformula T = ((.05V)/(-S log[sub a](1 - a[sub a])))]
, where T is reverberation time, V the volume of the room, S the surface
of the room, and aa the average coefficient of absorption of the
wall surface. In the past Sabine's formula,
[dformula T = ((.05V)/(Sa[sub a]))]
, has been used to calculate reverberation time, but for rather large
values of aa, that is, for "dead" rooms this equation gives too long
a time of reverberation, or in some cases, if the time is measured,
it gives a value of aa greater than unity. This we found to be the
case in the Sound Stage, Sound Picture Laboratory, Bell Telephone
Laboratories, Inc. Reverberation time measurements made in this room
support the new formula, however. Sabine's formula turns out to be
the special case of the more general equation and is to be used when
aa is small, that is, for "live" rooms, but not for "dead" rooms.The
new formula is of importance in the talking picture industry for
it indicates that a "dead" room may be obtained by the use of less
absorbing material than that calculated by the old formula.