Innovative Composite Materials for Sound Absorption and Insulation: Where We Are and Where We Are Going.

Materials with sound-absorbing or sound-insulating properties have been rapidly evolving in recent years due to several reasons [...].

Performance Characterization of Broad Band Sustainable Sound Absorbers Made of Almond Skins.

In order to limit the environmental impact caused by the use of non-renewable resources, a growing research interest is currently being shown in the reuse of agricultural by-products as new raw materials for green building panels. Moreover, the European directives impose the goal of sustainability supporting the investigation of passive solutions for the reduction of energy consumption. Thus, the promotion of innovative building materials for the enhancement of acoustic and thermal insulation of the buildings is an important issue. The aim of the present research was to evaluate the physical, acoustical, and thermal performances of building panels produced by almond skin residues, derived from the industrial processing of almonds. In this paper different mix designs were investigated using polyvinyl acetate glue and gum Arabic solution as binders. Air-flow resistivity σ and normal incidence sound absorption coefficient α were measured by means of a standing wave tube. Thermal conductivity λ, thermal diffusivity α, volumetric heat capacity ρc were measured using a transient plane source device. Finally, water vapor permeability δp was experimentally determined using the dry cup method. Furthermore, a physical characterization of the specimens in terms of bulk density ρb and porosity η allowed to study the correlation existing between the binder and the aggregates and the consequent acoustical and hygrothermal behavior occurring on the different mix designs. The achieved results suggested the investigated materials comparable to the main products currently existing on the market.

Composite Eco-Friendly Sound Absorbing Materials Made of Recycled Textile Waste and Biopolymers.

In recent years, the interest in reusing recycled fibers as building materials has been growing as a consequence of their ability to reduce the production of waste and the use of virgin resources, taking advantage of the potential that fibrous materials may offer to improve thermal and acoustic comfort. Composite panels, made of 100% wool waste fibers and bound by means of either a chitosan solution and a gum Arabic solution, were tested and characterized in terms of acoustic and non-acoustic properties. Samples with a 5 cm thickness and different density values were made to investigate the influence of flow resistivity on the final performance. Experimental results demonstrated that the samples had thermal conductivity ranging between 0.049 and 0.060 W/(m K), well comparable to conventional building materials. Similarly, acoustic results were very promising, showing absorption coefficients that, for the given thickness, were generally higher than 0.5 from 500 Hz on, and higher than 0.9 from 1 kHz on. Finally, the effects of the non-acoustic properties and of the air gap behind the samples on the acoustic behavior were also analyzed, proving that the agreement with absorption values predicted by empirical models was also very good.

On-site acoustical characterization of Baroque tapestries: The Barberini collection at St. John the Divine Cathedral.

Tapestries represent an important and valuable cultural heritage. In the past they played a significant role in enriching indoor environments, as well as contributing to change the indoor characteristics of the space. Considering typical mounting conditions, acoustics was likely to be strongly affected by hung tapestries. Even though their acoustic properties could be ideally inferred from their physical features, a proper characterization would be preferable to understand how they could modify sound propagation. However, given their dimensions and their delicacy, on-site measurements are often the only option. In the present paper, absorption coefficients of the Barberini tapestries were measured during a temporary exhibition held in the Cathedral of St. John the Divine in New York City. Measurements were carried out in a large chapel with and without the tapestries installed. The effect of acoustical coupling with the adjacent cathedral volume was investigated using Bayesian methods to exclude unwanted influence on the measurements. Results were obtained using either the classical reverberation time formula or the indirect approach based on the use of geometrical acoustic modeling. Finally, results proved to be in good agreement with predictions based on theoretical formulas, once the relevant non-acoustical parameters were estimated.

Acoustics of Italian Historical Opera Houses.

Opera houses represent a large group of performance spaces characterized by great complexity and, at the same time, versatility with respect to different usage (from opera to symphonic music and ballet). This kind of building originated in Italy during the 17th century and later spread across the country and then Europe and the rest of the world, slowly evolving into modern theatre shapes. As a consequence of the changes undergone by the interior space, the original acoustic features, which likely influenced many composers, experienced important variations. Thanks to acoustic measurement campaigns inside Italian Historical Opera Houses, promoted by National and Regional Projects, the distinctive features of these spaces were investigated in comparison to modern spaces. In this work, the newly acquired data are merged with data in the literature in order to present and discuss some of the distinctive acoustic features of historical spaces as regards their original function. Moreover, specific issues such as listening in stalls and boxes and the criteria governing the preference judgment of listeners are considered. The concept and the crucial role of the balance between stage and pit sources are also discussed by means of previous literature studies.

[Working conditions for supermarket employees: from experimental data to best practices]. / Comfort lavorativo negli ipermercati: dalla sperimentazione alle buone pratiche.

BACKGROUND: Thermal, acoustic and visual comfort conditions for hypermarket workers have never been investigated with scientific methods. OBJECTIVES: taking advantage of a case study, with characteristics capable of generalizing the results, analytically measure the actual comfort conditions to which workers are exposed and point out possible ameliorative proposals. METHODS: Carry out a detailed survey based on instrumental measurements combined with subjective questionnaires to assess the indoor environment. RESULTS: Even though the analysis pointed out no significant risk conditions, several smaller problems appeared in terms of local discomfort (such as cold limbs, higher sound level exposure, limited glare phenomena) for cashier workers. The origin of these problems appeared to be the pivotal position of the cash registers. CONCLUSIONS: Taking into account observed phenomena and their causes a list of "best practices" has been defined hoping that their adoption could further limit any impact on workers comfort conditions.

Optimizing stepwise rotation of dodecahedron sound source to improve the accuracy of room acoustic measures.

Dodecahedron sound sources are widely used for acoustical measurement purposes as they produce a good approximation of omnidirectional radiation. Evidence shows that such an assumption is acceptable only in the low-frequency range (namely below 1 kHz), while at higher frequencies sound radiation is far from being uniform. In order to improve the accuracy of acoustical measurements obtained from dodecahedron sources, international standard ISO 3382 suggests an averaging of results after a source rotation. This paper investigates the effects of such rotations, both in terms of variations in acoustical parameters and spatial distribution of sound reflections. Taking advantage of a spherical microphone array, the different reflection patterns were mapped as a function of source rotation, showing that some reflections may be considerably attenuated for different aiming directions. This paper investigates the concept of averaging results while changing rotation angles and the minimum number of rotations required to improve the accuracy of the average value. Results show that averages of three measurements carried out at 30° angular steps are closer to actual values and show much less fluctuation. In addition, an averaging of the directional intensity components of the selected responses stabilizes the spatial distribution of the reflections.

On the sound absorption by openings in rooms (L).

Sound absorption by openings has been rarely considered in room acoustics. In fact, information about small openings (such as ventilation grids) may sometimes be found, but nothing is said about larger openings, possibly as a consequence of the less likely occurrence in a design. In order to fill this gap, measurements were carried out in scale models, measuring the equivalent absorption due to different openings and comparing it with theoretical results. A "practical" model, showing a simple dependence on the opening dimension, was finally obtained and subsequently validated by measurements in a real room.

A comparative analysis of acoustic energy models for churches.

Different models to improve prediction of energy-based acoustic parameters in churches have been proposed by different researchers [E. Cirillo and F. Martellotta, J. Acoust. Soc. Am. 118, 232-248 (2005); T. Zamarreño et al., J. Acoust. Soc. Am. 121, 234-250 (2006)]. They all suggested variations to the "revised" theory proposed by Barron and Lee [J. Acoust. Soc. Am. 84, 618-628 (1988)], starting from experimental observations. The present paper compares these models and attempts to generalize their use taking advantage of the measurements carried out in 24 Italian churches differing in style, typology, and location. The whole sample of churches was divided into two groups. The first was used to fine-tune existing models, with particular reference to the "mu model," which was originally tested only on Mudejar-Gothic churches. Correlations between model parameters and major typological and architectural factors were found, leading to a classification that greatly simplifies parameter choice. Finally, the reliability of each model was verified on the rest of the sample, showing that acoustic parameters can be predicted with reasonable accuracy provided that one of the specifically modified theories is used. The results show that the model requiring more input parameters performs slightly better than the other which, conversely, is simpler to apply.

Identifying acoustical coupling by measurements and prediction-models for St. Peter's Basilica in Rome.

St. Peter's Basilica is one of the largest buildings in the world, having a huge volume resulting from the addition of different parts. Consequently, sound propagation cannot be interpreted using a conventional approach and requires experimental measures to be compared with statistical-acoustics and geometrical predictions in order to explain the interplay between shape, materials, and sound waves better. In previous research one of the most evident effects, the surprisingly low reverberation time, was believed to result from acoustical coupling phenomena. Taking advantage of more refined measuring techniques available today an acoustic survey was carried out and the results were analyzed using different methods, including Bayesian parameter estimation of multiple slope decays and directional energy plots, which showed that coupling effects actually take place, even though measured reverberation times were longer than those given in previous studies. In addition, experimental results were compared with geometrical- and statistical-acoustic models of the basilica, which showed that careful selection of input data and, in statistical models, the inclusion of phenomena such as direct sound radiation and non-diffuse energy transfer, allow obtaining accurate results. Finally, both models demonstrated that reduced reverberation depends more on increased absorption of decorated surfaces than on coupling effects.

A multi-rate decay model to predict energy-based acoustic parameters in churches (L).

Multi-rate decays are sometimes observed in room acoustics, appearing when markedly different volumes are coupled together and resulting in nonlinear decay curves. Such behavior appears in several churches at the very beginning of the decay process, although in conditions which cannot be explicitly referred to as coupling phenomena. Consequently, multi-rate exponential decays may be suitable to model energy distribution in this group of buildings, providing a more elegant and easily applicable set of equations in place of a previously defined "linear" model, used to adapt Barron's revised theory. The paper shows that the multi-rate approach ensures ease of calculation, without significant loss in accuracy in predicting energy-based acoustic parameters.

Sound propagation and energy relations in churches.

The results of an acoustic survey carried out in a group of Italian churches differing in style, typology, and location were used in order to study how the acoustic energy varies inside this kind of space. The effect of different architectural elements on sound propagation was investigated by means of three-dimensional impulse responses measured using a B-format microphone with sweep signals. Side chapels, columns, and trussed roofs appeared to scatter the reflections, so that the purely diffuse exponential sound decay begins after a time interval which grows with the source-receiver distance and with the complexity of the church. The results of the measurements were then compared with predictions given by existing theoretical models to check their accuracy. In particular a model previously proposed by the authors for a specific type of Romanesque churches was further refined taking into account the new findings and making some simplifications. Its application to the wider sample of churches under analysis showed that strength, clarity, and center time can be predicted with reasonable accuracy.