Noise control devices such as panels and barriers, when of high efficiency, generally are of difficult acquisition due to high costs turning in many cases their use impracticable, mainly for limited budget small-sized companies. There is a huge requirement for new acoustic materials that have satisfactory performance, not only under acoustic aspect but also other relevant ones and are of low cost. Vegetable fibers are an alternative solution when used as panels since they promise satisfactory acoustic absorption, according to previous researches, exist in abundance, and derive from renewable sources. This paper, therefore, reports on the development of panels made from vegetable fibers (coconut, palm, sisal, and açaí), assesses their applicability by various experimental (flammability, odor, fungal growth, and ageing) tests, and characterize them acoustically in terms of their sound absorption coefficients on a scale model reverberant chamber. Acoustic results point out that the aforementioned fiber panels play pretty well the role of a noise control device since they have compatible, and in some cases, higher performance when compared to commercially available conventional materials.

A description is given of sound absorption in small-furnished rooms at low frequencies. The objective of the work was to characterise room absorption and to develop a model of surface absorption appropriate for a modal description of contained sound fields at low frequencies. An empty test room was measured and numerically modelled, for use as a reference condition for later measurements with obstacles and absorbers in the room. A finite element model of the empty room demonstrated the need to account for even small irregularities in the room geometry. It is also confirmed that the vibration behaviour of the room walls contributes to the whole room absorption mechanism at very low frequencies. A parametric survey, involving model updating, yielded a frequency invariant value of absorption coefficient of 0.02, which produced the best agreement between prediction and measurement. The resultant predicted frequency response functions agreed with measurement within ± 5 dB. It is shown that it is possible to obtain agreement between measurement and prediction, after adjustments of wall acoustic impedance and loudspeaker frequency response.

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A description is given of sound absorption in small-furnished rooms at low frequencies. The objective of the work was to characterise room absorption due to contents such as furniture and to develop a model of absorption appropriate for a modal description of contained sound fields at low frequencies. In this paper, a preliminary study of the influence of furniture on low frequency room response was performed by introducing a "reference obstacle" at three positions within an enclosure which previously had been numerically modelled. Eigenfrequency shifts and selective modal damping were observed throughout the frequency range of interest, 20-200 Hz, and the numerical model was able to take these effects into account, showing overall good agreement with measurements. It is shown that when a large solid item is introduced into a room there is little effect on the room frequency response for frequencies below 50 Hz. Between 50 Hz and 200 Hz the effect is more pronounced and the influence of location becomes apparent. Obstacles placed along a wall or in room corners will produce a greater change in room response, when compared with a central location.