###### Abstract
A higher void ratio leads to higher and wider peak values in acoustic absorption coefficients, resulting in a shift of the peak of the coefficient towards the higher frequencies [12, 19, 20]. The effect on sound absorption of the aggregate size and different aggregate types by blending or combining them in multiple layers has been studied in [11, 13, 21, 22]. Aggregate size can be used to control the pore dimensions, as the median pore size increases for increasing aggregate size [19, 21]. With respect to aggregate size, it has been observed that most previous studies endorse the use of aggregate with dimensions in the range 1 to 10 mm [11, 21, 22], as smaller aggregates would clog the pores, thus reducing porosity, and bigger ones, despite increasing the pore volume, would reduce the tortuosity of the pore network. When considering the use of different aggregate types such as lightweight and normal-weight aggregates, the study in [22] suggests that lightweight aggregates can absorb cement paste from micropores on the surface of the aggregates. As a result, for an equal absolute volume ratio of aggregates, smaller size lightweight aggregates result in slightly higher sound absorbing performance, as they have a larger total surface area compared to bigger ones, and therefore the cement paste covering the aggregates can be better absorbed when smaller aggregates are used and result in higher void ratio values with respect to normal weight aggregates [22]. Other studies have investigated the use of different materials as aggregates, such as crumb rubber, nanospheres, and recycled aggregates [12, 13, 23, 24, 25]. The possibility of blending aggregates with different size in the concrete matrix has been studied in [19, 21], highlighting that, as a general rule, aggregate size should be selected in order to ensure that smaller aggregates do not enter the pores created by the bigger ones. Porous concretes with blends of aggregates of different materialshave been tested in [11], where expanded perlite aggregates were replaced by different percentages of slag, evidencing a nonlinear impact on the sound absorption performance of the panels. Concrete samples featuring aggregates of different dimensions or material have been combined into layers in [11; 13; 22], evidencing the coupling two layers of concrete with the external one featuring aggregates with lower bulk densities or bigger pores sizes compared to the back layer lead to increased sound absorbing performances. Slight differences in sound absorption have been reported regarding the shape of the aggregates; for instance, this was shown in [22], i.e., which has compared round shape (lightweight) and irregular shape (normal weight) aggregates with similar gradation. Moreover, as for all the acoustic porous materials, the thickness of the porous layer also results as important for the acoustic absorption coefficient spectra. The principal maximum peak of the absorption coefficient is displaced to lower frequencies when the thickness increases [13; 18; 21]. However, there is a threshold regarding the thickness of granular materials above which the absorption does not increase further [7]. Table 11 (Appendix A) briefly summarizes the details and the main findings of previous research investigating the effects of design factors on the sound absorption coefficient of porous concrete. The studies which analyzed different aggregate size, material and shape, and on panel thickness have been clustered evidencing if and the extent to which such variable was found to have an influence on the sound absorbing properties of the panels. Recent reviews on the strategies that have been proposed to enhance the sound absorbing performances of concrete have been presented in [17; 26]. However, these reviews highlight the fact that further research is required to provide larger datasets to refine and produce better estimation of the sound absorption of concrete materials.