###### Abstract  
The interest in the use of resistant acoustic materials has put further attention on the use of porous concrete in the building industry. This work investigates the acoustic properties of four different mix designs of porous concrete obtained with two types of aggregates, that is, normal weight and lightweight aggregates. The assessment of the sound-absorbing performances has been conducted in the small-scale reverberation room (SSRR) at Politecnico di Torino (Italy), in agreement with the procedure indicated in the ISO 354 Standard. For each concrete type, three panel thicknesses, i.e., 20 mm, 40 mm, and 60 mm, were tested. Moreover, different mounting conditions were investigated, considering the combination of single panels in multiple layers, adding an air gap between the panel and the backing, and inserting a layer of rock wool in the air gap itself. The results show weighted absorption coefficients (\(\alpha_{w}\)) in the range of 0.30 to 0.75 depending on the thickness and mounting conditions. These encouraging values make these materials useful for efficient practical applications in indoor and outdoor environments. acoustics; acoustic measurements; sound absorption coefficient; cement-based materials; building materials; pervious concrete; acoustic concrete +
Footnote †: journal:  
Porous absorbers are the most widespread type of sound absorbers. According to their microstructure, porous absorbers can be further classified into granular, cellular, and fibrous [6,7]. The most fundamental properties of porous materials influencing their sound absorbing properties are flow resistivity and porosity and second, pore shape factor and tortuosity [7]. Conventional concrete is generally characterized by poor sound-absorbing properties, as the prevailing phenomena occurring are sound reflections. In general, normal concrete has an absorption coefficient value of 0.05-0.10 [8]. However, porous concrete has the capability to work as a porous sound-absorbing material as it is characterized by high porosity, i.e., open pore structure on its surface and an interconnected network of pores. It is also known as pervious, gap-graded, permeable, or enhanced porosity concrete [9] and is currently widely used in urban environments as paving material to support environmentally sustainable rainwater management [10]. It has been also exploited for its acoustic absorbing properties in traffic noise barriers and railway noise reduction [11, 12, 13]. Pervious concrete mainly consists of normal Portland cement, coarse aggregates (aggregate dimension greater than 5 mm), and water, which generate a void content that generally ranges from 15% to 35% [10, 14, 15, 16]. Pervious concrete acoustic panels belong to the class of granular sound absorbers with pores created by the presence solid aggregates which are bonded together by a cementitious binder. The key factor to allow sound absorption to occur is the accurate definition of the quantity of binder to ensure that there is enough binder to keep the aggregate together without clogging the pore network and still allow for an appropriate resistance for its use and handling. Overall, the sound-absorbing properties of granular materials tend to be uneven in frequency and to be characterized by peaks [17]. Different strategies have been proposed with the intent to improve the sound absorbing properties, i.e., with the aim to enhance the rate and the evenness of the sound absorption provided by altering the microstructural properties of the pervious concrete. Note that these strategies applied to the mix design aim to vary the fundamental properties, i.e., flow resistivity, porosity, pore shape factor, and tortuosity, which control the absorbing performance. Indeed, the sound absorption properties of porous concrete are strictly related to the void ratio of the concrete. Therefore, it is important to adequately control the void ratio and aggregate type, which influence tortuosity and flow resistivity [18], pore size, and pore aperture size, which are used to control porosity [8].