# Tunable Perforated Panel Sound Absorbers for Variable Acoustics Room Design
## 4 Results and Discussion  
### Parametric Study of the Different Solutions in Terms of Sound Absorption  
First of all, the tuning capabilities of the proposed perforated panel absorber solutions were explored. To this end, the effect of varying their geometrical characteristics was assessed by conducting a parametric study for several configurations measuring the sound absorption coefficient using an impedance tube. In the case of the variable open area ratio solution (TS-A), the circular perforated panels were rotated to attain five different hole distribution patterns, whereas in the case of the iris-type solution (TS-B), the diaphragm was regulated to obtain eight different aperture sizes. In both cases, the backing air cavity depth was set to 10 mm. Figure 3 shows schematic representations of the hole distribution patterns and the diaphragm openings analyzed for both tunable solutions. Open area ratio data (in %) for each of the studied configurations of both types of solution are also provided in Table 3. Results of the experimentally measured sound absorption coefficient of both solutions for each configuration are shown in Figure 4.  
Figure 3: Schematic representation of the possible configurations for the tunable solutions assumed in the parametric study: (**a**) TS-A (hole distribution patterns A#1 to A#5) and (**b**) TS-B (diaphrag openings B#1 to B#8).  
Figure 2: Geometry of the case study room, including the identification of all the lining materials and the location of the source and receivers (created using SketchUp 3D design software). In this figure, a case with one-third seat occupation is illustrated.  
The sound absorption curves provided by solution TS-A (Figure 4a) indicate that a reduction in the number of perforations aligned through both panels from fifty holes (A#5, with open area ratio of 5.6%) to five holes (A#1, with open area ratio of 0.6%) shifts the resonance peak to lower frequencies from around 1500 Hz to 650 Hz, with the three intermediate sound absorption curves achieving higher peak values. This trend is also observed for solution TS-B (Figure 4b) as the size of the aperture gets reduced from an open area ratio of 14.1% (B#8) down to 0.7% (B#1). As mentioned above, reducing either the number of perforations in TS-A or the aperture size in TS-B implies a decrease in the open area ratio and hence a lower resonance frequency. Moreover, reducing the aperture size in TS-B further increases the dissipation phenomena in most analyzed cases and, as a result, enhances the amplitude of peaks in the sound absorption curve. As was mentioned in Section 2.3, the use of 3D printing methodology in the fabrication of the TS-B may lead to additional visco-thermal losses due to the microporosity resulting from this fabrication process. However, there are other factors which may also influence the attenuation of the panel, as is the case of the surface roughness. In this regard, some authors have investigated these effects using computational models [27] or analyzed the case of a coiled-up resonator system using different additive manufacturing techniques [28]. All the same, the previous results show the good absorption performance of the proposed solutions and highlight their tuning capabilities, which are their main advantages when compared to traditional variable air cavity systems, as they avoid the misuse or loss of room space.