## 4 Results and Discussion  
### Measurement Results  
To perform the measurements of the sound absorption coefficient, the pieces of giant need were assembled for each of the three types (mixed, only wooden part, and only bark), obtaining six samples with thicknesses of 4 and 8 cm, respectively. The sound absorption coefficient was measured by housing the pieces of giant reed inside the Kundt tube, after which the sample was fixed to the device using an acoustically transparent grid with large meshes. Sound absorption measurement results are shown in Figure 6. Different assembled giant reed samples were measured as indicated in the Materials and Methods section. Figure 6: Sound absorption coefficient values of 4 cm and 8 cm thick samples and for mixed, only wooden part, and only bark. Analyzing the results of the measurement of the sound absorption coefficient shown in Figure 6, it is possible to note that the 4 cm thick samples have, under the frequency of 800 Hz, a low sound absorption coefficient value; this result is confirmed in all three types of material. On the contrary, the 8 cm thick samples show significantly higher values of the sound absorption coefficient in the same frequency range (200-800 Hz); also, in this case, this result is confirmed in all three types of material. In the frequency range 800-1500 Hz this trend reverses, in fact it is possible to notice that the sound absorption coefficient is lowered for the samples 8 cm thick and rises for the samples 4 cm thick which have higher values. Finally, at high frequencies of 1500-2000 Hz, there is a further turnaround. This time it is the values of the sound absorption coefficient of the 8 cm thick samples that have higher values. Now let us analyze the results obtained individually for the three types of materials. Samples consisting of fragments with only one part in wood generally have values lower than 0.7 of the sound absorption coefficients. Only the sample with a thickness of 8 cm shows values higher than 0.7, and this happens starting from the frequency of 1800 Hz onwards, with a maximum value around the frequency of 1900 Hz. The samples with a thickness of 4 cm have an increasing trend at low frequencies with a maximum at the frequency of about 1200 Hz and then showing a decreasing trend at high frequencies. Samples with a thickness of 8 cm have two maximum values. High values of the sound absorption coefficient at low frequencies have a maximum near the frequency of about 650 Hz and then present a decreasing trend at medium frequencies. Starting from about 1300 Hz, this trend reverses and the sound absorption coefficient returns to increasing, with a maximum at 1950 Hz. The samples made up of a mixture of fragments with part wood and part bark show a similar trend to the previous one. In this case, the samples with a thickness of 8 cm have two maximum values. High values of the sound absorption coefficient at low frequencies have a maximum near the frequency of about 750 Hz and then present a decreasing trend at medium frequencies. Starting from about 1500 Hz, this trend reverses and the sound absorption coefficient returns to grow with a maximum at 2000 Hz. The samples with a thickness of 4 cm show an increasing trend, with a maximum at the frequency of 1500 Hz, approximately, to then show a decreasing trend at high frequencies. Near the extreme frequency, the sound absorption coefficient starts to grow again. Finally, the samples composed only of bark have values comparable with the previous one, even if the sample with a thickness of 8 cm shows values higher than 0.7 starting from the frequency slightly higher than 1500 Hz onwards, with a maximum value around at the frequency of 1700 Hz. After performing the measurements on the sound absorption coefficient, we performed the resistivity and porosity measurements as indicated in the acoustic feature's measurements section. Table 1 shows the results of the measurements. Repeated measurements were performed on four different samples.