## 3 Materials and Methods
### Acoustic Feature Measurements  
As a first activity, measurements were made of the following acoustic properties of the material under analysis: sound absorption coefficient, resistance to air flow, and porosity. As we said, the shredded material was grouped into three types: mixed, only wooden part, and only bark. For each type, the bulk material was inserted into the sample holder with a thickness of 40 and 80 mm, respectively. Finally, six measurement sessions were performed. For the measurement of the absorption coefficient at normal incidence, the method of the standing wave tube method (Kundt tube) was adopted. The method is described and standardized by ISO-10534-2: 1998 [31; 32] and consists of generating a plane wave and measuring the sound pressure at the points where they are placed the microphones. The standard describes in detail the procedure to follow to carry out the measurement of the acoustic properties using samples of small dimensions to be assembled. The measurements were performed using a Kundt tube with a length of 56 cm and an internal diameter of 10 cm. The diameter allows measurements up to 2000 Hz while the length of the Kundt tube and the position of the two \(\frac{1}{4}\) inch microphones placed at 5 cm allows measurements starting from 200 Hz. (Figure 4). Figure 3: Detail of the shredding operations (to the left), and the average size of the crushed material (to the right). The direct wave and the reflected wave, which travel in the opposite direction inside the tube, add up vectorially by being in phase in some points and in phase opposition in others, thus generating a stationary wave. The presence of the sample inside the tube will determine the absorption of part of the incident energy. The impedance of the sample alters the way the sound is reflected and, by measuring the resulting standing wave, it will be possible to calculate the absorption coefficient at normal incidence. Subsequently, the resistance to air flow was measured. When air passes through a material it encounters resistance; this parameter is precisely called resistance to air flow. For this measure, the procedure contained in the international standard ISO 9053: 1991 [33] was adopted. The standard offers two different methodologies for measuring this parameter. One method uses a one-way direct air flow through the material. Appropriate sensors measuring the pressure difference between the two sides of the sample return the resistance to the air flow. The other method involves an alternating air flow, with a frequency of 2 Hz. The pressure difference on the two sides of the sample positioned inside a measuring chamber is measured. The alternating air flow method was used in this study. Figure 5 shows the instrument used to measure the resistivity of the air flow. The device consists of a closed cylindrical tube with a housing placed at the end capable of containing the sample of the material. The alternating air flow inside the tube is created by a piston system moved by a rotating cam. The difference in pressure induced is measured by a microphone placed inside the tube. Finally, porosity measurements were performed for the different sample configurations. The ratio between the volume of the fluid contained in the pores and the total volume occupied by the sample represents a definition of porosity. From this measure it is therefore possible to refer to the fraction of the volume of air inside the material. Porosity can be measured through different approaches, depending on the material used for the saturation of the pores which can be air or water, and for the procedure adopted for calculating the volume of air contained in the sample. Figure 4: [PERSON]’s tube for the absorbent acoustic coefficient measurement at normal incidence (to the left), and giant reeds loose material in the sample holder (to the right). Figure 5: Air flow resistance measurements equipment with the alternating air flow method.