## 2 Materials and Methodologies
### Acoustic Measurements  
The surveys were carried out with the following equipment:  
-  Dome tweeter sound source (RCF TWT 50);  
-  Condenser microphone (Audiomatica 1/4-inch) [29];  
-  Audio interface (Clio CP-01);  
-  Computer connected to the compact audio interface. The selection of the sound source was a flat response (under free field conditions) in the frequency range where the analysis was focused. The excitation sound signal was an Exponential Sine Sweep (ESS) having a time-invariant sound pressure level. A compact Clio CP-01 audio interface was used to feed the loudspeaker with the sound signal coming from the computer and to record the impulse response (IR) [30]. The sound source was placed at 45 mm from the finish floor, while the microphone was 80 mm high. Figure 4: Schematic layout of the equipment location during the third set of measurements. Figure 3: Sectional scheme of the equipment location during the measurements without (**A**) and with (**B**) the metamaterial acoustic barrier. In accordance with [PERSON]'s law and with the dimension of scatterers, the theoretical acoustic attenuation (or gap band frequency) occurred at 3.1 kHz. The data acquisition of the recorded signal was based on a 65,000-point FFT and on a frequency sample rate of 48 kHz. The frequency range being considered was found to be between 1 kHz and 12.5 kHz, equivalent to 100 Hz and 1.25 kHz on a real scale. Figure 5 shows the configurations of all the measurement settings. The goal of testing different configurations of the metamaterial acoustic barrier is to obtain a robust data output and to assess the sound attenuation by establishing the number of rows attributed to the cylindrical scatterers. A summary of all the configurations is given as follows:  
**Configuration 1**: the metamaterial acoustic barrier is composed of three rows of type A diameter bars, with a staggered bar diameter in order to interrupt the direct line of sight between the source and the receiver. All the other configurations can be considered as variations of this one. **Configuration 2**: this setting consists of adding a row of type B diameter scatterers on one side of the barrier. **Configuration 3**: the acoustic barrier is composed of three rows of type A diameter bars and two rows of type B diameter sticks installed on the same side. Figure 5: Out-of-scale schemes of all the configurations assumed during the acoustic measurements: 3-row type A structure (**1**); 3-row type A and 1-row type B structure (**2**); 3-row type A and 2-row type B structure (**3**); 4-row type A structure (**4**); 4-row type A and 1-row type B structure (**5**); 4-row type A and 2-row type B structure (**6**); 5-row type A structure (**7**); 5-row type A and 1-row type B structure (**8**); 5-row type A and 2-row type B structure (**9**). * **Configuration 4**: the acoustic barrier is composed of four rows of type A diameter bars only. * **Configuration 5**: the acoustic barrier is composed of four rows of type A diameter sticks and one row of type B diameter bars. * **Configuration 6**: the acoustic barrier is composed of four rows of type A diameter sticks and two rows of type B diameter bars. * **Configuration 7**: the acoustic barrier is composed of five rows of type A diameter bars only. * **Configuration 8**: the acoustic barrier is composed of five rows of type A diameter sticks and one row of type B diameter bars. * **Configuration 9**: the acoustic barrier is composed of five rows of type A diameter sticks and two rows of type B diameter bars. Figure 5 schematically shows all the described configurations, indicating in green the cylindrical elements of type A diameter, and in orange the cylindrical elements of type B diameter. Indications of sound source (blue square) and receiver (red circle) positions are also given. The pictures included in Figure 6 shall be intended as schemes. Table 1 summarizes the parameters that characterize each configuration. Figure 6: Insertional Loss (IL) results related to the first set of measurements.