## 3 Measurements and Methodology
### Real-Scale Room Models and Instrumentation  
Two measurement campaigns were conducted in two real-scale rooms selected for testing the diffusers and described as follows:  
* A semi-anechoic room having dimensions 9 \(\times\) 10 \(\times\) 4.5 m [L \(\times\) W \(\times\) H] (Room A), one of the facilities of the SCM Group located in Rimini. This room has a hard finish floor and absorbing panels applied on walls and ceiling.  
A reverberant room having dimensions 16 \(\times\) 20 \(\times\) 7 m [L \(\times\) W \(\times\) H] (Room B), one of the laboratory facilities of the University of Parma. This room has hard material as a finished floor, plastered bricks on walls and an exposed concrete slab. Volume size is considered big enough to ensure the condition of a free field. The equipment used for the acoustic measurements is composed of the following items:  
* An equalized loudspeaker (Genelec 8351 SAM), Finland;
* 25 microphones pre-polarized for free field conditions (B&K 4188), Denmark;
* 8 converters for data acquisition (Behringer ADA-8000), Germany;
* Firewire interface M-Audio (Profire Lightbridge), USA.  
MacBook Pro 15\". Because the diffusion coefficient indicates how the quantity of the reflected energy is distributed in space, both sound source and receivers were placed in a certain way to determine the spatial distribution of the polar response of the reflected sound. In particular, 25 microphones were positioned at a constant radial distance from the sound source, recording the one-third-octave bands between 100 and 5 kHz. The anechoic conditions for investigating the panels' early reflections were obtained by setting the microphones at floor level to prevent receivers picking up waves reflected from the floor, which would distort the measurements.  
The conditions of the experimental measurements followed the rules of the far-field, by having 80% of the receivers out of the specular zone. In this research, the sound source was located only in front of the sample. The methodology for measuring the diffusion coefficient used during the two campaigns followed ISO 17497, part 2.  
The disposition of the instrumentation was different between Room A and B. In particular, inside Room A, given the limited dimension of the space, the 25 microphones were installed at a radius of 4 m from the test sample, which was located at the center of a virtual semi-circumference, and the sound source was at a radius of 8 m distant from the test sample, as indicated in Figure 2.  
Differently, in Room B the 25 microphones were installed at a radius of 5 m from the test sample, always located at the center of the semi-circumference, and the sound source was at a radius of 10 m distant from the test sample, as indicated in Figure 3.  
The excitation signal employed for the RIR was an exponential sine sweep (ESS), having a frequency range set between 40 Hz and 20 kHz. In each room and for each panel configuration, the measurements were repeated three times with a silence gap of 5 s in between.  
The size of the speaker was big enough to cover the diffuser dimensions, while all the microphones were set to have the same sensibility for any reflection going from the diffuser to the receivers, with a drift of \(\pm\) 1 dB.  
The sets of measurements were undertaken to capture the 25 RIRs with and without the test sample.  
Figure 3: Equipment disposition inside the reverberant laboratory of the University of Parma (Room B).  
Figure 2: Equipment disposition inside the semi-anechoic chamber available at SCM Group in Rimini (Room A).  
The type of diffuser utilized for this experiment was a single plane, having an acoustic impedance constant along the \(x\)-axis. On this basis, the diffusion coefficient can be measured on a plane orthogonal to the direction x, calculating the maximum diffusion.