## 2 Description of the Experimental FIVE@ECL Test Bench and Data Acquisition  
A detailed description as well as the main characteristics of the experimental FIVE@ECL test bench can be found in [25]. More specifically with respect to the brake system under study, it is composed of one disc with a 0.16 m external diameter, 0.033 m internal diameter and a thickness of 0.002 m, two F03C pads and a caliper XT M785 from the Japanese manufacturer Shimano. Figure 1a shows the braking system for the experimental FIVE@ECL test bench. In order to analyze and characterize the squeal phenomenon and the associated radiated noise, braking tests are conducted through the programmed variation of the pressure to the pads and speed to the disc rotation. These two parameters correspond to the two operational parameters monitored during a braking test whereas the motor torque and the temperature are non-controlled and measurable operational parameters directly resulting from the braking test conditions. The choice is made to perform a constant speed braking test, a protocol classically used in the field of friction-induced vibration and noise on academic test benches. The main objective is to limit the number of operating parameters (such as rotation speed) and to illustrate the fact that brake squeal occurs not only when the rotation speed of the disc is slowing down, which has already been discussed in [17; 25]. Figure 2 illustrates the experimental protocol with the different phases and the time evolution of the four operational parameters (i.e., the rotating speed of the disc, the motor torque, the brake pressure and the pad temperature) during a braking test. The protocol for one braking test is defined as follows:  
* \(t=0\) s: braking system at rest and the start of data acquisition;
* \(0\) s \(<t<5.3\) s: Phase 1--no rotation;
* \(5.3\) s \(<t<9\) s: Phase 2--increasing the rotation of the disc from 0 to 200 rpm (keeping the brake pressure at zero);
* \(9\) s \(<t<13.2\) s: Phase 3--system rotating at 200 rpm without brake pressure;
* \(13.2\) s \(<t<15.5\) s: Phase 4--rise in brake pressure from 0 to 9 bars. This period corresponds to the pads and the disc coming into contact along with the emergence of friction-induced vibrations and squeal noise;
* \(15.5\) s \(<t<36\) s: Phase 5--braking phase by keeping the brake pressure at nine bars and the disc rotation speed at 200 rpm;
* \(t>36\) s: Phase 6--brake pressure release. Figure 1: Experimental bench FIVE@ECL. (**a**) Braking system; (**b**) Digital microphone antenna. The different phases of the protocol are indicated on the Figure 2 by the notation Pi for the ith phase (with \(\text{i}=1,\dots,6\)). In the rest of the study, we focus on the analysis of the radiated noise in the time interval \(t\in[16;35]\) s, which is the phase of the braking test where the squeal phenomenon is established and the brake pressure and rotation of the disc are constant. As previously explained, one of the main originalities of the present work is to provide a detailed and in-depth analysis of the radiated noise during a braking test to characterize squeal noise. In order to achieve this objective, it is necessary to set up a specific experimental system to accurately measure the radiated field over a large number of test points. The measurements are carried out in collaboration with the company MicrodB by using an LMS SoundCam 1.5 m large digital antenna with 117 microphones, as shown in Figure 1b. The position of each microphone in the plane of the antenna is shown in Figure 3. This digital microphone antenna is modular enough to cover a wide frequency range (from 100 to 20,000 Hz). The position of the digital microphone antenna in relation to the braking system is chosen at 0.12 m. More precisely, it corresponds to the distance between the microphone array plane and the disc plane, i.e., the distance between the center of the antenna (aligned with the the center of the brake disc) and the brake disc system.