## 1 Introduction  
The problem of friction-induced self-excited vibrations and acoustic radiation has been widely studied for many years as it is one of the major problems encountered in the design of industrial brake systems. General reviews on this subject can be found in [1; 2; 3; 4; 5]. Despite the large number of studies that have been carried out on this subject, understanding the physical phenomena involved in brake squeal and predicting squeal noise remain a major concern in many industrial applications related to the automotive [6; 7; 8; 9; 10; 11], aeronautical [12; 13] and railway [14; 15; 16; 17] sectors. Indeed, the study of brake squeal is a multi-physics problem that requires advanced analysis in various fields such as vibration, tribology, thermodynamics and acoustics, for example. As a result, academic test rigs [18; 19; 20; 21; 22; 23; 24; 25; 26; 27] have been developed in order to propose experimental contributions in order to better understand friction-induced vibrations and noise. Although many experimental studies have been carried out to investigate the problems of friction-induced vibration, only a few experimental studies have so far been conducted to investigate the impact of squeal noise on the basis of the resulting radiated sound field. In most cases, a single microphone is used to measure squeal noise, which does not allow the complete and consistent analysis of radiated noise during squeal events. Although a study of the link between friction-induced vibrations and squeal noise using acoustic measurements with a nine-microphone antenna was recently carried out by authors of [26]; these previously proposed results do not allow detailed analysis of the radiated field. Furthermore, specific studies on the acoustic holography of brake squeal have also been performed to measure the movement of the disc and the associated travelling waves [28; 29]. Thus, [PERSON] and [PERSON] [29] used acoustic measurements to estimate the parietal velocity field of a braking system to characterise its dynamic behaviour. The near-field acoustic measurements were processed, and an inversion algorithm was used to reconstruct the parietal estimates. This study highlighted wave propagation phenomena, showing the appearance of travelling waves, whose direction of progression changes during squeal. More generally, the problems of source identification can be addressed by performing near-field pressure measurements [29; 30]. However, the direct acoustic field in the surrounding space has never been studied in detail, i.e., in terms of temporal evolution, sound level, as well as directivity characterization. This question is one of the contributions of the present study, the main objective of which is to characterise the squeal phenomenon from the point of view of acoustic radiation by deploying pressure measurement devices in front of the braking system subjected to vibrations induced by friction and acoustic noise. With regard to this objective, although the braking system and operating conditions considered are similar to those already proposed in previous studies [25; 26], here, we focus more specifically on the analysis of the radiated sound field and the characterization of the squeal frequencies in terms of amplitude and acoustic directivity, by carrying out pressure measurements for a large number of test points. In order to achieve this goal, a 1.5 m diameter antenna with 117 microphones is used to determine the acoustic field and describe it over a large surface and not just at a limited number of spatial points. Thus, it is possible to carry out an analysis of the acoustic radiated field and obtain the characterization of squeal noise by the direct exploitation of these pressure measurements. To the best of our knowledge, this type of acoustic characterization for the problem of friction-induced vibration and noise for brake systems has not yet been addressed in the literature.