###### Abstract
Given that the incompressible pressure wave induced by the turbulent eddies within the boundary layer is transported approximately at the mean flow speed and the compressible pressure wave propagates at the vector sum of the sound speed and the mean flow velocity, the transmission characteristics of hydrodynamic and acoustic pressure waves through the wall panel of the cabin are pretty different from each other. [PERSON] et al. [15] computed the flow field around a high-speed train at the speed of 300 km/h in an open field by employing a highly accurate LES technique with high-resolution meshes of more than 300 million grids. The unsteady surface pressure fields were then decomposed into the incompressible and compressible ones using the wavenumber-frequency transform (WFT), and the surface pressure power spectral levels due to each wall pressure fluctuations were estimated and compared for the selected wall panels of the train. These results showed that the incompressible pressure components contribute more to the total surface pressure levels in the low-frequency range, whereas the compressible ones do more in the high-frequency range. This fact implies that the interior sound of a high-speed train also follows the same spectral characteristics. Accordingly, the measure for reducing the interior sound may differ according to the frequency range because the speeds of compressible surface pressure waves are much higher than the incompressible ones. There are many tunnels for high-speed rail in mountainous areas. For example, the Seoul-Busan high-speed line in Korea consists of 412 km of double tracks, including 191 km (46%) of tunnels [16]. Given that cabin interior noise generally increases when a high-speed train passes through a tunnel, developing high-speed trains with quieter interior noise in such an environment is more critical. Figure 1 shows the time histories of the overall sound pressure levels measured inside coaches of two high-speed trains running both in open-field and tunnels with integration time 0.5 s. The averaged running speeds of the trains when the measurements were carried out are listed with averaged SPLs (sound pressure levels) in Table 1. It has been observed that the sound pressure levels measured in cabins while the trains are running inside tunnels are higher by about 5 dB to 10 dB than those in open-field [17]. The sound sources contributing to high-speed train interior noise can be classified into three categories: equipment noise, structure-borne noise, and airborne noise. HVAC(heating, ventilation, air conditioning) systems are mainly responsible for the equipment noise. Structure-borne noise is caused by the reradiation from the structures excited by vibroacoustic sources like wheel/rail contact and engine/auxiliary equipment. Lastly, as described above, the airborne noise is due to external flow disturbances that can be further divided into incompressible and compressible pressure fields. The surface pressure fields drive the vibration of the body panels and windows of a high-speed train, thereby causing interior sound. The strength of the source causing the equipment and structure-borne noises is not different between open field and tunnel, and thus the difference in the sound pressure levels measured in a cabin is mainly due to the external flow disturbances. This study aims to investigate the leading cause of the difference observed in the sound pressure levels measured inside the cabin of high-speed trains cruising in an open field and in a tunnel. The characteristics of surface pressure fluctuations are analyzed by assessing relative contributions between the incompressible and compressible wall pressure fluctuations. First, compressible LES (large eddy simulation) techniques with high-resolution grids are employed to accurately and simultaneously predict the exterior flow and acoustic fields around a high-speed train.