Viscothermal effects are the main source of acoustic dissipation. In many problems, those effects are neglected in the modeling of acoustic wave propagation at ordinary pressures and frequencies. However, when dealing with problems in which some geometrical characteristic dimensions are of the same order than boundary layer thicknesses, the viscosity and heat conduction effects become important and should be taken into account in the model for accurate calculation of acoustic quantities [1, 2]. The common example of such problems is the transmission of sound in narrow ducts but viscothermal effects can also be important in irregular geometries. Recent studies reveal that the energy losses in irregular geometries are related to mode localization phenomenon induced by this irregularity [3, 4]. According to these studies, the more a mode is localized near the boundary, the more the acoustic dissipation is increased. The localization is often measured by the existence surface (2D) or existence Volume (3D) the eigenmodes which quantifies the volume (case of the 3D cavity) in which the energy of the mode is localized. A localized eigenmode is characterized by a very small “existence volume” compared to the total volume of the cavity. Most of these studies are numerical and it is difficult to experimentally show the localized modes because of the smallness of localization area and the high confinement of such modes energy. In this paper we’ll try to propose a solution to this problem by an innovative mean using Laser scanning vibrometer technique. A simple example composed of rectangular cavity coupled to a quarter-wave resonator as irregularity is investigated to numerically and experimentally observe the localized mode and the relative viscothermal effects.