Inherent cavity resonant modes often lead to significant degradation of shielding effectiveness, responsible for unwanted electromagnetic coupling. Cavity resonant modes of the metal shielding enclosure can produce two adverse problems: the mutual coupling among different RF modules and shielding effectiveness reduction of the metal enclosure. The cabinets serve to shield certain components from electromagnetic interference (EMI). However, these cavities present some resonance peaks at 5 GHz, making it impossible to use them at higher frequencies. By incorporating absorber sheets inside the lid of the cabinet these resonances are attenuated or even eliminated, allowing the use of the cabinets as a protective element in PCB at higher frequencies. In this study, it has been calculated theoretically, simulated using commercial EM field solver and tested in laboratory in different cabinets to find their resonance frequencies, as well as tested incorporating a ferromagnetic material inside them to see if this can help to attenuate the effect of resonances. In addition, thermal simulations and laboratory tests have been carried out to prove that this material can have a dual function as a resonance attenuator and as an element to direct the heat flow from inside the cabinet to the outside. The calculated, simulated and tested data in most cases differ between one to seven percent error and a significant reduction in resonances has been demonstrated. Extending the frequency range of these cavities will make a difference for IoT applications, 5 G and future functionalities that require the use of a higher frequency range
