What is the resonant frequency of a shielded compartment on a PCB and how do I avoid problems at that frequency?

A shielded compartment (shield can or frame) on a PCB forms a rectangular cavity resonator with resonant frequencies determined by its internal dimensions. At resonance, electromagnetic fields are amplified inside the compartment, which can: increase coupling between circuits within the compartment (defeating the purpose of the shield), create feedback paths that cause oscillation in amplifiers, and reduce the shielding effectiveness at the resonant frequency. The resonant frequencies of a rectangular cavity are: f_mnp = (c / (2 sqrt(Er))) x sqrt((m/a)^2 + (n/b)^2 + (p/d)^2), where a, b, d are the internal dimensions of the cavity (width, length, height), m, n, p are the mode indices (integers, at least two must be non-zero), Er is the dielectric constant of the substrate filling part of the cavity, and c is the speed of light. The lowest resonant mode (TE101 or TE110 depending on the dimensions) has the lowest frequency. For a typical PCB shield can (20 x 15 x 3 mm on FR4, Er = 4.4): the lowest resonant frequency is approximately f_101 = c / (2 x sqrt(4.4)) x sqrt((1/0.02)^2 + 0 + (1/0.003)^2) approximately 24 GHz. This is well above most digital and RF circuit frequencies. However, larger compartments (> 30 mm) or higher-frequency circuits (> 10 GHz) can have resonances within the operating band.