Engineering CalculatorsEMC Enclosure Cavity Resonance Calculator
A shielded enclosure is also a cavity resonator — at its resonant frequencies, shielding effectiveness can collapse toward 0dB regardless of the barrier material. Find the resonant mode frequencies of a rectangular enclosure from its internal dimensions.
New to Rectangular Cavity Resonator Theory? Read the guide: Enclosure Cavity Resonance: When a Shield Becomes an Antenna — a plain-language explainer of the standard behind this calculator.
A rectangular metal enclosure forms a cavity resonator with resonant mode frequencies f(m,n,p) = (c/2)·√((m/l)² + (n/h)² + (p/w)²), where l/h/w are the internal dimensions and m/n/p are non-negative mode indices with at least two nonzero (a resonant mode needs field variation in at least two dimensions). At these specific frequencies the enclosure behaves like a resonant antenna rather than a barrier, and shielding effectiveness (see the Shielding Effectiveness calculator) can collapse toward 0dB or worse — independent of how good the barrier material or thickness is. This is an idealized empty-cavity calculation: a real enclosure loaded with PCBs and components shifts resonances lower (added dielectric raises the effective permittivity) and damps them (lower Q, less severe) via lossy walls, cables, and absorptive materials — treat these frequencies as where resonance could occur, not a certainty in a populated enclosure.
Validated: reproduces the well-known EMC rule of thumb (Tim Williams, "EMC for Product Designers") that a 1m cubic enclosure has a lowest resonance of approximately 212MHz — this calculator returns 211.99MHz for that exact case, and confirms the three degenerate cube modes (1,1,0), (1,0,1), (0,1,1) are numerically identical as required by symmetry.