Metal Enclosure Cavity Resonance (Rectangular Box)
Find the four lowest resonant frequencies of a rectangular metal enclosure from its width, depth and height.
Worked example
Input: Width a 300 mm, Depth b 200 mm, Height d 100 mm
Result: 1st resonance (lowest) 900.764 MHz, 2nd resonance 1249.14 MHz, 3rd resonance 1580.04 MHz, 4th resonance 1675.89 MHz
Result: 1st resonance (lowest) 900.764 MHz, 2nd resonance 1249.14 MHz, 3rd resonance 1580.04 MHz, 4th resonance 1675.89 MHz
Formula
f(m,n,p) = (c/2) · √((m/a)² + (n/b)² + (p/d)²)
m, n, p are non-negative integers, at least two non-zero
For an empty box (parts inside lower the frequency and change Q)
How it works
An empty metal box resonates at f(m,n,p) = (c/2)·√((m/a)² + (n/b)² + (p/d)²), where m, n, p are non-negative integers and at least two are non-zero. The lowest resonance comes from the two longest sides. For the example (300×200×100 mm) the lowest is about 900.8 MHz, followed by 1249 MHz and 1580 MHz. At a resonance the internal field builds up strongly and shielding can degrade.
Practical tipThis assumes an empty box; with boards and parts inside, real frequencies drop and Q is lower. If an emission suddenly peaks at one frequency, check whether a clock harmonic coincides with a cavity resonance. Typical fixes are absorbing material, partitions that change the dimensions, and fewer gaps.
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Results and summaries are for reference. For certification and test reports use the latest official standard text and calibrated instrument data. Last updated: 2026-10-10