A computer with labeled dimensions (40 cm by 38 cm by 16 cm)

EMC Question of the Week: June 7, 2021

The first (lowest) resonant frequency of a 40-cm x 16-cm x 38-cm rectangular metal enclosure is 

  1. below 10 MHz
  2. approximately 380 MHz
  3. approximately 540 MHz
  4. above 1 GHz


The best answer is “c.” The resonance frequencies of an air-filled rectangular metal enclosure are given by the expression, 

f m n p = c 2 m a 2 + n b 2 + p d 2

where a, b and d are the dimensions of the enclosure; c is the propagation velocity; and m, n and p are non-negative integers (at least two of them non-zero). The assignment of dimensions corresponding to a, b and d is arbitrary, but for a<b<d the lowest resonant frequency is f011. For this enclosure, 

f 011 = 1.5 × 10 8 m/s 0 0.16 2 + 1 0.38 2 + 1 0.40 2 = 544 M H z

Of course, we have assumed that the question is asking about the first electromagnetic (not acoustic) resonance, and that the enclosure is mostly empty (e.g., not filled with a dielectric). 

Have a comment or question regarding this solution? We'd like to hear from you. Email us at This email address is being protected from spambots. You need JavaScript enabled to view it..