The only difference between the two models is in the housing of the electronics module. The CM-30 electronics interface module installs in a standard single-gang electrical box. The CM-31 has a cylindrical (tube) electronics interface module.

The CM-30 would be a better choice for a permanent install that will not be moved (example: most house of worship choir areas). The CM-31 would be a good choice for applications that need to flexible in location (example: theater stages, temporary sound systems). In both choices, the microphone remains a professional-quality, supercardioid condenser designed for inconspicuous miking.