Strutt|Environmental|Semi-Reverberant Courtyard (Room Constant) calculates the room constant for an outdoor enclosure between 2, 3 or 4 walls (e.g. a courtyard or 'canyon' between buildings). The noise level in this situation will consist of both the direct sound component and a semi-reverberant component, which may be calculated using the room constant and the Reverberant Level (From Room Constant) calculation.
Reference: Miller, L.N. Noise Control for Buildings and Manufacturing Plants, Section 6-5