Fitzroy Equation

The Fitzroy equation is a reverberation time prediction formula designed for rooms where absorption is concentrated on one or two pairs of surfaces rather than uniformly distributed. Published by D. Fitzroy in 1959, it calculates separate reverberation times for each pair of opposing surfaces and combines them as a weighted average based on surface areas. The equation addresses a key limitation of both Sabine and Eyring: neither handles non-uniform absorption distribution well. For example, a room with an absorptive ceiling and reflective walls has different decay rates depending on the sound propagation direction. The Fitzroy equation predicts longer RT60 values than Eyring in such cases because it accounts for the slower decay along the reflective surfaces. It is particularly useful for classrooms (absorptive ceiling, hard walls/floor), industrial spaces, and open-plan offices where absorption is directionally concentrated.