A modified form of the Eyring equation is derived which explicitly takes account of the anisotropy of a sound field due to the deviation of the energy reflection (absorption) coefficient of individual surfaces from the mean value for all surfaces. When all surfaces have the same energy absorption coefficient, the equation simplifies to that of Eyring; whenin additionthese absorption coefficients are small, the usual simplification to the Sabine equation is valid. Using a simplified version of this modified Eyring equation, one can show that the effective absorption coefficient of a single absorptive surface in a hypothetical reverberation room is a complicated function of the location and total area of randomly oriented scattering panels. Apart from any correction for diffraction, the effective absorption coefficient is also a function of the area of the absorptive surface. In typical situations, this theory predicts that the effective coefficient will be greater (0 to 40%) than the energy absorption coefficient. In general terms, this agrees with measured values. In extreme situations not usually found in reverberation rooms, effective absorption coefficients are predicted to vary more widely and to be either greater or less than the energy absorption coefficient.