Free-field versus diffuse-field transmission gains differ by under eight decibels up to 8 kHz

Evidence

The gain factor, ao, holds for the free-field condition. The other extreme condition is a diffuse field, for which we have to use a different transmission factor leading to the value aOD. Both values are given in Fig. 8.18 as a function of critical-band rate and frequency. Up to about 8 kHz, both values deviate less than ±4 dB. This seems to be not very much; however, for calculating loudness as precisely as possible, such values become important. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1272

Categorization

• QUESTION: Above 8 kHz the deviation presumably grows - worth chasing the diffuse-field vs free-field divergence in the higher critical bands.
• corroborated: 2 independent passages support this claim — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1272