Widely separated tones add by loudness, not pressure or intensity

Evidence

For a frequency separation larger than about 160 Hz, the level of the equally loud I-kHz tone, i.e. the loudness level, increases strongly. This increment shows a continuous rise that leads, at a frequency separation of 2000 Hz (1 kHz is the geometric mean of h at 400 Hz and h at 2400 Hz), to an increment of 10 dB relative to the single I-kHz tone. Because an increment of 10 dB of the I-kHz tone corresponds to a doubling of loudness, we can represent the development of loudness for large frequency distances of the two tones by assuming that the loudness of the complex is the sum of the loudnesses of each tone. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1209

For small frequency separations up to about 10 Hz, the hearing system is able to follow the beats between the two tones that lead to a loudness sensation corresponding to the peak value reached within the beating. The peak corresponds to twice the sound pressure of the single tone, and leads to a loudness level of 66 phon. This means that for narrow frequency separations, the loudness level is determined by the peak value of the sound pressure reached within the beating period. In order to estimate the loudness level of such a two-tone complex, we have to add the sound pressures of the two tones. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1207

Categorization

• QUESTION: Worth chasing: the ear transitions among pressure addition, intensity addition, and loudness addition as frequency separation grows — a clean three-regime model of loudness summation worth fully internalizing.
• corroborated: 2 independent passages support this claim — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1209; Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1207