Medium-separated tones add by intensity once the ear stops tracking envelope

Evidence

For larger frequency distances between the two tones, i.e. in the range of 20 Hz up to about 160 Hz, the equally loud tone is constant in level and matches at about 63 dB. This corresponds to an addition of the sound intensities. In other words, in that region of frequency separation in which the ear no longer takes into account the temporal variation of the envelope of the sound pressure, the sound intensity is the value that creates loudness. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1208