why subs ultrasonic not infrasonic? (or normal sonic)

[Shannon]

In other words: the overall volume is greater in the masked tracks than the buzzing track (for equal script volume) since ethe buzzing track has no loud masking sounds. Is this logic right?

The volume of the - we'll call it B-track - is about the the same as that of the ultrasonic track, and the same as that of the trickling stream mask, and the same as the ocean surf mask.

When the B-track is mixed down with the masking track, it's volume is reduced considerably. The masking track becomes louder. But if the masking track were vocoded such that volumetric variations for the masking track were matched by the B-track, and that B-track volumetric adjustment was not made, the volumes would be about the same for both tracks.