Human ear should be the ultimate audibility detection threshold

[dbetchkal]

Right now, ambient levels can be provided that set a detection threshold more sensitive than the human ear. This represents a potential source of error, especially given we are using residual ambient $L_{Z90}$ spectrally, and in some bands sound level meters can exceed the sensitivity of a human ear.

To fix this problem we simply need to use the maximum of the ISO 226:2003 equal loudness contour for the threshold of human hearing and the ambience curve in each band.