mean it literally - just as we perceive within a sound spectrum and a light spectrum, so we perceive within a density spectrum.
I ask the question, because taken literally the idea that we would perceive within a density spectrum seems inane assuming that density is taken to mean mass per unit volume. You must mean something different than what I expect by the term.
Under a government which imprisons any unjustly, the true place for a just man is also in prison. Thoreau: Civil Disobedience (1846)
The apathy of the people is enough to make every statue leap from its pedestal and hasten the resurrection of the dead. William Lloyd Garrison