Structurally, the auditory system is able to distinguish different pitches (sound waves of varying frequency) via the complementary vibrating of the eardrum.
This was followed in 1947 by the widespread introduction of methadone, another structurally unrelated compound with pharmacological properties similar to those of morphine.
In this manner, occupancy separations are treated similarly to fire walls which are structurally stable in case of a fire, thus limiting fire-induced building collapse.