Real and apparent dissipation of vibrations of structural systems

Discussions of ‘‘real’’ and ‘‘apparent’’ dissipation of the vibrations of structural elements to which are attached substuctures, have suffered from a lack of precision in terminology. Identifying a real dissipation with the transformation of mechanical energy to heat and an apparent dissipation with the transformation of mechanical energy from a form that one observes to a form that one doesn’t, would clarify issues. A further classification of a real dissipation as either ‘‘resonant’’ or ‘‘nonresonant’’ is suggested. A resonant dissipation is obtained for vibration frequencies that are nearly coincident with the natural frequencies of a subset of the attached subsystems. Finally, a further classification of an apparent dissipation as either ‘‘reversible’’ or ‘‘irreversible’’ is also suggested. The physics underlying the different type dissipations; their modeling; and, the dependence of measures of dissipation on more fundamental measures of the attached substructures will be discussed.