Stiffness-based instruments space their modes wider apart than tension-based ones.

Evidence

Nonstretched instruments, such as plates (cymbals), bars (glockenspiel) or three-dimensional structures (bells) get their physical restoring force from their stiffness. In contrast, stretched instruments such as strings (violin) or membranes (drums) get their restoring force from the stretching itself. Stiffness-based instruments tend to have their modes more widely spaced in frequency than do tension-based instruments. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1310

Categorization

• DEFINE: Clarifies the stiffness-vs-tension dichotomy that organizes how percussion instruments are classified acoustically.
• XREF: Connects to how mode spacing determines harmonicity, which determines perceived pitch strength — links to kettledrum pitch discussion.