Idiophones produce sound by vibrating the instrument itself, not strings or membranes.

Evidence

The marimba is an example of an idiophone—a musical instrument that produces sound by initiating vibration of the instrument itself. (Strings or membranes are ruled out by definition in idiophones.) — Eric J. Heller, Why You Hear What You Hear- An…, 1892

Categorization

• DEFINE: Defines ‘idiophone’ as a category of instrument that vibrates itself.