The marimba is used among many tribes, but in particular fancied among the people of the Cental Region around Dodoma. The small wooden box is the resonator for a row of metal springs of different lengths. They are touched with the thumbs to produce the required sound.
Source: Wembah-Rachid & kirknaes J.A.R. & Jesper dans TANZANIA – 1990 – p. 29