The surdo is a large bass drum used in many kinds of Brazilian music, most notably in Axé/Samba-reggae and samba and its variants, where it plays the lower parts from a percussion section. Surdo sizes normally vary between 16" or even 14" and 26" or even 29" diameter. Surdos may have shells of wood, galvanized steel, or aluminum. Heads may be goatskin or plastic. A Rio bateria will commonly use surdos that have skin heads (for rich tone) with aluminum shells (for lightness).