The surdo is one of the most important and characteristic instruments of samba in Rio de Janeiro, especially as a part of the drum groups in samba schools, where one can find anywhere from 25 to 35 surdos. The surdo is a large, low-pitched drum that comes in three different categories and sizes, and which marks the beat in samba music.
The surdo de primeira (first surdo), with a 30-inch rim, has the lowest pitch. It is played on the upbeat of the rhythm to keep the main beat of the samba going. The samba singers are guided by its sound, and it keeps them from speeding up or slowing down.
One of the main characteristics of samba is the low-pitched sound played on the second beat by the surdo de primeira. In most samba schools, this accentuated sound gets a response on the first beat with the higher-pitched, 55-inch rim surdo de segunda (second surdo). Nonetheless, this call and response pattern is not a rule. At the Estação Primeira de Mangueira school, for example, the drum group is characterized by the absence of a response from the surdo on the first beat. The surdo de terceira (third surdo), on the other, has a 40-inch diameter and is the most high-pitched. It plays a number of syncopated rhythms that fill in the regular beat of the other drums.