Wikipedia yielded this information:
"Calling the Cabaça a gourd is technically a mistake. As far as Brazilian berimbaus are concerned, the fruit used for the berimbau's resonator, while still known in Brazil as 'cabaça', is not technically a gourd (family Cucurbitaceae); instead, it is the fruit of an unrelated species, the tree Crescentia cujete (family Bignoniaceae), known in Brazil as calabaça, cueira and cuia (ref.: O Estado de São Paulo, 6–12 April 2011, Suplemento Agrícola, page 2) or cabaceira (Houaiss Dictionary)."
Calabash Tree (Crescentia cujete)
Hope this helps!