On a berimbau, the gourd is used as a resonator of the piano string's vibration, when struck by the stick.

There are two holes on the upper part of the gourd to slip a piece of twine into it and use it to fix it to the lower part of the bow.
The higher the gourd is positioned on the bow, the higher-pitched the berimbau is tuned.
Still, the gourd has to remain on the lower half of the bow.
In order to produce bass sounds, the player should use a large gourd, its size being proportional to the height of the desired sound range.
Small gourd for a "Viola" (smaller) or a "Medio" (medium) berimbau 
Height: about 4.7 to 5 in 

(Largest) diameter: about 4.7 to 5 in