Sound begins blasting ear drums and rattling bones even before the speakers — hauled by big rigs creeping their way through Brazil’s thronging Carnival crowds — draw near. The behemoth sound trucks known as electric trios are a Brazilian innovation that amplified music and effectively did away with front-row seats — making Carnival more accessible. In the seven decades since the first one hit Brazil's streets, they have become a fixture of the country's annual pre-Lenten festivities and draw millions to the streets.