A singer calls out to the crowd. An electric bass thumps out a beat. Horns blare and strings swirl. These are the sounds of R & B. Rhythm and blues music evolved from all sorts of sounds: swinging jazz, gritty blues, and African American spiritual songs. The music''s smooth mix of styles made it unique, and its passionate performers made it a sensation. Ever since Ray Charles hit the charts in the 1950s, R & B fans have held it down on dance floors. And R & B singers have belted out messages of love and calls for social change.