Roseburg’s free summer music series closes on an acoustic note Tuesday with two female musicians who offer a more intimate show than earlier performers. Georgia-born musician Madeleine Peyroux headlines Music on the Half Shell, and British actress and musician Rebecca Pidgeon will open. This will be the second time this season the Half Shell series has offered two performances on one night. Salem-based Ty Curtis opened for San Francisco funk group Monophonics in July. Peyroux, 39, is a singer-songwriter and musician whose somber melodies and slightly raspy voice are reminiscent of 1930s and ’40s soul vocalist Billie Holiday. A contemporary …