A portable acoustical shell for the performing arts and the like, and which includes a plurality of generally quadrilateral, and preferably trapezoidal, sound reflecting panels arranged together as an assembly along adjacent long side edge margins in substantially abutting juxtaposed relation to form a forwardly open, arch-like configuration. The arch-like panel assembly is supported and held in place by a modular frame assembly formed from a plurality of pyramidal frame modules, each of which is associated with a respective one of the plurality of sound reflecting panels. The modular frame assembly further includes a number of cross members which connect the apexes of adjacent pyramidal frame modules to one another. In an alternate embodiment of the invention, the cross members are length-adjustable and thereby permit the arch-like curvature of the acoustical shell to be flattened or steepened as desired.