An acoustical panel assembly has two channel members at oppostite sides each having a face portion and two flanges extending from opposite extremities thereof, thereby providing a channel between each set of flanges. A septum extends between the channel members being joined to said face portions intermediate each set of said flanges so that each channel member provides two channels to receive sound insulating material which may be in the shape of thin boards or packs. At the top and bottom are covers which close my panel assembly. A method of forming the assembly first provides the septum with reinforcing means which are joined to the channel member face portions.