in the form of a large horn 10 defined by a lower wall 11 and an upper wall 12 which are horizontal and parallel and curved side walls 13 which are dimensioned and follow a line dependent on the expansion of the areas required for the correct operation of the horn itself all the parts may be produced on site in brickwork o with the use of prefabricated elements the upper wall 12 of the horn may itself constitute a surface that can be walked on as an alternative this upper wall may be overhung by a support cover 14 represented by or forming the floor of a stage the curved walls 13 of the horn extend outwards away from a space 15 of suitable volume in which a driver unit 16 is housed this unit comprises one or more loudspeakers 17 which are arranged in a cabinet 18 fitted with wheels 19 the driver unit 16 can thus be moved in the manner of a trolley to be easily inserted into the dedicated space 15 and be easily removed and transferred as required the horn 10 may be built for example at the base of a screen in a room used for the reproduction of music and speech as is schematically shown in fig4 and 5 or of a concert stage fig4 b in combination with complete low medium high and screen loudspeaker systems respectively the walls 20 of such a room may be covered according to the present invention with a diffusing reflecting absorbing structure 21 which consists of basic modules or panels 22 in a large variety of.