Complex sentence: This early acoustical knowledge informed the design of opera houses and concert halls in the 17th, 18th and 19th centuries; these were deliberately built to create internal reflections in order to enhance and project sound from the stage in the days before electrical amplification.
Split into simpler sentences: This early acoustical knowledge informed the design of opera houses and concert halls in the 17th, 18th and 19th centuries. Architects designed these to create internal reflections that would enhance and project sound from the stage in the days before electrical amplification.