For a 3-channel (L,C,R) system, L + R speaker channels should be placed
equidistant from center of screen image area, so that they are just inside the
screen area when screen is masked for the widest format (typically 2.35:1).
Acoustically transparent side masking should be used to prevent blocking of HF when side masking for a “flat” picture is moved in format.
Horizontal “Spacing” of Screen Channel Systems