Rack panels. The common format of audio racks is 19 inches, which is 48.26 cm wide. The height of the rack module is (rack unit, U) equal to 1.75 inches, or 2.54 cm. Amplifiers, effects, interfaces, UPS, everything else useful in a professional studio is built for rack mounting. Rack panels, in multiples of 1 U, are for example useful for covering empty spaces when a cabinet is not completely occupied.