Our R-series mics have a patented offset ribbon design, created to help them do a better job of handling high SPL's. Because the ribbon is in a different physical space when you use the back side of the mic, there is a slight tonal difference between the two sides (see Recording With The Back Side). This only takes place at close distances - at distances of more than 3 feet, both sides of the mic sound identical. At three feet or less, the back side will be a little brighter than the front, which can be nice for vocals, acoustic guitars, and other applications. SF-series mics use a traditional design - the ribbon is centered and the front and back sides sound the same at all distances.