A: The Shure SM7 dynamic mic was introduced in 1973; according to Shure, seven years of field research went into designing what was intended to be the finest unidirectional dynamic microphone ever. The SM7 uses the same Unidyne III acoustical principle as the venerable SM57 and SM58, but has a thinner and more flexible diaphragm. Plus, the SM7 has three times as many turns of wire on its voice coil compared to the SM57/SM58. This reduces the resonant frequency of the mic and, combined with the larger housing, provides rich low-end response. In fact, the filter switch on the SM7 technically works in reverse of how you might expect; the “boost” position is actually the unfiltered low-end response, while “flat” inserts a filter to reduce the low-end response.
As for differences among the SM7, SM7A, and SM7B, the SM7A featured a humbucking coil designed to reduce noise picked up from computer monitors and an improved mic mount. The SM7B has a larger foam windscreen, which is included with the mic.