Both the SM58 and the SM58a are housed in a rugged, enamel painted die-cast metal case. The microphones are designed for use on stage, and the solid casing and steel mesh grille offer a great degree of protection from the "rigors of the road."
Both the SM58 and SM58a have a standard three-pin XLR connection, making the microphones interchangeable. The XLR cable enables both microphones to operate on phantom power, wherein the cable draws current from the connected device, such as a mixer or amplifier. Both mics are compatible with Shure wireless cartridges, which convert them into wireless microphones.
The polar pattern determines the area from which the microphone will pick up sound. The SM58 is a unidirectional cardioid microphone, meaning it collects sound from all directions. The pickup area is heart shaped, hence the term "cardioid." The SM58 captures more sound from in front than it does from behind, meaning you must point it toward the sound source for accurate sound detection. It typically captures 6 fewer decibels of sound at a 90-degree angle from directly in front of the microphone. The SM58a has a super-cardioid polar pattern -- the same shape as the SM58 but a setup that covers a slightly larger area, making the microphone more sensitive. Depending on the application, sensitivity is not always desirable.
Frequency response dictates the type, rather than volume, of sounds the microphone captures. The SM58 has a frequency response range of 50 to 15,000 hertz (Hz). The SM58a has a slightly greater range: from 50 Hz to 16,000 Hz. As such, the 58A is more suitable to applications requiring stronger response in high frequencies -- for example, amplifying brass instruments.
The SM58 weighs 10.5 oz, while the SM58a is marginally lighter at 9.92 oz. While this difference may seem negligible, it does carry practical implications. For example, if you use the microphones interchangeably, you must adjust the tension of the microphone stand collar to account for the extra weight of the SM58.