AS I just bought a SM58 I spent some time in the Shure knowledge base, this is quootes from there:
"The SM58 and SM57 do use the same cartridge. The grill, though, does effect the frequency response to some extent. At a distance, the difference is probably not noticable. But at extremely close distances, the difference is noticeable. Due to the grill, a person's mouth can get closer to the diaphragm of an SM57, thus causing more proximity effect and increased bass when compared to the SM58.
Note that the cartridges are not interchangable because the mounting scheme is different for the two microphones."
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"It is true the SM57 and SM58 microphones are based on the same cartridge design. The main difference between them is in the grille design. The SM58 was designed for vocal application and it uses a separate grille with a very effective pop filter. The SM57 was designed as an instrument microphone where smaller grille size is preferred. In this application the pop and wind are not usually a concern. The SM57 uses an integral resonator/grille assembly, where grille is actually a part of the cartridge. These two grille designs place the diaphragm of the microphones in a different acoustical environment. First of all, the distance from the top of the grille to the diaphragm is significantly shorter on the SM57 compared to that of the SM58. This allows for closer sound pickup with even more pronounced proximity effect. Secondly, a different resonator/grille assembly design of the SM57 is responsible for its slightly higher output above 5 kHz."
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"Q. On the Shure FAQ "SM57 vs SM58 - proximity effect", the answer contained the following sentence:
"Remove the ball grill from the SM58 and it will be more similar to the SM57 in its low frequency response."
Can you actually use an SM58 this way, or was that only a theoretical statement?
A. Yes, but be very careful! With the ball grill removed, there is very little protection for the SM58 diaphragm. Take the ball grill off and examine the SM58 mic element to see for yourself. As long as you take great care, the SM58 can be used with the ball grill removed."