Here are some answers that should satisfy any question about the SM57/58. This was taken from the shure website faq section:
Question:
Do the SM58 and SM57 mics have the same frequency response curves? Is a 58 simply a 57 with an integral pop filter/windscreen?
Answer
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.
Question:
What are the real reasons an SM58 should be used for vocals, and an SM57 be used for instruments? Of course, everybody seems to use these mics as mentioned. No one has given me a convincing reason for this, other than 'That's just what you do- everyone does it this way'. Please ease my anxious mind!
Answer:
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.
Doug