In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system.
Microphones themselves noise floor is practically immeasurable, adding in a preamp stage to make it function is were thing begin to change and 99.99% cause of hiss / hence, noise floor.
Other noise floor causes: thermal noise, black-body, cosmic noise as well as atmospheric noise.
57db refers to the lowest measurable sound pressure on the diaphragm that's accurate within manufactures specifications.
Us sound engineers refer noise floor as "AIR" stemming from electronics circuit hiss.