Understanding Noise Voltage, Equivalent Noise, Signal to Noise Ratio

From the Sennheiser micro revue 70 71 page 14:
http://lcweb2.loc.gov/master/mbrs/recording_preservation/manuals/Sennheiser%20Micro-Revue%2070-71.pdf
(There is nothing better than reading from the best)


If you bring a microphone into a completely noise free room and connect it to a very good amplifier, you will notice a hiss or rustling sound which originates from the microphone. In dynamic microphones this noise is created by the thermal agitation of the electrons in the resistance of the moving coil of the microphone. In condenser microphones this noise derives from several sources...