A mike in a completely silent room still generates some residual noise. This generated noise can be measured and is figured by taking a mike’s sensitivity and relating it to the threshold value of hearing. A more common method is stating the signal-to-noise ratio.
"I just heard my father's voice 11 years after he passed away. I cannot express my joy enough. He read poems that he wrote and sang his favorite hymn. Your restoration and transfer of an old 8-track made that possible. Thank you is not enough."