Rockvirgo - you are correct. S/N is the measure of noise below 0db level, whether the music is at that level or not. Typically music rarely plays at 0db. In an analog system such as a tape deck, 0db is already into some distortion. At +3db most tape recorders are well into distortion. For digital recording 0db is totally pegged ie its the maximum output for the 16bits being put into a word on the CD. Now most music will be layed down on the recording medium below these maximum values. The music you listen to might have a dynamic range of 50db and the loudest moment in the music recorded at -5db. This means to softest music is at -55db. But that's not to softest signal on the recording. Backgroud noise (outside street noise) musicians shuffling music, and good stuff like the ambience in the recording hall all come in below (hopefully) the softest music. Now if the device doing the recording has a S/N ratio of 100 db then there is 45db S/N ratio for the softest passage and 95 db S/N for the loudest. This is the S/N for the recording, not the equipment. It's that 45 db of floor that really makes the recording sound good. You pick up more "air" and "ambiance" in your listening experience. The recording process can also shift the balance by micing near or far from the instruments. A close micing will allow less ambience to be recorded because the ratio of instrument to hall ambience shifts many db. It does however give you more detail in the instruments near field sound that includes inside-the-instrument harmonics and overtones as well as instrument and musician noice (value clatter on a horn or the musicians snorfing - ever heard a violinist or pianist humming along with his own playing ala Glen Gould)
