thanks ethan, i see your point but perhaps there is still some confusion as to what i'm saying. i will use an example:
1. say you had a source on tape/disk that varied between a -5dB and -20 dBFS signal - just say you did. without knowing the actual SPL levels that went to tape, would this information at least still represent a 15dB dynamic range in the original SPL?
for example: the SPL might have been between 60dB and 75dB if it was measured with an SLM originally?
so what i am saying is does a 1dB SPL difference = a 1dB difference on tape/disk. if not, is there some calculation that tells me the difference between a 1dB difference on tape in relation to that difference in SPL?
therefore, i am not talking absolute levels here, just ratios i guess.
2. back to absolute levels, i understand that there is no way to know the SPL levels that went to tape, but i still imagine it could be done with some kind of calibration tone or something that had a pre-defined level? would this be possible and if so, how?