|MadSci Network: General Biology|
You are asking quite a complex, yet poorly defined question.
The closest quantitative measure of what you are describing is a signal to noise ratio (differentiating different intensities in varying degrees of background noise). Unfortunately, the answer is dependent not only on the frequency content of the signal (as the dynamic range is mentioned above), but also on the frequency content of the noise content. It is also the subject of quite a bit of current research.
Due to a phenomenon called masking, sound energy at one frequency can mask or drown out sound energy at a nearby but different frequency in a time and frequency dependent manner. Frequency based masking is closely related to critical band theory which is the basis of subband coding and MP3 commpression. Temporal masking is its own unique phenomenon and has been used in higher order audio codecs.
There is a huge literature base attempting to define these concepts and their boundaries. Much of the literature spans psychology and basic science; but, if you are quantitatively minded the Journal of the Acoustical Society of America would be a good place to start.
Try the links in the MadSci Library for more information on General Biology.