Bruno, 85dB and 105dB are absolute loudness level numbers(at the listening position), they're not in "reference" to something else. The reason I happened to pick those two was that 85dB(at most)is what can be used for a long term average listening level without causing permanent hearing loss(note the EPA/WHO suggested limits
here for "community" sound, not in the workplace(first two tables) where some long term loss is considered acceptable(by some). The most dynamic recordings may have peaks as much as 20dB above the average level and this is the reason for the 105dB number. Note that it's largely classical items(some of which I have)that may have that wide a dynamic range. Many almost uniformly loud pop recordings have a very small dynamic range. Rather amusing was an article in my AES Journal a few months ago which described tests of various recordings to measure the thermal stress their dynamics imposed on speakers. One very loud pop recording measured out with a dynamic range of essentially zero dB.
The dB volume can't typically be referenced to 0dB in most cases. Movies are supposed to have a maximum level in the speaker channels of 105dB when the receiver is properly calibrated at the 0 point. Of course, most of the time the level of movie sound is far below that maximum and during a relatively low level segment the sound level with the receiver at 0 might be 50dB. Typically, few listen to movies at the "reference" 0 level at home, but at 10dB or so lower than that. Music, as compared to movies, has no established reference level and different discs can have widely different sound levels if someone listened with the volume at 0.
So, as was said, volume can be measured with an SPL meter, but can't be very accurately judged from receiver volume settings.
