There is a setting called "reference volume" that calls for the average SPL (Sound Pressure Level) to be at 85dB for movies with peaks as high as 105dB in the speakers and peaks as high as 115dB in the LFE (Low Frequency Effects) subwoofer channel.
The idea is that you are supposed to set your speaker trim levels (the individual + or - dB levels of each speaker individually in the receiver's menu) so that when you have the master volume set to "0 (zero) dB", that will be "reference volume" at your primary listening position. Thus, the volume goes to negative dB numbers to indicate how many dB below "reference volume" you are currently listening to. Or you can go above "reference volume" as well, which will result in positive numbers on the volume display. This is the standard "relative volume" display that you'll find on most receivers these days
FYI, when you are setting the individual speaker trim levels, pretty much all receivers will output a test tone from the speaker that is being adjusted at that moment. Most receivers are set by the manufacturer so that the test tone should read 75dB on your SPL meter. So start by setting the master volume to 0 (zero) dB. Then set your SPL meter up for "C"-weighting and slow response with the range set so that 75dB will fall in the middle of the needle's range (you don't have to do this with a digital SPL meter usually). Position the SPL meter at your primary listening position. Then go into the receiver's menu to the speaker levels section. Cycle through all of the speakers and adjust the trim settings so that every speaker reads 75dB on your SPL meter. You now have "reference volume" properly set! 0dB on your master volume dial will now represent reference volume and show you how far above or below reference volume you are with positive or negative dB numbers, respectively
One other tidbit, if you are using an auto-setup program like Audyssey, YPAO or MCACC, those programs sometimes screw up their readings if you do not set the master volume to zero dB before you start. So it's a good idea to always set the master volume to zero dB before running any auto-setup program or running the individual speaker trim level test tones manually. I haven't seen many (if any) user manuals ever mention this. And technically, you're not supposed to have to set the master volume to zero dB before hand. But I've had enough experiences and had enough other people share those experiences to know that, for what ever reason, setting the master volume to zero dB before running the test tones is sometimes the only way to get proper readings from those programs.
Hope that helps!
