Originally Posted by BeeMan458
What am I misunderstanding>
Pretty much everything.
At least in this post you asked a question vs posting grossly inaccurate info as factual.
With the addition of 2 more channels in 7.1 lossless formats, there is a potential of 105dB peaks from each channel being redirected to the SW channel and added to the 115dB peaks of the .1 channel.
Although this is a theoretical possibility of actually occurring in a soundtrack, it is an extremely unlikely one because of the requirements for making it happen.
1) All channels in the 7.1 sound track would have to have all content in the subwoofer range.
2) All channels would have to have coherent content simultaneously.
3) The LFE content would have to be coherent with all 7 channels content.
4) all 7.1 channels would have to have the above required content encoded at 0dBFS (Zero Decibels from Full Scale)
IF that were ever to occur on a disc, here is how it would be tabulated:
Adding 7 coherent and equal levels adds +17dB. Adding that 17dB to 105dB peaks from each of the 7 channels = 122dB. Adding 122dB plus 115dB (a logarithmic formula) is calculated in the graphic above, summing to a 125.207dB peak.
Finally, that maximum level is at the Listening Position, not 2M Ground Plane (which is the equivalent to 1M Free Air). So, every doubling of distance from 1M to your Listening Position requires +6dB of output over your subs 2M GP or 1M FA output.
The requirement of your subwoofer system, in the never-gonna-happen theoretical 7.1 lossless format recorded sound scenario described above, is lessened by the phenomenon referred to as Room Gain. This phenomenon occurs in every room and has nothing whatsoever to do with the size of the room, how "sealed, as in air tight" the room is or what the rooms longest dimension is. Rather, it has to do with a) the sound transmission losses due to the construction method of the boundaries in your room subtracted from b) the gain realized by the reflections of the sound off of the boundaries in your room. Sound reflected off a boundary reflects back into the sound arriving later that hasn't yet reached the boundary. That interaction may be constructive (an increase in level), or destructive (a "dead spot" of no sound) and everything in between, depending on the wavelength and the distance from subwoofer to boundary. Sound pressure waves are spherical and the sphere increases in diameter as frequency decreases. So, as frequency decreases and wavelength increases there is less and less chance for the reflections to be destructive and more and more chance that the reflections will be constructive across multiple reflections off of multiple boundaries.
Since most every house built in the past 100 years in the US used 1 of 3 basic construction methods and your listening space may be predictably on one of 3 floors in the structure a simple formula can be applied to find the capability of your system.
As the metric for reproduction of recorded audio has been 20 Hz on the low end for decades, most electronics have been filtered below 20 Hz at arbitrary points for all of those decades to protect against DC offset (a bad thing). DC offset protection is easily accomplished by adding FR filtering capacitors on either the input or output ends of the analog signal. The more pieces in the signal "chain" (player, AVR, outboard EQ, amplifier), the more potential for an early rolloff in FR below 20 Hz because the roll offs are additive. The bottom line here is "If it ain't goin' in, it ain't comin' out". Obviously, if part of the spread of frequencies that make up this max SPL scenario is below the roll off of your system, you'll never be able to reach the max SPL because some of the energy is simply missing from the equation.
Finally, be aware of the limitations of a hand held SPL meter. If your in-room FR has a peak at 'x' Hz, the meter will register that peak and add everything below the peak. What does that mean? Let's say your FR at the LP is flat save for a +10dB peak at 40 Hz, induced by the room. If your SPL reading would have been 100dB, it now will read 112dB on the meter. Say you get your flat-to-2 Hz EQ in the chain and flatten the peak. Most SPL meters are 'C' weighted and roll off the low end dramatically. So, if you use a 'C' weighted meter and the content of the peak you're trying to capture has most of its energy below 30 Hz, your reading will be considerably lower than reality.
Out of time. Hope this helps.