Originally Posted by Adamg (Ret-Navy)
All this talk about MV and calibration of one's AVR to a specific level. Something I wanted to discuss here. A point that is not always clearly communicated. At least not in my mind anyway. Please correct me if I get this wrong. Always open to learning and tweaking my own knowledge.
When you set speaker trim levels, either manually or by employing Audyssey, MACC and/or other automated calibration software interface. What you are doing is setting an "Average" playback loudness level for each speaker/sub. Not a Maximum or Minimum volume level. But an "Average" Volume SPL.
All audio content is dynamic in terms of volume at particular points of time in the Film. This Dynamic Volume is what controls how loud each channel can get on its own. Setting the "Reference" Average for each speaker is the baseline used to manage how low or loud your system will play at various scenes in a Movie.
So remembering this when you go to tweak certain channel trims, like boosting the Sub channel. Because it sounds too low at Reference using test tones. Reference test tone SPL is "Average" and can and will get much louder when playing actual content.
Having what we call "Reserve" or "Head room". This is another long topic. But the short of it is this. In a perfect world we are able to keep all AVR channel level Trims in the negative range. (-1 or more). Putting a trim in the positive rage can lead to clipping and/or distortion of that channel. This happens when the AVR tries to dive this channel to a dynamic loud scene and you are starting in a "positive trim" range at Ref average.
I see this happen mostly on Sub Level trims. On most subs, you now have two sets of trims to manage. The AVR LFE channel trim and the Sub Amp Trim itself. A rule of thumb here is you want the AVR LFE Sub Trim to be in the -4 to -8 range. This leaves adequate Sub Trim headroom. You use the Sub Amp trim to set the Ref Average Level. There is mush more about this in the Audyssey Part II Thread and in @mthomas47
Guide to Subwoofer Levels.
You asked me to comment on this. The issue of trim levels can be a little complicated, and the regular channels don't follow exactly the same best practice rules as the .1 subwoofer channel does. Any channel can distort if the volume demands exceed the capabilities of the speaker in question. But, it is low frequencies which may especially strain our system capabilities, in that regard, because it is the low frequencies which require so much more amplifier power compared to mid-bass and higher frequencies.
As noted, in your post, it is a good idea to always try to keep subwoofer AVR trims in the negative range--ideally at least -3 to -5. But, as I understand that, and as I quote some audio experts as saying in the subwoofer guide linked below, that is because you specifically want the voltage which powers your subs to come from the subwoofer amp and not from the AVR amp. The higher your AVR trim level is (and particularly above about -3) the more it is your AVR amp which is powering the subwoofer. Keeping the AVR trim level low for the subs, and using the subwoofer gain control to increase the sub boost, keeps the signal cleaner and helps to prevent clipping or other issues.
But, the situation is different for the regular channels. They are actually supposed to be powered by the AVR (or by an appropriate external amp) and there is nothing inherently wrong with having positive trim levels for those channels. This isn't really something that is subject to user control anyway. It will depend entirely on the relative sensitivity of the speakers, and on their distance from the MLP. When an AVR runs a calibration routine, using a 75db test signal, it will measure the FR of each speaker as it is at the MLP. And, it will set trim levels accordingly.
With a sub, if you don't like your trim levels, and want them to be deeper into the negative range, you can easily get them there by raising the gain control on your sub. That will increase the measured SPL of the sub and cause the automated calibration program to set a lower corresponding AVR trim level. But, we don't have a separate amplifier to allow us to do that with our regular channels, so our AVR trim levels for those channels are whatever they are. And, that is usually fine.
In my room, for instance, one pair of my speakers are (deliberately) 21' away from my MLP. There is no way, that I am getting solidly negative trim settings from that distance. But, that's okay. As long as I exercise reasonable bass management, I still have plenty of headroom to play as loudly as I want to. People who really want to play at Reference (0.0 MV) or higher, need to be mindful of both good bass management protocols and of the sensitivity of their speakers. But, for the vast majority of HT systems (and users), Reference volumes will be much too loud anyway, and the speakers will be perfectly capable of playing with sufficient headroom, even at positive trim levels.
Again, the whole positive/negative trim level dichotomy is based on the advisability of using subwoofer amplifiers for the subs, rather than AVR amplifiers, and it applies only to the subs. Of course, if someone has excessively high trim levels for the regular channels, and is exceeding their capabilities, then larger, more sensitive speakers may be required. Horn speakers are especially capable of producing high SPL, for instance.
I hope that this explanation is helpful.