I just wanted to coment on some of Roger's last post about bass management here. I am sorry this got far longer than intended. I hope you read it as intended. This is not about DOLBY ATMOS, as I feel it applies to any multi channel sound system. All my opinion from 30+ years of tuning rooms, your feelings and experience may differ.
Carefully used Bass Management can certainly work very well. The problems we run into are very inconsistent quality of sub woofers with wildly varying amount of distortion and even frequency range.
The older IMAX film format was actually a 6.0 system with all 6 channels bass summed into a common hue sub woofer with about 10 db on in band gain. So if you fed any of the 6 channels with flat pink noise, you would measure pretty close to X-curve from 16K down to around 100 Hz or so, but from there down, it would ramp up to + 10 below 80 HZ from the common sub. Play correlated noise into 4 channels now and the electrical summing makes the low end go to more than +20 db while the mid range is at just +6. But for IMAX mixes this worked just fine because they were mixing the tracks knowing this was the room they were going into. But if you take a 5.1 mix into an IMAX room, it turns into a pile of monster bass. And here is the problem. Noone is mixing in a room with bass management like that for normal cinema. On that 1812 Overture recording you talked about.. are the canon blasts mic'd with 5 mics, or was it mono mic'd and panned across the 5 channels, or something in between? This will greatly change how the bass summing will mix. Was the mix done on a system using bass managemant also? Was the room where the mix was done, and where it was played both Eq'd to the same curve and cross over point to the common sub-woofer?
All these questions and many more make it difficult to keep the results consistent across hundreds, or thousands of rooms of different sizes and shapes. Of course there are many people who like a bit of extra bass, so having the bass summing create the artificial boost makes it sound "better" than the creators original mix. We have even had some content creators admit that they liked an artifact like that, even though they knew it was not faithful to the original mix. My personal view is that I am in the business of RE-production, not PROduction. Once the content creator signs off on how it sounds on the dub stage, it is my job to make it sound IDENTICAL in the final theatre environment, not richer, crisper, boomier, etc. I have had some excellent results in smaller rooms using bass management when the owner chose to use some smaller main front speakers, but it took extra tuning time and some trade offs that made it sound a little different than the original intent.
When I do have to make a compromise on a system setup, the goal is to make any deviation from perfect playback be on the side of "pleasing to the ear". So in most cases we end up with a bit more bass in a bass managed system, but it is not as accurate to the original mix.
I love huge dynamic range when used well, but there have been some recordings that are just plain silly. I am sure you will know one I am speaking of. It is a 2 trk version of the 1812 Overture with real mic'd canons, but the producer made a choice where the canons are 40 db louder than the orchestra playing the music. To me this sounds like it was recorded to be the point of view of the man standing at the canon to fire it, while the orchestra was a half mile away. I was not the mixer or producer, I try to play it faithful to the creator's vision, but I just can't enjoy this version myself. I know that has little to do with the discussion, but you mentioning a recording of the 1812 Overture made me think of how much that recording bothered me. Just because we can make those canons that loud does not make it a good choice. There is a schene in a film called "Smokin Aces" where a girl fires a 50 cal. In one theatre, I felt it bounce the entire back wall of the auditorium over an inch from the pressure wave. Yet it was far more pleasing than those silly 1812 canon blasts. I screened that film at 4 different locations, and no two sounded alike. It was a very good use of the LFE track, but it also showed how difficult it is to match the lowest octave performance. This was not a case of low end responce, it was more about the attack speed of the pressure wave on the first cycle. Ported cabinets do not have any resonanse help on a shock like that. I never did get to hear it on an infinate baffle or Bag End sealed system, that would probably stop your heart for a second from the pressure pulse. As it was the bank of 18 18 inch woofers felt like getting punched in the chest, even at about 100 feet from the speakers. In a much smaller room, a bank of 6 18's with over 3000 watts behind it just didn't have near the thump on your ribs, even though my analyzer showed nearly identical 1/12th octave measured frequency response to well below 20 Hz. In fact, the thump seemed to have far more energy up at 80 to 100 Hz. In any case, this kind of a sound could never be produced from the main channels due to the 10 db gain difference below 125 hz. This is why I think the idea of 3 sub woofer feeds will work much better. Keep the screen speakers separate, use a dedicated high output LFE subwoofer, and then use smooth no in band gain bass managed sub woofers to augment the smaller surround speakers. The signal and demand placed on the two systems are quite different.
I also do understand that 60 cycle buzz will have higher harmonics, but you have to agree, even the 60 Hz sine wave can be located by human ears. True, it is not as easy to localize, but I know many people who can walk right to the source of a clean 60 Hz hum. I wanted to show an obvious example where bass summing would detract from a directors vision.
I am not saying to never use bass management, I agree it can be helpful, but I also feel it is over used and set up poorly far too often. I would rather have independant screen channels with a smooth roll off around 30 Hz rather than bass manage them below even 40 and go down to 20 Hz. Of course that is my personal preference. In the end, I will install what the customer wants and tune it as accurately as possible to re-create the original intent of the content producer. And in more than a few cases, the system owner then went in and cranked up the surrounds and low frequency levels to their taste. Oh Well, might as well pay for the ISF display calibration then crank up the blue to make the sky and water prettier too.