Originally Posted by garygarrison
Here is my understanding (or misunderstanding) of the reasonable recommendation by THX and others to use a Main Volume level below Reference in a small (i.e., home size) room. While its true that smaller rooms increase the physical intensity of the sound, they shouldn't after Audyssey gets through with them. After Audyssey (or similar, professional) calibration to Reference Level is done in any room, small or large, they should have the same SPL, if everything worked correctly. Audyssey will have taken into account the reinforcing nature of the small room ("room gain"), and adjusted the volume to make the maximum peak SPL ("full scale") 105 dB from each regular speaker, and 115 dB from the sub, no matter what the room size. I think that's Reference Level, period. Any increased intensity due to small room size is turned down by Audyssey calibration until the test tones produce Referenc Level. But, small rooms have many early reflections, and are small enough to have standing waves within the audible spectrum. Both can be unpleasant --- unpleasant enough to sound "louder" than Reference to the human perceptual mechanism, but not to a microphone hooked to a standardized measuring device. Some of the early reflections in a home theater can be early enough to seem to be part of the original sound coming out of the speakers, and can alter the apparent frequency characteristics of a speaker. These early, often specular, reflections may well sound like unpleasant increases in SPL, just as increased distortion makes music sound louder. So, to cook up a worst case, if one were to put a high distortion amplifier in the line, and have many early and specular reflections in the room, everyone would agree that it sounds louder, even if we had adjusted the SPL to be the same. If that same SPL occurred in both a small and large room, most people would say that it was "louder" in the small, reflective room. So, on the average, the MV in smaller rooms should be set somewhere below where Audyssey sets it (Reference Level), not because the SPL would otherwise be too high, but because it sounds too loud at Reference Level in most small rooms.
Indeed, SPL comes from the world of physics, while "loudness" is a perceptual phenomenon, dependent on many factors. "Volume" originally referred to ... well, volume. The AV equipment and radios of the early 20th Century had a control marked "volume" to adjust to room size. The same SPL in a living room or a big hall would require very different volume control settings, because those rooms were of very different volumes.
I enjoyed reading your viewpoint on Reference volume, and I thought that you made several good observations. I particularly liked what you were saying about the impact of early (and late) reflections on perceived loudness. I think it is important to have a nominal Reference standard, even though we know that the standard is a maximum, and that not all movies are actually recorded using peak volumes of 105db for the regular channels and 115db for the LFE channel.
And, I also think it is important to try to explain Reference in simple terms, which I did in my post above, whenever someone asks on the thread about the relationship between Reference and Audyssey. But, in reality, the concept of an absolute Reference standard in cinematic production, much less in home theater, is much more complicated than that. Frankly, I among others, believe that Reference volume is a chimera--useful, but never the absolute that it is often taken to be. There was a recent thread on Reference volume in which I posted the following:
"I was late to this party, but enjoyed reading the discussion. There have been other threads in which several film mixers have stated that 0.0 MV (Reference), for movies they have mixed, sounds loud to them in their personal home theaters. This is for the small room effect reasons observed by Holman, and cited by LTD02. They consequently said that they usually watch movies at about -5 MV.
One additional point which has been skirted, but not addressed directly, is the difference between the sum of 7 speakers playing at 0.0 MV, and only 5 speakers. A 7.1 system should measure slightly louder at the MLP, than a 5.1 system, at the same MV, in the same room. I believe I remember reading that the difference would be about 2db.
I particularly liked Mark Seaton's post, and LTD02's posts, that explained that Reference volumes are actually a bit of a chimera in home theaters, depending as they do on so many variables, including things like size of room, room treatments, bass boost, and number of speakers. The Reference standard gives us a great starting point in calibrating our systems, and understanding their objective capabilities. But it is doubtful if any random two of us, listening at exactly the same MV setting in our respective home theaters, would measure, or perceive, exactly the same SPL."
Toole, Holman and others, have written about small room effects, which magnify SPL, and not simply the perception of SPL. And there are multiple variables which affect Reference level as any kind of absolute number. Audyssey is a useful tool for approximating Reference, but it is just an approximation. That is for at least two reasons which are independent of the room itself. The first reason is that Audyssey is measuring the SPL of each speaker individually, based on a roughly 75db test tone. But, Audyssey is never measuring the sum of the speakers in a system. And, the sum of the speakers in a system (each playing at an individual level of 75db) will be louder than 75db. Most audio experts I have read on the subject, attribute about +1db for each additional speaker in a system. So, a 5.1 system, calibrated with a 75db test tone, would natively play louder than a 2.1 system, at a Reference volume of 0.0 MV, and so on.
The second reason that Audyssey can only approximate Reference, irrespective of individual room factors, is the Audyssey microphone itself, which has an error factor of about +/- 3db. So, when Audyssey plays a 75db test tone, the Audyssey microphone could register that tone as low as ~72db, and boost every channel in a system accordingly, or the microphone could register the tone as high as ~78db, and cut every channel in a system accordingly. This has never been perceived as a problem, because the Audyssey microphone would be operating consistently for all of the channels, and would be achieving its intended goal of making each channel play at the same volume at the MLP. We also typically accept that an in-room Reference setting is really only an approximation anyway, and not an absolute number.
And then to name another major variable, which is independent of the Audyssey calibration, what about a bass boost? Any user who is boosting his subs above the nominal Audyssey setting is automatically playing at above Reference levels, relative to the other channels in a system. Some of that boost will be at frequencies which most SPL meters will have a tough time registering (depending on the type of sub, for instance) but the increase in total SPL will nevertheless be real. And, according to Holman, Toole, and others, that rising curve on the low-end is not only preferred by the vast majority of users, it is a natural result of the Equal Loudness Contours. Human hearing is simply not as acute in the very low frequencies, and more SPL is required to make those frequencies audible. It is entirely possible that some people hear those low frequencies more or less acutely than others do, and it is also entirely possible (highly likely, in fact) that some people enjoy hearing more bass than other listeners do. But if we increase the sub trim, the SPL is there, whether the individual listener entirely registers it all or not.
If anyone is interested in reading a really good discussion of Reference volumes, this is a link to the thread I mentioned: