Home Theater Sound vs Cinema Dynamic Range
A while ago, an interesting discussion on perceived dynamic range at home vs. movie theaters arose on the Axiom forums, prompted by a question from "bdpf" (Bruno), one of our forum regulars. I responded on the forum and noted that Iíd ask Dr. Floyd E. Toole* for his comments. Floyd replied with comments on my response as well as a further general response to Brunoís question. He also sent my email and his response to Dr. Sean Olive*, who also commented. The discussion is enlightening and thought-provoking and I thank Dr. Floyd E. Toole and Dr. Sean Olive for their contributions.
Brunoís question (slightly edited):
It's something I never understood. Why, in home movies, DVDs or Blu-ray, is there such a big dynamic range? You go through a soft passage where you can barely hear the dialog and a second later... BANG! . .the whole house is shaking with extremely high volumes. The reason I say home movies is because this high dynamic range doesn't seem so high at the theaters for some reason---or at least not to me. The dynamic range of movies viewed at home seems to be much more extreme than in the cinema.
For clarity, Iíve labeled the responses with initials (AL), Floyd (FET) and Sean (SO):
AL: Bruno, there's no question that the dynamic range of Dolby Digital soundtracks on lots of special effects blockbuster movies is too great for a lot of domestic environments. The mix is certainly the same as the one used in commercial movie theaters. I think there are a couple of factors that somehow make the huge dynamic range more acceptable in a large cinema than at home.
The size of the space and number of bodies in the seats plus all the absorptive upholstered seats in big theaters somehow "soaks up" the dynamic range better. At home, our relatively small rooms just generate too much immediate and very loud reverberant energy that can't go anywhere and get absorbed, so it's just too much acoustic energy impinging on our ears.
FET: Actually it is probable that the reverberation time in typical homes is lower than that in typical cinemas. Some of what we experience may be related to expectations -- big room, big screen, big sound: OK. Small room, small screen, big sound: maybe not. Personally I have never had a problem, but I never play movies at "0" dB "reference" level. Most people play movies several dB lower at home.
AL: Also, the ambient noise level in a house from the heating/AC system plus a refrigerator/freezer and outdoor city urban noise is usually significantly higher than in movie theaters. Cinemas are quite well insulated to keep the ambient noise at a very low level so the available dynamic range can be played without having the peaks at deafening levels.
FET: When I saw "Avatar" at an IMAX, I could hear the air conditioning units on the roof, and hum from the amps through the loudspeakers -- cinemas screw up too! The bass was obviously "one note", but I don't know whether that was the playback or the recording. Have you ever noticed the similarity in explosive sounds in movies and on TV -- I think they all use the same "boom" clip.
AL: At home, ambient noise causes us to shift the range higher so the softer sounds can be heard above background levels, then when a peak comes along, it's way too loud.
FET: Yes, this could happen, but the background noise would have to be quite high -- and I guess in some homes it could be.
Finally, FETís formal response to Brunoís question, followed by SOís comments:
FET: First of all we cannot assume that the soundtrack on a disc and that in the cinema is identical -- the sound on the disc has gone through an additional mastering operation during which an engineer has the opportunity to alter a number of variables, including dynamic range. We don't know what, if anything has been changed, which makes discussion of this observation a bit uncertain. . .
However, assuming that the soundtracks are identical, there is another variable -- the audio system in the cinema itself. If we are talking about top-line cinemas the dynamic range and bandwidth can be substantial. If we are talking about older cinemas or your local multiplex, the audio system may well not be up to the highest standards in terms of being able to reproduce the lowest frequencies, or the highest sound levels, resulting in less "impact" during big sound effects. So-called "standards" for setting up cinemas are sadly inadequate, and individual operators are sometimes known to go in after calibration and set things the way they like them. Frankly, if you have truly good loudspeakers, including subwoofers, at home, and have properly set up your own system, you may well be hearing something better, and closer to what was intended than you might in a garden-variety cinema.
Thinking on the "dark side" perhaps the home system is overloading (amplifiers and/or loudspeakers), causing distortion, clipping, or other sorts of unpleasant misbehavior during crescendos.
If your home system is up to standard, and properly calibrated, and you don't like the "startle" effect of a wide dynamic range, that is a totally different matter, for which there is no technical solution.
SO: I donít have much to add except that in homes there are other factors that may come into play that influence the "preferred" dynamic range of movies.
Differences in active attention of listener Ė in cinemas, you are usually giving your full attention to the movie; at home we are often multi-tasking and playing movies at lower playback levels. In this situation, the wide dynamic range may be a distraction to multi-taskers and present a problem with speech intelligibility at lower playback levels. Solutions like Dolby Volume and their dynamic range limiter (for late-night viewing when you donít want to disturb the neighbors) may be a solution for this situation. I have not actually tested Dolby Volume so I canít comment on how well it works: http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/dolby-volume-tv-techpaper.pdf
Floyd is correct that there are specialists who remix films slated for DVD release. Bob Margouleff and Brant Biles at Mia Casa do this primarily as their primary work: http://www.micasamm.com/newsite/
In Zacharov and Bechís book, "Perceptual Audio Evaluation," they claim that for most program material, the preferred playback level for most people in their homes is about 10 dB lower than "reference level" when their system is properly calibrated.
So there you have it. If readers have further comments, please post in the Axiom Forums.
Dr. Floyd E. Toole is the former vice-president of engineering for Harman International, a former president of the Audio Engineering Society, and for 25 years was an acoustical scientist at Canada's National Research Council. He is the author of "Sound Reproduction: Loudspeakers and Rooms".
Dr. Sean Olive is the Director of Acoustic Reseach, Corporate R&D, at Harman. He did extensive work at Canada's National Research Council with Dr. Toole prior to joining Harman. He authors a blog, Audio Musings. ---
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