Q: Why is THX so adamant about having only one source in a multi channel system producing bass below 80Hz while also having a fixed 80Hz global crossover setting? Our blind listening tests revealed that placing a subwoofer near a listener is localizable when crossed over above 60Hz (24dB/octave). In addition, many users enjoy large bass capable main speakers and prefer the advantages of stereo bass while also supplementing their speakers with a subwoofer and adjustable crossover setting. Higher crossovers limit where the subs can be placed to properly cross to the satellites and not be localizable.
A: Here's another of those "how much rope" questions. 80Hz is a very thoughtfully and scientifically chosen compromise, but is indeed compromised for some people in some situations. An interesting and useful experiment is to listen to a sub by itself, playing program material at various levels, moved from place to place with the crossover changed from point to point-all done blind. The listener cannot know ahead of time what he's listening to. Other crossover points can work just fine but place more restrictions on the system than 80Hz.
Some of the observations/issues are as follows:
Unless you're sitting right on top of it, a sub properly crossed at 80Hz, 24dB/octave and free of stray resonance, rattles, port chuffing, coil bumping and the like cannot be localized. 160Hz is the lowest frequency that is consistently localizable by most people in most situations.
Lower crossovers require larger screen speakers that load the room's bass modes at inflexible locations to the left and right of the screen. This makes it more difficult to get smooth bass in all the seats in the theater.
Larger speakers are sometimes more difficult to place properly in relation to the picture, particularly the center speaker, which is the most important speaker in movie sound.
