In sync movie sound and image question

henrylrjr

Member
2012-10-26 7:49 pm
Ct.
Lets assume one is watching a movie and one scene is a view, from the perspective of a stationary actor, watching a loud, low pitched, slow moving locomotive coming into view from the distant right, then passing in front of the actor and continuing into the distance to the left. If the low frequencies are below the capabilities of your front speakers, wouldn’t a right and left channel subwoofer be needed to convey the sound moving with the image?
 

eriksquires

Member
2013-05-10 4:11 pm
Hi Henry,

There are two things at play. The 0.1 channel, which carries extra bass information for the movie, and the bass management in most receivers. The 0.1 channel has no association with any of the 5 (or 7) channels in the rest of the movie presentation. Your receiver however is sopposed to figure out what to do with the bass, depending on your configuration. Whether you have a subwoofer or not, how many, how many full range speakers, etc. The receiver needs to handle not just the 0.1 channel, but when you are not using full range speakers, it needs to cut the bass out of any small speakers and route it to speakers which can handle it, either full range or a subwoofer. In my home only my L and R speakers are full-range, all others cut off a little earlier than that, so my Center and Surrounds are set to a crossover of 70 Hz.

Below 120Hz (exactly where is debatable) bass becomes impossible to tell where it's coming from. You hear the overtones which let you know where the train is intended to be, but the bass itself could come from a single subwoofer tucked in a corner. The THX specification is conservative, and says that satellites should have a -3dB point of 80 Hz and be sealed. This allows for a 2nd order active high-pass filter at 80 Hz to be applied, and 4th order low pass to be used to feed the subwoofer(s). In practice most receivers and players like Oppo allow more flexibility in the crossover point, but you get the idea. My point is, 80 Hz is the standard, and we can reasonably assume that sound below that is non-directional.

Having said this, integrating a subwoofer well is a giant pain in the butt. Done well, like I finally got in my living room, it's impossible to tell where the sub actually is, and effects are localized by the individual speakers. Like I said, few have actually heard this done right, so few think it's possible. :D :D :D In addition to proper tuning, things like rattling picture frames or other room resonating items like dinner plates, etc. may destroy the non-directional illusion and should be dealt with first.

Best,


Erik
 
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Lets assume one is watching a movie and one scene is a view, from the perspective of a stationary actor, watching a loud, low pitched, slow moving locomotive coming into view from the distant right, then passing in front of the actor and continuing into the distance to the left. If the low frequencies are below the capabilities of your front speakers, wouldn’t a right and left channel subwoofer be needed to convey the sound moving with the image?

Hi Henry,

Interesting question. You may be interested in this paper:

http://www.filmaker.com/papers/RM-2SW_AES119NYC.pdf

FYI, I run dual-subwoofer system in linear-phase mode, and the woofers and ports are facing forward. The system has both: punch and seismic rumble, shaking the walls.

Best Regards,
Bohdan