LAW OF TWO-AND-A-HALF (Walter Murch)
Which sound designer hasn’t tried to sync a lot of the same objects in a scene to conclude it turned out to be a mess? I did once with a visual effects shot in which dozens of light beams graciously flew out of a body in less than 3 seconds. Every one of them had different trajectories, volume and speed to which I wanted to associate different tones, volume and EQ’ing. It sounded great in the first 2, not bad with 3, just some small adjustments I needed to make to provide variety, but I couldn’t advance further without sounding too confusing and eventually I lost the initial purpose. How could this be possible if I was spending so much in detail? I tried later again in this logo intro:
It didn’t work again so I treated the coloured light beams more as a group evolving in time instead. And why does this work better?
On his lecture Dense Clarity – Clear Density, Walter Murch points a similar example when, back in 1969, syncing the footsteps of the robots on George Lucas’s THX-
1138. He notices how there is no need to sync footsteps perfectly if there are more than 3 similar character to sound design: “any sync point was as good as any other!”
“Somehow, it seems that our minds can keep track of one person’s footsteps, or even
the footsteps of two people, but with three or more people our minds just give up – there
are too many steps happening too quickly. As a result, each footstep is no longer
evaluated individually, but rather the group of footsteps is evaluated as a single entity,
like a musical chord. If the pace of the steps is roughly correct, and it seems as if they
are on the right surface, this is apparently enough.”
This clearly extends to other areas as well. The balance relies on our mind’s ability (or limitation?) of perceiving the various objects as a group instead of focusing instantaneously on each element, going along with the Gestalt Theory of Proximity or Grouping. This theory states that objects that are close to one another appear to form groups, even if there are differences between them. Their collective presence becomes more meaningful than their individual identities, thus Murch calling “the chord” to the “feeling of the whole”.
This Law seems to apply only to elements with the same “colour” (“sounds from the same part of the conceptual spectrum”). Walter Murch nick-named this law after concluding that the “balance point occurs most often when there were not quite three layers of something”.
http://www.youtube.com/watch?v=pgrt8H4DucI&feature=player_detailpage#t=746s
Melissa Pons 
This is interesting, however, it may have to do with the interval between sounds more than the number of people walking. Note that sound events that come too quickly always stop at some point being perceived as individual sound events – sound pulses that become too close will begin to sound like a tone. In a similar way, our ears (brains) can’t identify individual echoes that are too close – we perceive this effect as reverb, rather than individual echoes. This seems to happen around 40-50ms intervals or so.
When synchronizind sound with moving images, another element comes into play: the moving images are, in effect, fixed images fired at a rate of ~50 per second, i.e., one every 20ms.
It does seem like audio or visual events separated by less than 20-50ms get blurred in to one by our brains.
I would speculate that if the mis-synchronization between video-audio is in the range of 20-50ms, it will be difficult to detect.
Great point! I think those two factors come to play here. I’m doing the sound design for a short cut in which there are several people (all the same) walking in the street in various directions, some closer and some more on the background. I’ve been noticing that is easy to percept individual footsteps of a character at a point where they are at the extreme panning of stereo image (figure / ground) , if I try to synchronize some 3 or 4 in the background, it starts to get blurry; so there’s a need of playing with good timing and taking advantage of the space. Thank you for your comments, keep it coming!