If I have calculated this correctly, if you are 5m from the speakers it would only take 25ms to cause an audible echo then.
I'm not sure I follow that exactly, can you elaborate a little? The distance between the speakers is the key issue, along with one's positioning in relation to them. If you were 5m from both speakers you'd still need a minimum of 40ms of drift to get an echo. It's also important to stress that 40ms is about the lower bound for detecting echo; some people can't perceive them until you get closer to 80 or 100ms. But 40ms makes a useful target for discussion.
This is why I wonder if it actually matters to have properly synchronized multi-room audio at all.
Obviously the sync errors would be noticeable if you have all units playing together in the same room - but if they are in separate rooms, surely the distance between yourself and the speakers is going to cause echo effects anyway, and that will change as you move around?
It depends on the setup. For example, my kitchen sound source is about 5m from my living room sound source. You're right that, even if they were synced flawlessly, there would be still be
some drift between them, that's unavoidable due to physics. But bear with me for a thought experiment.
Imagine a circle that has both of the sound sources on its perimeter. If I'm outside of the that circle, there would be drift due to distance, but it would be, at most, about 15 ms because 5m is the maximum differential distance that sound will need to travel to reach me. But if I'm inside that circle, the unavoidable sound travel delay will be highly variable, but always less than 15ms because I will never be more than 5m from either speaker.
So in my 5m separation case, the maximum unavoidable drift is 15 ms, which is not enough to get an echo. If there were a hypothetical, magic, perfect zonesync available, I would never experience echo with that setup no matter where I was.
Now let's take a different example: imagine the speakers are 13m apart, and imagine my imaginary circle with both speakers on the perimeter again. If you're well outside of the circle, the sound from one of the speakers will be delayed by as much as 40ms which means you'd probably perceive an echo. But you'd also be more than 13m from one of the speakers and SPL loses 6dB for every doubling of distance. So if you were 1m from one speaker and 14m from the other, the output from the second speaker would be more than 20dB quieter than the first one (assuming no obstructions; with obstructions, obviously much lower). That's not imperceptible, but it's low enough that it might not be distracting.
On the other hand, if you're inside the imaginary circle, you'd typically have less than 40ms of unavoidable delay. That means with my hypothetical magical perfect sync device, you probably wouldn't experience echoes when in between the speakers, etc.
Obviously with greater distances, avoiding echo becomes increasingly hopeless, but the echoing sound also becomes increasingly inaudible.
At a lot of the distances one would experience in a domestic setting, really strong syncing would effectively avoid echo. As a practical illustration, at the start of a track with zonelink engaged, I don't hear echo in my setup. And the problems of sync-drift become more pronounced when you have three or four sources playing in a relatively small area.
Sync-drift is most noticeable when sound sources are closer to one another, and those are the exact circumstances where strong sync would be most likely to eliminate echo. In a small open-plan house, the sync drift/echo can be noticeable if you're moving around (ask me how I know). In a large house, with good separation between sound sources (e.g. three setups on three different floors with distances in the tens of meters) sync is almost irrelevant because you're very unlikely to be able to hear multiple outputs at once to begin with.
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Topic: Linked zones and audio drift (Read 9441 times)