I can tell you exactly how accurate it is: extremely accurate.

Sonos invented wireless multiroom audio and has patents for the technology that ensures effectively perfect synch.

I know that doesn't really answer your question........

Somewhere I read they had got sync to significantly less than 1ms (which is the equivalent of moving one’s head by 30cm/1ft). I suspect it’s a lot better than that in stereo pairs, as any substantial timing offset would soon be apparent. There may be the occasional slow drift of the soundstage but if so it’s pretty imperceptible, and smoothly corrected.

Sonos sync works by distributing the audio asychronously, then coordinating buffer playout via a shared clock. I’d imagine that the professional systems mentioned are synchronous.

Thanks for the replies guys. If I have some time I will make a measurement with pulsed sweep signals and record it with microphones directly in front of the speakers and update you!
 

Make sure to use identical speakers or you may well be measuring amplifier, crossover or speaker delays.

I miss the days we could see our Sonos internals, that made them a lot more fun to play with.

I use 2ms as the synchronization estimate, but it is probably better than that on average. Stock up with sandwiches and coffee, find a comfortable chair, and Group two speakers playing the same mono source. If you are very observant, you’ll hear some gentle shifts in the image from time to time as the system recalibrates. Another way to approach this is to play a mono tone and place the speakers face to face, almost touching. When they are exactly in sync, they will tend to null each other. Over time, you’ll notice that they’ll slowly drift out of sync, then correct. It’s also interesting to do this with a stereo pair. Actually, once they had Group synchronization working, stereo pair was a cheap trick.

I have found my Echo devices to sync just as well as Sonos does, and by accounts that I have read, so does Bluesound.

So, two questions:

1. Are they using a different approach to achieve the same results?
2. If not, are they paying license fees to Sonos, seeing that Sonos has sued neither, as far as I know.

Likely, Google uses SONOS technology, as does HEOS.

And Lenbrook/Bluesound too.

Been a long while but I seem to recall Sonos gear accessing an NTP server, don’t see that in my DNS logs and too lazy to put in a firewall logging rule to monitor NTP and see if they are still using it.

That shouldn’t be patent-able but the way they synchronize the music being sent to the amplifier probably is. I’d guess Sonos patented the way they do it, and as many alternate ways as they could think of. That seems to be pretty standard practice to keep your competition from simply working around your patent.

External NTP access is required for straightforward setting of the system date/time. Internal sync is by regular distribution of a local clock using SNTP.