If the ceiling fan is located where the Arc or Era 300s reflect sound off of the ceiling, it will have a negative effect on the height channel audio, even if the fan is turned off. I would advise against installing a fan.

You could make a case that any object, even a hanging lamp will have some negative effects on audio, surround, or Atmos, but it will be minimal. I’d throw a non operating fan into this class, but you could argue that a fan is larger and more complex than a simple lamp.

An operating fan is much more involved in the audio. A Leslie is essentially a fan in a box, used to deliberately create sound effects.

how far is the fan from the wall where the speakers are?

Wonder wich is the audio cone of the firing up driver.

For instance if the fan is 1,50 m from the speakers at the back of the listener , maybe the reflection is not disturbed.

The fan will be center between the Arc and the listener. The distance between the Arc and the listener is about 2.2m. I would estimate a good 1m between the Arc and the centre of the fan. I believe the fan will be in the way of the sound cone. Cheers!

I seem opinions similar that a non moving fan may not have too significant impact. But no one actually have the experience to get a reasonable confident statement. Cheers!

Would be interesting having Corrys opinion.

The significance of a fan will be listener dependent. Similar to the horror (in some opinions) of dumping condiments onto a world class chef’s creation, “best” sound is in the ear of the listener.

Quick update on this if anyone is interested. I have posed this question direct to Sonos. They are confident that a ceiling fan should not affect the spatial sound, but advised to run trueplay first. The support person sounded confident but I am still slightly sceptical. But who am I to challenge them. I hope they are right.... Thanks everyone who has responded to this question.