One Sonos Era 300 as surround for Arc Ultra

As has been said you need two speakers to create surround effects.

The Arc Ultra is designed to produce an Immersive effect when watching movies with Dolby Atmos sound tracks and/or music recorded in Dolby Atmos. It also does equally well with Dolby Digital 5.1 sound. 

The main difference between the Era 300 and Era 100 is that the latter is a stereo speaker. The Era 300 is a Dolby Atmos; as well as a stereo speaker. 

Using the Era 100’s as surrounds the Dolby Atmos effects will all be processed by the Arc Ultra. You’ll still get an immersive effect with the Era 100’s; but not FULL immersion.

Using the Era 300’s you’ll get FULL Dolby Atmos immersion as they will out-put Dolby Atmos effects that are contained in the surround channels of a Dolby Atmos movie thus complementing the Arc Ultra.

Bottom-line

• If you watch a lot of Dolby Atmos movies typically via movie streaming services, AppleTV and streaming sticks, the Era 300’s are the best choice.
• If you watch mostly cable TV which is typically stereo and/or Dolby Digital 5.1 the Era 100’s will serve you well.