Is Arc or Beam the better choice for this shelf?

The Beam would be the better choice. There isn’t enough clearance for the Arc.

Even the Beam would be a bit compromised because it has side firing speakers, so putting it in an enclosure is not a preferred position. It just would not degrade the experience as much as it would do on an Arc.

Thank you both so much! I would have probably purchased the Arc, so I’m glad I asked :)

I don’t know how difficult it would be for wiring. If possible I’d try to figure a way to raise the TV and mount an Arc or Beam below it. As @106rallye stated the performance of the Beam will be degraded.

To me it doesn’t make sense to purchase a speaker designed to deliver Dolby Atmos and cripple it due to improper placement; then later complain about the performance. If you intend to move forward with your  placement I recommend the Sonos Ray; which is a better choice.

Read the official Sonos recommended use for the Ray that actually fits your placement scheme by clicking the link:

https://www.sonos.com/en-us/shop/ray

@AjTrek1 thanks for the response, I really do appreciate it. I can’t raise the TV because the indent in the wall fits exactly a 77 inch TV, and I’ve already purchased that size TV. So mounting a soundbar below the TV, unfortunately, isn’t an option.

I am planning to make this room a home theater with a sub and two Era 100. Does that change your recommendation to get a Sonos Ray? I worry the Ray might be underpowered with respect to the other speakers. Knowing that this will be a home theater setup, do you still recommend the Sonos Ray instead of the Beam?

How big is the room? Ray is meant for smaller rooms but still would fit better in the location you’re proposing.

If you really want a good HT room, why not mount the tv and Arc on a different wall? Use that space for your Blu-ray collection? 
 

Rather than sitting the soundbar inside one of those shelves, use a TV bracket that can mount the soundbar at a proper distance below the TV. The soundbar will then be in front of the wall, not trapped in an inappropriate shelfspace.

Indeed hang the soundbar underneath the tv in front off the shelf with a mount like this

OK, here’s the down and dirty 😅…

The suggestion(s) by @buzz and  @Eric1989 to use the integrated wall mount and sound bar hanger is by far the best scenario*. The design would allow the full potential of the Arc or Beam to be realized with Dolby Atmos movie audio. Besides the Arc offering the best experience the major difference is its upward firing speakers which the Beam does not have. Therefore, the Beam is slightly less effective with Dolby Atmos content.

If you stick with placing a Sonos speaker inside a shelf the Ray is the better choice. The Ray will match perfectly with any Sonos sub and the Era 100’s as surrounds. It’s the only speaker per Sonos marketing that is “designed to be tucked away”. Click here for more info.

The Ray requires an optical connection to your TV.  It does not accept a direct HDMI input of any type (i.e. ARC/eARC). Your audio will be Stereo PCM, Dolby Digital 5.1 and DTS Digital surround (depending upon audio content). 

To answer your question…the Ray is not an under-powered speaker; especially so considering your intended setup. The Ray is more focused to push all audio content forward to viewer/listener. The Beam on the other hand would not offer that type of emphasized directional audio push. 

Most retailers offer a 15 day return policy…Sonos is 45 days. If you don’t mind “fishing” cables behind the wall to connect HDMI to a Beam and later optical to a Ray; you might consider auditioning both. By doing so you can decide if the Ray or Beam will sound better in your environment.  

I’d suggest you listen to Dolby Atmos encoded material to determine how much degradation is experienced by comparison of the Beam to Ray. The Ray will downgrade the audio to Dolby Digital 5.1.  If you perceive no difference, then the Ray would be my choice based upon cost; barring connectivity.

I realize you came to this community hoping for advice to make a clear-cut decision to acquire an Arc or Beam.  As you can see Sonos designs speakers not just for listening enjoyment; but for certain use scenarios.  Your intended mounting scheme has brought other parameters into play that makes the decision not as clear-cut as one might have hoped.

Oh…one last thought about room size. Unless you intend to employ the suggested wall mount and sound bar hanger…room size is irrelevant (unless you intend to re-think your room choice).  A Beam or Ray placed in an enclosed shelf levels the field for both.

 * Click here to see 3rd party mounts

@nik9669a The room is 20’ x 14’ with the TV on the 20 foot wall. Would Beam or Ray be better for this room size?

This is the only wall (in the photos above) that will accomodate my TV.