Okay, this one is perhaps a tad nutty but bare with me a second. I came across a Reddit post where they claim with streaming music in ‘Ambient’ mode (Surrounds), they hear different program material perceived on the rears but not happening on the front ‘stage’. Replies were - they are clearly nuts or yes, DSP is tweaking the surround image. 

I assumed Ambient was simply rolled off a bit where Full meant the rear volume was hotter. Both being mirrors of L/R channels. In ‘Full’ that’s exactly what it sounds like. But… in Ambient, things seem to change. 

Standing in the center of the room, I caught what he’s talking about. I hear seemingly panned rear reverb trails and isolated bits like stick taps on the high hat from the rears only. Sometimes backup vocals are heard rear, not present or are weaker in the front. At first I thought it may be different size enclosures, different tweeter responses, reflections or placement. But in Full or when TV audio plays in PCM 2.0, it sounds mirrored as expected.  Nope, rule out the box difference doing this. 

I watched a YouTube with the Sonos CEO and lead engineer. They discuss DSP used in their products to ‘make a small box sound big’. I think there may be a splash of "pseudo-acoustic" treatment and/or “psychoacoustic” processing going on here with Ambient mode. They quite proudly use it in the Beam. I have a Harmon Kardon info unit in my car where if you increase the Surround’ effect, it sounds very similar to the noticeable stuff I catch in rear Ambient mode.

I know Sonos claims no ‘Surround’ effect is artificially generated using a TV input. That’s a good thing for movies. Sports fans may disagree as Yamaha DSP can sim a way larger stage er, stadium. Pretty sure there’s pseudo/psycho stuff going on there. The Psycho element could be on the listener (me) in this case, but I swear there’s trickery at play. 

I believe they may have snuck in some DSP time-space enhanced goodness for music. Specifically used in Ambient mode. A dash of delay, a tad of psycho, with a sprinkle fresh pseudo spiced to taste.