Hi Peter, I like the sound with the curtains open. And, in reality, they cover a fair portion of that entire wall even when open.
When Bonnie visited, she said she could work with the wall with the glass, basically she could acoustically model with one reflective surface. The rest of the room is "treated" essentially all the walls are BAD panels using a computer generated random pattern of various sized holes. My room thread "Wilson & ARC for me" described that build out including pictures of the guys drilling the wall panels on-site. Then the fabric we chose to cover the walls was sent to Bonnie for testing so it would be incorporated into her algorithm. I'm sure our rooms sound very different—mine is certainly more 'damped' than yours. Folks will form their own opinions, but those who've visited generally agree its the best room they've heard. (I can't speak to the size of their sample set!)
Back to the curtains... these, and those in the rear, contains Lumitex acoustic fabric. (
http://www.soundsense.com/wp-content/uploads/2019/10/Lumitex®.pdf) The front wall curtains contain a single layer, those in the rear 3 layers. The folds in the fabric provide some diffusion and some absorption. Drawn across the glass, the room/sound loses sparkle. There are also some translucent Hunter Douglas honeycomb blinds that can drop in front of the windows. If the outside is really bright summer sun, sometimes the contrast vs. inside is too much for my eyes. These are essentially plastic film, honeycombed, and reduce the glare. They seem to have minimal effect acoustically. But... that is a rare day I am in the music room middle of a bright sunny day.
View attachment 83139