Turns out one of the horn players in class has an older whisperroom and he uses it for soft practice at night with his tenor. Brought the doghouse over for a test and it looks very promising. Managed to squeeze into this 3.5'x4.5' interior and it actually had a little room to spare. Comfy inside and feels like a private studio. His setup was the whisperroom inside a smallish room (almost like a closet) with a door which he closed while practicing. So it's a full on room-within-a-room setup. The building was old SF construction - almost victorian so the walls are probably not all that thick/insulated.
The verdict: it seems to work pretty well with just the single wall version. I played a blues while pulling on the strings fairly strong (stronger than I normally play) and he said it sounded very soft (read not bothersome) in the adjacent rooms and completely quiet on his porch. No mute no nada. I won't hesitate to get one if I need quiet.
FWIW: I did the same for him while he played tenor. His horn cut right through and was pretty easy to hear when he played loud. The whisperrom cut his sound in half basically but was still easily audible throughout his flat. He practices before 11pm so as to not bother the neighbors and plays softer too which helps.
Now I'm kinda kicking myself I didn't get that instead of the Yamaha EUB. hrm.
I think this is definitely the way to go.
