I live in a bi-level condo. My living room is about 11' x 15', and extends to the dining room through the kitchen. When I first purchased my used piano, I placed my used piano on the wall against the stairs. This was because I thought that this would minimize used piano sound transmitted to adjacent units.

This turned out to be a disaster. Since the space behind the stairs wall was hollow, the tone quality of my used piano was terrible. Sound waves created from the used piano shook the thin wall, creating unpleasing sounds only to be described as drywall rubbing against another drywall.

I also heard echoes bouncing from opposite side of the wall, which made the whole used piano sound pretty awful.

I then relocated my used piano against the dividing wall, and purchased some sound absorbing panels. I put three small 1x2 panels right behind the used piano soundboard, and placed the 2x4 are placed left and right of the used piano. Same size panels are placed on the opposite wall as well.

This set up eliminated the echos and vibration I heard from the walls. But, it created whole another problem where I heard rattle coming from used piano on certain notes. Additionally, used piano sound was very muted since, three small sound panels absorbed most of the high pitch sounds.

Not satisfied with the result, I removed the three small panels behind the sound panel. My used piano sounded much brighter but the rattle continued. I put small felts on every contact point imaginable and lubricated the joints and tightened every screws I could see. Despite of this, the rattles continued.

Finally, I put the small sound panels below the big panels, and hung a large painting in the middle in between the panels. Believe or not, this set up worked. I have no idea how it happened not I can explain scientifically.

The point of whole story, I don't think there is a way to predict how your used piano will sound in your environment. There are so many variables go into it. Only thing I can say for certain, the sound panels greatly reduced the echoes in my living room. I can only say through my experience. Please contact me if you have any questions.