[ericHosick]

I would like to make a 120 square foot (internal dimensions) max 10 feet high roof to foundation listening room in the back yard. Very mild climate so heating/cooling is not necessary. Wall thickness is not an issue.

Do you think that is possible while still getting good base response and have minimal sound escape?

[Slow_Hand]

What is your intended purpose for the room? Listening to records for pleasure? Critical listening for mixing and editing audio? Something else?

My first rule of thumb is don't make the ceiling 8' high. 12' high is ideal, but 10' high works too. When people sit in a chair their heads are located 4' from the floor, which means that with an 8' ceiling their ears are located halfway between the floor and the ceiling. This is the primary vertical room mode and will cause your low-end to cancel itself at 70Hz right off the bat.

A solid concrete slab in the floor is ideal so that your floor doesn't resonate.

120 sq ft is relatively small and not ideal, but if that's what you have then so be it. What you DON'T want is for your room dimensions to be square. The more rectangular that you can make your room without the length and width being integer multiples the better. Don't make the room 11'x11'. Make it 9'x 14', or something in that neighborhood. This will distribute your room modes more evenly, whereas a square room will have the same modes along the length dimension and the width dimension. This will give you a massive cut and boost at in your low-end.

If your speakers won't be soffit-mounted then place them as close to the wall as possible. Some people may advise you to place them 3' off of the wall because of a phenomena called Speaker Boundary Interference Response. Ignore this and place your speakers against the wall and save yourself some trouble.

Lastly, you don't want your listening position to fall on a position that is 1/2 or 1/4 of the length of the room from the front to back wall. Those are where your largest room modes (and therefore largest bass cancellations) are going to occur. Ideally your listening position should fall at the point 38% of the room length between the front wall or rear wall.

Unfortunately it's hard to advise you further on the room treatment and isolation without discussing the project holistically. Your room dimensions, speaker choice, level of isolation, room treatment, listening position, and furniture all interact with one another and I need more information.