Very real problem indeed. People are spending significant amounts of money to get equipment with a little bit better signal/noise ratios, but don't worry too much about ambient noise, basically rendering the investment pointless. Just try listening with/without the AC on, you will find a very important difference. In my case I found that my AC (2 hp) produces around 45/46 dba of noise at the listening position to cool down the 72M2 room. So when listening at 65-75 db level (listening position) the dynamic range would only be 20-30 db as well. Therefore, I have created a solution in order to reduce the AC noise significantly by using a false wall around the AC, but divide the upper part (=hot air intake) and lower part (cold air outlet) at the AC unit. (intake opening at the top, outlet at the bottom). I did create a channel inside this "box" so that the produced sound has to get around the corner a few times before being able to enter the room. Please note that this "box around the AC" is quite larger than the AC itself, so the air intake and outlet openings are much bigger than the original once at the AC unit. Air velocity is significantly reduced, hence not too much noise is produced. This works quite well, the room stays cool, and noise level has reduced by about 10 db at the listening position. The false wall has been made using 18mm plywood plus an 8mm thick foam finish, and is removable to allow for maintenance of the AC. I realize that this may not be for everybody, as space may not be available, work execution may be difficult, as you will have to do it yourself or hire a handyman. However, if you DO have the space, this is an investment (possibly a few hundred dollars) you won't regret.