“You have to open a door or window to let the pressure out of the room.”

Don’t try that if you’re listening to headphones on a plane lol.  

Although opening the windows or doors would release some sonic energy, the resulting sonic change would likely be negative including likely raising the noise floor by adding outside sounds

The only benefit of an open door in an audio room is that it prevents some of the sound from reflecting off the door that used to be there.

No walls at all would be the optimum situation. That's not achievable in a domestic situation.

It is true that the sound will decay as it propagates into the room. That's because radiated sound whether from cone speakers or panel speakers expands in size as tit propagates into the room, thus losing intensity as it moves forward.

This is true for most frequencies. Below about 200 Hz, things change. Some sound waves are actually reinforced because a modal frequency (or its multiple) is excited. Resonant decays can last for several seconds whereas non-resonant frequencies last for less than 300 milliseconds or less. I have a resonance issue at 40, 80, 120 Hz. I have extensive traps, but taming these low frequencies is very challenging.

Acoustic traps are far more effective than any other kind of treatment for the normal listening environment. Opening a window or door here and there will alter the sound somewhat, but will not be nearly as effective or consistent as traps (fiberglass is the material of choice. Foam "egg crates" are useless.).

If one doesn’t digest BEANS well, opening doors or windows might be imperative to avoid passing out from getting gassed 😉

I’ve got my system set up in the back family room of the house we’ve now had since November. At 13x17, it serves as a type of music practice and teaching studio space as well. I’m going to need to install a door as it connects to the kitchen. Currently only using a curtain to aid as a barrier, which really does practically nothing to benefit those outside the room tbh. I’ve got treatment and diffusion in place and it’s sounding SO balanced. First time I’ve ever been able to thoroughly enjoy getting into the 90’s db wise from time to time.  I’m regretting what a door might do? But it might improve some things for all I know. At least I’ll be able to open said door and stick a panel on it for when no one else is at home.