So we recently adopted an australian shepherd mix (9-11 months old). We've had him for about a week. He's really smart and super friendly, but lately he's become territorial around our part of the apartment complex. He doesn't have a compulsive barking problem, but he barks at every loud sound or people's talking outside the house. I've tried working on the quiet command and waiting until he's quiet to give him treats, and it semi-works, but if the neighbors beside us is going to their apartment and you can hear them clearly, he'll go crazy and run to the door howling and barking. He'll also bark at anyone going near or up our stairwell to get to their own apartment. Once we leave that area, he's really friendly with every dog and person, he tends to bark more at people when we go on night runs. I know he's just being protective and alerting us, but I don't know how to effectively stop his barking. It's really interrupting our sleep, I have to wake up two or three times just to get him to calm down when the neighbors below us get home (they get home late every night). I know it's stranger-directed, he stopped barking at loud noises inside the house and he doesn't bark when my family goes up the stairs, but I've asked the neighbors and they said he barks when we aren't there, but only when they are coming and going. He's food motivated, but not as much as our chihuahua and he's not very toy motivated. It really sucks that he's influencing our chihuahua to act the same way, the other day when I was putting them in the car their leashes got tangled and when I tried untangling hers, she took off barking at our neighbor. She's very submissive and I know she can't do any real damage, but I'm afraid of what would have happened if it were our aussie that got loose. Any ideas on how to stop this type of barking? I'll probably to have to make a separate post about stopping him from bullying our chihuahua when it comes to toys, but this problem is more important because we don't want the neighbors to report us to the apartment managers. He gets exercise everyday, taken out multiple times. Inside the house it doesn't seem like he has tons of excess energy, he's really chill unless he's playing or bullying our chihuahua. We've tried turning on the radio when we leave but even if we have the radio or TV on, he still hears everything outside and will stop growling or barking.

He is neutered.