He's an indoor dog but is allowed into the backyard whenever he wants. He gets a mid-day/late-morning for a walk in a park for about a hour-and-half.
Now he has a quite loud bark so it is mainly we don't want to bother the neighbours. Is there any training techniques to stop him barking, or perhaps a deterrent from going to the very back of the yard (like how some scents keep animals away)?
First off, good job on the exercise, though it does remove my most common advice. Dogs that bark a lot in the backyard are often just suffering from boredom and excess energy because they don't get regular walks. That's not your case, though.
There's a few options you have here. First up is simple management: keeping Tripp away from the area that makes him bark. At a garden supply store or maybe a hardware store, you should be able to buy a cheap short wire fence that you just stick into the ground, and provide a barrier between him and the back of the yard. (This is dependent on whether he's likely to just jump over/barrel through, or if he'll respect it.)
The fence will allow you to control when Trip gets to the part of the yard that sets him to barking. And you'll want to let him into that part of the yard, because you're going to use that to teach him proper behavior, so that you don't have to use the little fence anymore...
Now, there's an interesting characteristic to training animals - if you train an animal to do something on command, it becomes less likely to do it without the command. This isn't fullproof - my dogs sit on command, but they also sit on their own. But I find it works well for barking. In short, if you want your dog to stop barking, sometimes the best method is to teach it to bark on command. Huh.
I'd take Trip (and a baggie of treats) out to the back fence where he likes to bark (and at the time he likes to bark), and say "Speak!" and wait for him to bark. Encourage him/get him worked up if you need to. When he does, say "Good boy!" and treat him. (If you're doing "clicker training," which I'll describe here soon, you'd click instead, but for now we'll just use "Good boy!")
Say "Speak" again and wait for a bark, then say "Good boy!" and treat. In short order, Trip should be barking right after you say, "Speak!" and then probably looking for his treat. Now you start varying the intervals between giving the command. Start with a few seconds, then work up to random lengths of time between a five seconds to a minute or more (this may take a few days). Do not get into the habit of repeating the command over and over if he doesn't bark right away! Wait at least 30 seconds before saying "Speak!" again. (Although, in the very beginning, you can go ahead and use body language to get him worked up enough to bark if necessary.)
Any time Trip barks when you haven't told him to, say "Nope!" and turn your back on him for 10 seconds. Ignore him completely. Then turn back around and continue the game.
You see what you're doing here? You're training Trip to bark when you say "Speak," and only when you say "Speak." Start working other parts of the yard, then inside.
When he's got this pretty good, within the first couple of days, start phasing out the treats. Say "Good boy!" but every so often, leave out the treat. Then only treat about every other time, then less and less often. Try to stay random. Eventually, you won't be treating at all except on very rare occasions just to surprise him.
Now, the stronger you build this command, the less likely he is to bark without it, but I'd be surprised if it was 100% - sometimes he's still going to hear something outside that fence and get to barking. So, let's build a couple of other behaviors on top of that.
If there's anything else that gets Trip to barking, let's use that to teach "Quiet." (For a lot of dogs, ringing the doorbell does the trick. You can either have a friend hang outside and ring your doorbell, or for about $15-20, you can buy a doorbell set from a hardware store and just ring that inside your living room.)
Whatever works, get Trip barking. After two or three barks, say "Quiet!" in a firm (not yelling), low voice. Now wait for him to stop barking. Don't do anything else while waiting. The moment he stops barking for a second, say "Good boy!" and give him a treat. Ring the doorbell again and repeat. Before long, Trip will stop barking when you say "Quiet" and then you treat him.
Don't make the session more than 5 minutes or so. You can have a few sessions through the day, but keep them all very short. He's barking because the sound stresses him a bit, and you don't want to make that worse.
Once he's got that down solidly (may take a few sessions), start saying "Quiet" softer and softer. Be consistent about saying it always after the first two barks. Probably, Trip will only ever bark twice after a while, because that's always when the command comes.
Eventually, you try not saying "Quiet," and see if Trip stops barking after just two barks. If so, praise him mightly and treat him. Give him a another try, and then quit that session and pick up later.
Start working the Quiet command from different parts of the house, or standing farther and farther away from Trip. Advance quickly, but in baby steps.
If you catch Trip barking at the back fence, grab a couple of treats and go to him and say, "Quiet." When he stops, say "Good boy" and treat. After a few times like that, he'll get the connection that "Quiet" in this context means the same thing as it did with the doorbell.
This one's pretty easy. Get some REALLY good treats - pieces of meatballs, or diced up steak. Call Trip from the other side of the room with a loud, friendly "Come!". As he comes towards you, turn partially away from him and move away from him, like you're starting a game of chase. As soon as he gets to you, give him the awesome treat and lots of praise.
Surprise him with this multiple times during the day. Furthermore, make an effort not to make a habit of calling him to you only when he has to stop doing something fun. If he's playing in the yard, call "Come!", give him a treat, and then immediately let him go back to playing. He will start coming to you much more responsively and eagerly.
Anytime you see him by the back fence, but he's not in a barking jag, call him to you and praise him and play with him and give him the awesome treat.
Just as with "Speak," phase out the treats slowly, but make sure to surprise him every so often with one.
If Trip is typical, within a couple of weeks, you can have a dog that
- barks less because he thinks its only worth it to do it if he's asked to,
- knows that "Quiet" means he needs to stop barking if he wants any chance of a reward,
- and will come running to you when you call him away from the fence.