Thanks Mr Steam
The dogs have various yards in which they roam free, but they have some kennels within them where they can go to sleep. The biggest yard is 20 m x 20 m (that's about 60' x 60')
There is a gateway which is central to all the yards, and I have a power cord running to this gateway, currently for a floodlight.
There is about 20 m from the gate to the farthest point the dogs could be in any particular yard.
If a sound was made there it would need to be at least the loudness of an average voice (not yelling) - they are all young dogs and have good hearing.
Another alternative to the buzzer is a flashing light (though it might be a little harder to train with it during the day if it's not so visible, but is a nice silent option I won't hear).
The sprinkler option would work very well to send dogs to bed, but due to us being in severe drought with water shortages I wouldn't currently risk having a sprinkler hooked up in case it did not turn off and used up a lot of water. However, I'd still be interested in that option for other times.
There is no wifi available out there. It is too far from my office.
The siren/buzzer/light etc, I think 10 seconds is about right to get their attention without going on too long. (I am on a farm so none of these things will disturb anyone else). I assume if they keep barking, after the 10 seconds, it will trigger it again.
FYI, the training would involve the dogs being in the yard (done either one at a time or in little groups), sound the siren (or lights etc), shoo them into the kennel and hand out some treats in there. Repeat a lot and then the siren becomes the trigger for them to go into their kennels. Once they are disturbed like this and go to kennels they stop barking.
The barking I want to disturb is the boredom "pack" barking/howling. One starts a howl or bark and the others join in like a pack of wolves - just standing barking or howling into the air. As opposed to play barking which tends to be less uniform with more running around.
If only on at night then the sensor could be set to be quite sensitive, as they don't need to be doing any barking at night.
I look forward to your ideas. You know, if it works well, it could be extremely popular. There are commercial ones that make an ultra high frequency whistle which i could try too - but in theory these are supposed to work with no training - i.e. the noise is supposed to be unpleasant, but lots of reviews show they don't work. However, combined with the training I described they might. Sprinklers would be good - they don't really need to wet the dog more than a few drips, as most dogs won't stay out in the sprinkler or rain (It's always quiet at night when it's raining).