Go speak to them in a friendly way (and being a bottle of wine or something to ingratiate yourself and make them feel guilty for bothering you), and explain how annoying this is. Maybe they don't know that their dog barks so much when they're away, so you might audio tape the dog for a awhile and play them the tape so they can hear it for themselves. Then ask them to try some of the products out there made to deal with this, like one of these bark collars. http://www.petstreetmall.com/bark-collars/# Maybe you can buy them one and give it to htem so they have no excuse not to try it.
There are noise ordinances you can seek to enforce, and maybe you'll need to contact the property owner to see if these renters are even allowed to have a dog, but maybe this can be handled as a simple dog training issue and you can keep things on good terms with the neighbors without escalating it to the owner or to law enforcement.
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I agree with Ms. Koslyn - it's best to try and handle this in as friendly a way as possible since you're pretty much stuck with each other. If they refuse to cooperate then you should call whoever handles animal control in your area and ask for their advice. You might also want to speak with the owner of the house.
I completely agree with my fellow lawyers. Be careful with getting or suggesting a bark collar. Many dog owners consider this cruel and will get upset. Just the opposite of what you are trying to accomplish.
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