Anti dog barking equipment legal advice.....(10 points best answer!!!!!!!!!)?

Hi i am just wondering if an anti dog barking birdhouse that sends frequencies to my dogs ears from the neighbors is illegal.... The neighbors are insinuating that my 2 dogs are barking constantly non stop and they are not barking at all. anyways they hung this anti bark frequency device that emits frequencies to my dogs when the bark and its pointed from their back yard straight into our backyard..... and we don't live in a neighborhood so theres no barking rules/laws enforced. im just wondering if this is illegal thanks!!