A bark collar is a device that works by stimulating the dog negatively if the dog starts barking. There are collars out there that administer this negative stimulant by delivering an electric shock by using a rather small amount of electricity; just enough for the dog to realize that he has done something wrong. Other collars do this in a different way; for example, by using an annoying sound that only the dog can hear. It is very effective, because the dog will only hear it when he barks; meaning that in order to stop the sound from appearing again, the dog will understand that he needs to stop barking. Other collars include the smell method, which uses citronella (a smell that the dogs dont like) in case the dog barks.
PetSafe® Citronella Pet Spray Control Refill
Citronella bark collars work on small/medium to medium sized dogs that don’t have a strong personality. The size of the control box on the citronella collar can sometimes be a bit too large for very small or toy dogs, while large or stubborn dogs can sometimes not be effectively curbed from barking by the citronella spray.
Stop Your Dog from Barking with a No Bark Dog Collar - Canine Journal
In summary, while the small sample size precludes unequivocal evidence of the efficacy of citronella collars, it appeared effective for three and ineffective for four of the seven dogs. Thus, citronella collars may be effective for reducing or eliminating the vocalisations of some dogs but not others. In our study, it appeared that the collars were more effective with dogs that had a shorter vocalisation history. For other dogs, an initial reduction in vocalisation when the active collar was applied was not maintained with continued use.
Best Bark Collar For Small Dogs - Dog N Treats