Barking is a common way for dogs to communicate, and they can bark for many reasons, depending on the situation. Territorial protection is the most common cause.
When a person or an animal enters what your dog considers his/her territory, excessive barking can be quickly triggered. The volume of a dog’s bark is 60 decibels on average, and it can sometimes exceed a whopping 110 decibels.
The breed that holds the record for the loudest bark is the Great Dane. As much as we love our canine companions, the volume barking can reach can be unbearable at times. This woman trained her dog to do something that makes this problem a thing of the past.
Source : http://www.wimp.com