Here is an article from Whole Dog Journal on the subject.
Understanding Your Dog’s Vocalizations
Barks, growls, howls, whines, and whimpers – your dog is talking to you, and he’s got a lot to say! Here’s a quick run-down of what your dog might have to say.
Barking
Dogs bark for many reasons, including alert (there’s something out there!), alarm (there’s something bad out there) boredom, demand, fear, suspicion, distress, and pleasure (play).
The bark of a distressed dog, such as a dog who suffers from isolation or separation distress or anxiety, is high-pitched and repetitive; getting higher in pitch as the dog becomes more upset. Boredom barking tends to be more of a repetitive monotone. Alert bark is likely to be a sharp, staccato sound; alarm barking adds a note of intensity to the alert.
