Most dog owners have experienced their dog barking when a visitor comes to the house and often this is the dog alerting the rest of the household that someone is here.
Rather than being seen as an act of aggression, the dog is alerting his housemates to the fact that someone from outside the home is entering their territory, and they need to know about it to get ready.
There are generally two types of bark that you will hear whenever your dog becomes aware of a visitor. If the dog doesn’t know the person or is unsure then the bark will be like a ‘woo, woo, woo’ sound, which is a classic alarm bark.
On the other hand, if the dog knows the person then the barking will be higher pitched, excited barking, often accompanied by other excitable behaviour as he tells the rest of the house that ‘Dad’ or whoever is back.