Barking when guest come

Our little shih tzu is 15months old and for the last few months he barks really loudly at guests when they come over. He knows the quiet command. I tell him quiet. He stays quiet for a few seconds then begins to bark again. He will bark on and off until the guests leave. His bark sounds like a stranger invading our home. I allow him to sniff the guests first but I would like him to be calm when guests come over. Any tips.