Blocking out unwanted sound sources Suppose you are in a room with two other persons, both of them talking. If you want to hear one of them, you are able to block out the other even though he is talking (although not very loud). How our brain does this as if the other person is not talking, even though the sound waves from both the persons are entering your ears?
You're not really blocking them out. It's more that you're processing the language of the person you're focusing on. 

You could do exactly what you describe at reasonably high volume levels. 

Though the level is not equal for all people. Young children and people with attention difficulties 
Why it's Difficult to Listen to two People