You are standing in an open field near a train track as a train goes by at a constant speed blowing its horn in all directions (except down, into the ground) at a constant frequency. You are first able to hear it when it is approaching you 1 km away. When it is a quarter of a kilometer past you, by how many decibels has the train's sound level intensity shifted?
(options: 6, 12, 58, 105, 108, or 114)