Aircraft sound heard by an observer can be influenced by a number of factors. As previously stated, meteorology is one such factor. Another factor is the propagation of sound over the ground and as it is affected by terrain. When an aircraft is directly overhead, the sound experienced by an observer is only affected by meteorology. However, as the aircraft passes by or is at lower elevation angles, the sound heard by an observer is both the sound that travels in a straight line from the aircraft plus the sound reflected off the ground. Although the interaction of sound traveling over the ground is quite complex, recent research has provided new insight.