3D audio sources Harsh Panning problem

In a 2D project I am using a mono audio source in 3D and everything is working correctly except the panning. There seems to be no controllable curve for panning intensity? This results in harsh switching between left and right when the listener passes the audio source.

Example: When the listener is to the right of the source the volume in the left channel is much louder (as it should be) but there is almost no volume in the right channel. As you move closer the right channel only increases in volume slightly, until you pass the source, where it then flips drastically to loud volume in the right channel and little in the left.

Am I missing something? I can't find a curve or slider to adjust this?