I was looking for a study showing the difference between ground plane measurement and (VS) anechoic measurement but I was not able to find anything. Obviously I know that the GP measurement will indicate higher SPL compared to the anechoic that will have lower sensitivity. I am referring to the shape of the response curve. How is that affected? I will also assume that the high frequencies will be affected less than (compared to) the low frequencies.

Let me give an example of what I am hopping to find;
For sake of discussion lets assume that we have a 3 cu ft tower speaker (in the upright position) that measured (ground plane) flat 20Hz to 20Khz. Then suspend the same speaker 50ft in the air, take another measurement and compare the two curves to get a picture of how the shape of the curve is affected. I am hoping to find a compatible type of study.

Any thoughts / suggestions appreciated