Density gradients, which refract laser light within the cavity, degrade beam quality. In addition to wall influences and viscous effects which cause density gradients, there is another mechanism. This mechanism, which is due to wakes and compression waves from heat (vibration energy to translation and rotation) addition in a supersonic stream, appears to have been overlooked. This appropriate equation is stated and discussed. A semigraphical solution procedure is outlined. Contours of constant density have been calculated for circular and rectangular cavities. Graphs of the isodensity contours are given. (Author)