and what is kinetic energy spectrum?
Energy density is the amount of energy per unit of volume. So if I have 10 joules of energy stored in a capacitor, and the capacitor has a volume of 0.001 cubic meters, then the capacitor has an average energy density of 10 / 0.001 = 10 000 joules per cubic meter (for the electrical energy it stores, that is).
As for "kinetic energy spectrum", you're going to have to explain a little bit more. Where did you see those words together?
Separate names with a comma.