So, although I have discussed something some few points I have listed here directional waves may come surface spectral function is proportional to the energy of waves. Hence directional spectrum gives energy of the waves per unit plan area against different frequencies as well as directions. I explained that that is volume represented by S (f comma theta into D f) elemental frequency and elemental theta directional is now proportional to the average wave energy per unit horizontal area and the variables are mentioned there frequency band with D theta is implemental of directions and S (theta) is known as the directional spectral density function or directional waves spectrum spreading function angular spreading function or directional distribution. So, you have a number of names assigned to S (theta).