We learned about radiation characteristics to essential radiators. One is radiation from breathing sphere. Breathing sphere means that surface moves in phase like this. And at the other essential radiator, we learned about, is trembling sphere. The trembling sphere trembles. The sphere is a trembling like this. Therefore, the phase of this side and the phase of this side is different. Using this to essential radiators, we introduced the concept of far field and the near field. Those are expressed in terms of ka and kr. What is ka? K is wave number. Therefore, two pi over lambda, number of waves per unit length multiply by two pi. And the ka, a is radiator size. Therefore, if ka is very, very large, then either trembling case and breathing sphere case goes to far field. Far field, the radiation characteristics. And then, what is kr? What measures kr? Compared with the ka, kr is two pi over lambda multiply by r. Therefore, two pi over lambda multiply of r essentially measures where we hear or observe sound compare with the wavelengths. So, this measures the radiation characteristic at observation point, and this measures radiation characteristics compare with radiator size. Those two measures are essential measure to characterize to essential radiators. And those two measures are what you have to understand in this lecture essentially.