Abstract: Using the concept of complex sound intensity, we discuss in this paper the rotational character of the active intensity which is the real part of the complex sound intensity. It is shown that it is the irrotational component of the active intensity that transfers sound energy in sound field. The measurement of sound power by use o the two-microphone technique will be affected by the rotation of the active intensity. Because the irrotational component of the active intensity depends on the surface sound intensity, measuring the surface sound intensity can avoid the effect of the rotation. Formulas about the surface sound intensity which are based on the auto-power spectra and provide a theoretical basis for measuring the sueface sound intensity are given in this paper.