Phase shift between gas velocity and pressure in an orifice pulse tube

In order to investigate how the phase shift between pressure $P (t)$ and velocity of gas $V (t)$ affects the net refrigeration power of an orifice pulse tube, we have recorded at the cold end of the regenerator, when the temperature is constant at 173 K and 259 K, the net refrigeration power and the two signals of dynamic pressure and velocity of gas. The phase shift is computed from FFT of the two signals. If we maintain the amplitude of pressure $\Delta P = (P_{\max} -P_{\min})$ constant, the net refrigeration power varies as a function of the phase shift between pressure and velocity of gas