A primitive understanding of fluid oscillation in resonator tube is based on the standing wave approximation. Recent progress in studies on thermoacoustic spontaneous oscillation requires us to make a more realistic image of fluid oscillation in resonator tube. Finite Q-value means some dissipation of work flow in the resonator. Analogous discussion to a transmission line with finite damping successfully leads to empirical equations on the resonance curve and distribution of pressure and velocity amplitudes. The result, however, does not exclude the possibility of negative damping, since this treatment abandons completely information on phase difference between pressure and velocity. The phase difference and the work flow are included in the thermoacoustic theory. Recent progress on experimental techniques makes it possible to observe the phase difference and the work flow, which is rarely discussed in traditional acoustics.