Sound power emission of a simple source in a reverberation chamber is derived from the normal mode theory and compared to that in free space. Both pure-tone and noise sources are considered. The acoustic impedance of the sound source itself is deemed important in its power emission and incorporated into the analysis. The acoustic impedance presented to the source by the sound field produced in the chamber varies significantly from point to point, and this leads to the large variability of the power output of the source in the room. But the average obtained over the whole room approximates well the sound power produced in free space, provided that the source impedance is large compared to the field impedance and that the frequency is high enough so that the normal modes overlap. A statistical formula is derived for the power emission as a function of the space coordinates in the room. It is found that the power emission is a maximum on the boundary, and undulates about the free-field value when the source is moved toward the center of the room. The average power emission is less than the free-field value if the averaging avoids the high-power region near the boundaries, as is usually the case. The difference is derived that seems to explain the earlier results of sound power measurements.