To gain an insight into the acoustic properties of a stage floor, the sound reflection of a spherical sound wave from an infinite elastic plate, which is driven by a point force, is theoretically analyzed. According to asurvey of a stage floor's constructions, most stage floors have an air back cavity. Therefore, effects of an air back cavity on the reflection from stage floors should be clarified. Furthermore, the analysis is generalized to extend the theory to a back cavity filled with absorbent media. The resultant field takes the form of the sum of the field reflected by the plate without a point force excitation and the radiated field caused by a point force excitation. The total energy fluctuates around the sum of the reflected energy and the radiated energy, and the fluctuation range is theoretically determined by the reflected pressure and the radiated pressure. The cavity causes a dip which is similar to the one seen in the panel absorbers, and some periodic small dips and peaks due to the resonances of the acoustic mode in the cavity.