By separating the Wiener spectra of screen mottle of a medical screen/film system for various tube voltages into those of quantum mottle and structure mottle, we experimentally determined the tube voltage dependence of the detective quantum efficiencies (DQEs) of the front of back screens. For the purpose, using the DQE, the equation of the Wiener spectrum of the quantum mottle has been expressed, and using the equation, our method for separating the Wiener spectra of the screen mottle was modified. Both the DQEs of the front and back screens decreased with tube voltage. The DQE of the front screen at each tube voltage was greater than that of the back screen. We explained the tube voltage dependence of the DQEs using the dependence of the first and second moments of absorbed x-ray energy distribution in the screen obtained using our data of incident x-ray spectra for various tube voltages and Swank's noise-equivalent absorption spectra.