By using the detective quantum efficiency (DQE), which represents the transfer efficiencies of the signal and noise, we expressed the Wiener spectrum of the quantum mottle. Using the new equation of the quantum mottle and the equation of the structure mottle proposed in 1992, we modified our method for separating the screen mottle, and proposed a new method for obtaining the DQE. Using the new method, we experimentally obtained the tube voltage dependence of the DQE, and the density and tube voltage dependence of the Wiener spectra of the quantum mottle. To explain the tube voltage dependence of the DQEs at a spatial frequency of 0 mm-1, we theoretically obtained the tube voltage dependence of the first and second moments of x-ray photon spectra absorbed by the screen. We found that the tube voltage dependence of the DQEs was caused by the fact that the output signal from the screen varied only slightly with tube voltage, while the output noise increased. Using the tube voltage dependence of the DQEs and the number of x-ray photons incident on the screens, we explain the dependence of the Wiener spectral values of the quantum mottle.