We attempted to determine the number of frequency components required for efficient turbulence generation. A random-phase mode was applied for generation of turbulence where an input signal was fed to each fan with quasi-random phases. We used an input signal composed of two frequencies by this method and investigated characteristics of the turbulence. The turbulence far enough downstream generated by two-frequency mode has almost the same characteristics as that by the forty-frequency mode, but the development of power spectrum is much slower for the two-frequency mode than for the forty-frequency mode.