Schematic representation of the quasi-monodisperse regime. The top half of the figure deals with the gas, while the bottom half deals with the dust component. The horizontal axis indicates (wave) speed. The red solid arrow denotes the propagation speed of a gas sound wave in the absence of dust, and the blue arrow indicates the resonant dust drift speed. The backreaction of the dust modifies the propagation speed of the sound wave, leading to the dashed red arrow. The blue rectangle indicates a range of drift speeds if there is a distribution of dust sizes. As long as this rectangle does not come close to the dashed red arrow, we are in the quasi-monodisperse regime.