The authors study the effects resulting in the asymmetry of combination frequency spectra observed when high-frequency and low-frequency oscillations interact in a plasma column. Asymmetric spectra are the result of amplitude and phase modulation of a high-frequency wave. The phase modulation is due to the dependence of the wave number on the plasma density and velocity of motion of the electrons in the low-frequency wave field. The amplitude modulation is produced by the action of two mechanisms, one of which is associated with transverse motion of the plasma (with respect to the axis of the system), while the second is related to the instability of the system relative to the high-frequency oscillations.