In this article, ultrasound-modulated two-fluid (UMTF) atomization of viscous Newtonian liquids at a lower ultrasonic frequency (25 kHz) is compared to that at fundamental frequencies of 54 and 110 kHz The experimental data obtained show a shift to smaller drop sizes at a higher air velocity and a larger ratio of air-to-liquid mass flow, rate, consistent with those obtained previously at the higher fundamental frequencies. Also consistent with earlier findings, the drop-size distribution becomes narrower as liquid viscosity increases. The experimental results of drop-size distribution are consistent with the theoretical predictions of greater-amplitude growth rates for the capillary waves generated by higher (third and above) harmonics than by the first and second harmonies based on the modified Taylor's dispersion relation. However, since the harmonics differ in frequency by 25 kHz only, more than one higher harmonic (up to fifth) make significant contributions in UMTF atomization. Compared to the acoustic-modulated pressure atomization that also operates at 25 kHz, the UMTF atomization requires an ultrasonic drive two orders of magnitude smaller.