versus sound frequency. It can be clearly observed that at sound
frequency values higher than 2 kHz the decrease of moisture ratio is
proportional to the increase of sound frequency. According to Fig. 7,
the drying time becomes shorter as the sound pressure level increases.
As mentioned before, increasing sound pressure level resulted in higher
fluidized quality of lignite. Furthermore, sound waves also produce
rapid oscillating velocity and alternating stress which may remove the
moisture.