Short answer
Low frequencies around 150–200 Hz work best; 165 Hz is the most widely used. Low tones make the speaker membrane move further on each stroke, pushing water through the grille.
At the same volume, a lower tone forces the speaker membrane to travel further. That long stroke pumps air — and water — out of the grille. Go much lower than ~100 Hz, though, and a small phone speaker can't reproduce the tone well. The 150–200 Hz band is the practical sweet spot, and 165 Hz became the standard thanks to the popular Siri shortcut.
Sessions that vary the tone slightly can help dislodge stubborn droplets. Afterwards, sweep higher frequencies with a tone generator to confirm the speaker sounds clean across the range.
