This study has established specific ultrasound parameters in the use of low-intensity, low-frequency ultrasound stimulation (LILFUS), designed to mimic the brainwave patterns of theta and gamma oscillations observed during learning and memory processes to induce predictable and long-lasting changes in brain function.
https://www.science.org/doi/10.1126/sciadv.adk3198 (Open access)
I realized my mistake a few minutes after typing that and was hoping people would just take it as a meme I was thinking the gamma and theta frequencies but that 500khz is the actual important part that needs special tech.