40 Hertz

https://deudascienciaysalud.com/2019/05/25/la-cientifica-que-usa-luz-para-combatir-el-alzheimer/

Does listening to a 40 Hz tone “clean up” the brain in Alzheimer’s patients?

https://blog.szynalski.com/2018/03/40-hz-tone-alzheimers/