It's awesome how easy this kind of experimentation is today. Thirty years ago, I worked with a mathematician at JPL who used fairly specialized equipment to fin

It's awesome how easy this kind of experimentation is today. Thirty years ago, I worked with a mathematician at JPL who used fairly specialized equipment to find a frequency notch where he had lost his hearing -- at roughly 8k, IIRC, in addition to his total inability to hear anything over ~12k. (The latter was probably just normal age-related high-frequency hearing loss). In those days, an accurate signal generator was an expensive piece of lab equipment... .