I'm a old student(baby boomer) but have not stopped learning. I read this and was wondering what leds vibrate at.
"Ear cannot respond to all frequencies in an unbiased manner. Audible range of human being is commonly quoted as from 20 Hz to 20,000 Hz, which in fact will vary with age, health, past exposure to noises, and so forth. Ear also acts like a filter and will favour certain frequencies over the others. Ear is most sensitive to sounds at the range of 1,000 to 5,000 Hz, and particularly at about 4,000 Hz."