The Neurophone was invented by Patrick Flanagan and used radio transmitter that injected sound into the human nervous system. Antennas coupled a one-watt 40kHz transmitter to the body to inject the signals. Seems that Patrick was quite the brain since he developed and sold a guided missile detector to the U.S. military when he was 11 years old.
Andreas Hahn has make a modern version of the Neurophone using modern electronics to help him out. Andreas uses ultrasound that is modulated with sound and can be sent directly into the brain.
“To make the ultrasound signal, we’ll use a widely-available TL494 pulse-width modulation controller. This isn’t a perfect solution, so you won’t hear the signal as well as with one of Flanagan’s designs. But it’s a lot simpler than messing around with DSP. And it gives you a chance to experience and experiment with the “Neurophone” effect.”