In 1958, Patrick Flanagan invented the Neurophone, a device patented in 1962 that allows radio signals to be picked up by the human nervous system. The skin is the organ that receives the signal, converting it into a modulated molecular vibration, which the brain interprets into sound. Basically, it gives one the ability to 'hear' through the skin, making it sound like the audio you're hearing is actually in your head. It's kind of like having headphones in your brain.
The only problem was that it used such high voltages that it was too dangerous for normal use. Fortunately, while he was developing a language translation device for communications between humans and dolphins (yes, dolphins), a device that the NSA suppressed for four years under a national security order, he came up with a way to use ultrasound instead.
Turns out, you can make your own DIY Neurophone at home with some parts that are pretty easy to find online or at electronics stores. It uses a pulse-width modulation controller, two potentiometers, a sine wave signal generator, an oscilloscope or frequency counter, and two piezo disks.
It sounds complicated, but what all this fancy equipment does is create a weak electric circuit that's picked up by a part of your brain that translates it into sound. This one won't sound as good as Flanagan's, but it's a lot simpler and probably a lot of fun. You can see the schematic below.
Read more about the DIY project here.