In 2008 I saw a concert by Pomassl in Austria. At some point he pressed to his head a bare jack plugged in the loud sound system, generating a deep buzz.
I liked the raw, analog connection between the action and the sound, and experimented with my own version. I quickly realised that just applying a jack connector to the skin was not producing reliable results, and not loud enough. I got round the difficulty by making a miniature monotone tunable oscillator, controlled by a lever switch. When the user presses the switch on their head or any suitable surface, a square wave tone is generated, the pitch of which can be adjusted before use.
I called the device a Cranial Oscillator, and used a set of them on stage while performing with B.O.R.N.
Frank Pomassl in concert, Ars Electronica 2008, Linz, Austria.