Reading about a 'necklace' coil inducing a strong alternating audio current (using an LM386 f.i.) into a magnet inserted in the ear canal set me thinking. The problem is that the magnet has to be inserted in very close proximity to the eardrum, so removing it can be a problem. Would it be possible to make use of a 27 MHz AM transmitter to induce a strong signal into the tank circuit of a tiny crystal set? That crystal set could be built with some ingenuity into a wireless crystal earpiece. The transmitter would use a 'necklace' loop antenna to induce the signal, containing the audio.
It is certainly possible to build a 27 MHz receiving crystal set small enough to fit in the ear. A 27 MHz crystal has both effective series inductance and parallel capacitance so it could serve alone as the tank circuit. Another "crystal" is the diode detector that demodulates the AM and a third piezoelectric crystal converts the detected voltage to sound. That eliminates your bulky magnet! However more ingenuity is needed to couple adequate RF signal into the receiver and you probably want some kind of tuning control, which could be a tiny variable capacitor in parallel with the 27 MHz crystal.
Having seen your question, I can not leave it without cautioning you against "experimenting" with putting things into the ear canal.
If you cause an ear infection, depending on the health care system in your locality, you could find yourself having to pay thousands of dollars for treatment.
Be good, and if you can't be good, BE CAREFUL.
Seems like a good idea to me. Using a crystal earpiece avoids inserting anything "into" the ear that worries MartinRH - at least no more than you would with Walkman earworms etc. The reciever does not need to be tuneable, as you make the transmitter necklace tuneable. The only limitation that I see to miniaturisation, is there needs to be some element which receives the signal. Without an antenna this is usually the coil, so even if you can have a complete tuned cct with a crystal, I think you need to have a coil to act as antenna.
When I was a little kid I lived about ten miles from AM station WLW near Cincinnati Ohio which is a 50kW station. I had a crystal radio set and I found I could never tune anything but WLW. Then after some experimentation I found I could just use the crystal earpiece and the germanium diode across the leads of the earpiece and since WLW had such a dominant signal I needed no tuning circuit at all nor did I need any antenna.
It seems encouraging that you could attempt what I succeeded to use forty years ago.
You can do this .The coil of your crystal set will act as the receiving antenna .The necklace is your transmitting loop antenna . Your crystal set could be a class E resonant rectifier or some other technology borrowed from wireless charging. Your propagation is in the near field not the far field . 27 MHz is your choice of operating frequency . Sure crystals are easy to find but it may not be optimum . A lower frequency increases detector efficiency . I think you should experiment with a sig gen so you will minimise battery drain on your proposed production device .