I was reading the Wikipedia article on tinnitus, and came across this pearl of a sentence:

A common and often misdiagnosed condition that mimics tinnitus is Radio Frequency (RF) Hearing in which subjects have been tested and found to hear high-pitched transmission frequencies that sound similar to tinnitus.

Hmm, what? Yes, humans, under special circumstances, can hear radio-frequency pulses in the range of 2.4MHz to 10GHz (corresponding to radio frequencies and microwave) as buzzes, clocks, hiss or knocking at apparent auditory frequencies of 5kHz and higher (very high-pitched). That doesn’t mean that you can hear talk radio by receiving AM waves; it just means that when it’s very very quiet, you can hear a faint high-pitched noise from RF sources.

But how could electromagnetic waves be detected as sound, which is a pressure wave? After all, light is an EM wave too, but we don’t hear light! It’s a long story, but basically, you’re a microwave bongo head. Elder and Chou (2003) offer a thorough overview of the phenomenon.

RF hearing was first reported in the 1940s by people working with radar, but reports were dismissed as illusions or hallucinations. The phenomenon was investigated scientifically by Frey in 1961, who concluded that RF hearing is a real thing. It can be stopped, for example, by placing a piece of aluminum between the RF source and the ear.

RF sources can only be heard by people with working audition above 5kHz. This would imply that RF sources create an acoustic vibration close to the cochlea that gets detected as high-frequency sound. Indeed, one can record electrical potentials inside the cochlea evoked by RF pulses that look just like potentials evoked by sound waves.

The authors further report that the apparent acoustic frequency of the RF pulse is independent of the EM frequency of the actual pulse but dependent upon head dimensions. So EM energy gets absorbed by the head and somehow this energy is transformed into pressure waves that get reshaped by the head. Thus, microwave bongo head.

The most likely explanation for this is the thermoelastic expansion theory. When RF pulses are created near a container of water, it is possible to detect evoked sound waves in the water; the acoustic frequency of these waves is similar to that of the sounds heard in RF hearing. When an RF pulse is absorbed by water, it locally elevates the temperature, which causes a rapid local expansion which then gets propagated as a pressure wave. The local elevation in temperature can be quite small: the authors give a figure of 5 x 10^-6 degrees Celsius (!). This sound wave gets transmitted by bones to eventually make its way to the cochlea, where it gets detected as just another pressure wave. This is related to the well-known photoacoustic effect (illustration above), where laser pulses generate to ultrasound via thermal expansion; the laser source gets replaced by RF pulses in the RF hearing effect.

The authors point out that this is neither dangerous nor useful. It’s just kinda cool. Ain’t science neat?

Elder, J., & Chou, C. (2003). Auditory response to pulsed radiofrequency energy Bioelectromagnetics, 24 (S6) DOI: 10.1002/bem.10163