(Amended to correct major factual blunder – whoops – and more details added from original post of earlier today).
I was fascinated to read in the Herald this morning about the anti-teenager sounds that are being used to deter graffiti artists. High-pitched sounds that only the young can hear are being used to deter people away from places that tend to get subjected to vandalism. A quick bit of research shows that these systems have existed for a while. Whether they are ethical or not, I guess is a moot point. It makes some kind of implicit assumption that all children are criminals.
Now, I’m not too sure of why presbycusis (loss of high frequency sounds) affects the high frequencies first, but what I do find fascinating from a physics perspective is how the ear can distinguish different frequencies of sound in the first place. It’s done in the inner ear, in the cochlea. A sound sets up a wave on a membrane, that moves into the cochlea. However, the properties of the membrane and surrounding fluid are what physicists call ‘non-linear’. This means a wave only travels so far before it is attenuated (dies down), and it turns out that the lower frequency waves are able to travel further into the cochlea. (Eeek – on the original post I said higher frequency. That was wrong – sorry). Tiny hair cells on the surface of the cochlea then vibrate, and the position in the cochlea where they vibrate the most is one mechanism that tells the brain what frequency is being listened to. This biological (basically mechanical) method of frequency discrimination is very different from electronic versions of the same process. (Contrast that with the obvious similarities between the camera and the human eye).
But, before teenagers protest too strongly, they can get their revenge though with the adult-proof ring tones.
[Postscript – added after original post, for those who want more details. The basilar membrane in the cochlea has the property that its springyness reduces with increasing distance into the cochlea. This means that a wave travelling along it will slow down. As the wave slows, its amplitude increases (like a tsunami idecreasing in speed but increasing in height as it moves into shallow water). This increase in amplitude means that the membrane moves up and down more quickly. The fluid surrounding it behaves in a non-linear way in that the drag it exerts on the membrane increases rapidly with increasing velocity. That means small amplitudes don’t get attenuated, but large ones do. So overall, there is a position along the membrane where the amplitude is greatest, and this position depends on the frequency of the vibration. There are lots of physics concepts here – a bit rushed through I know. ]