[quote name='thinman' timestamp='1334945583' post='1624002']
I thought I'd have a go at answering the original question...
As previously pointed out if a speaker was producing a simple sine wave then the cone would move back and forth smoothly at the wave's frequency.
If it is having to produce a complex wave, say the result of two sine waves of different frequencies, the form might look like the lower frequency but with ripples on it of the higher frequency.
The cone will therefore follow the shape of that wave - in general moving back and forth at the lower frequency but rather than smoothly it will be making much smaller back and forth movements at the higher frequency.
As I understand it that is what makes speakers imperfect (but not impracticably so) because the above behaviour introduces Doppler distortion. (The effect of a fire engine's horn apparently changing pitch as it passes you). Imagine the cone is the fire engine and its horn the higher frequency - the fact the higher frequency is being produced by something that is itself moving.
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Now this is getting close to answering the original question I asked. There's no doubt however that the answer goes much deeper than I first realised, as it encompasses sound generation by the speaker, reception by the ear drum, neurological conversion and finally, interpretation by the brain. There's also the scientific analysis of sound waveforms.
The BBC could make a really interesting programme exploring the journey of discovery that led to our current understanding of this topic. Could be an interesting academic paper too.
Thanks for that answer thinman