Thanks for all your help everyone ... I have found a reference to the
paper by Yeowart, Michael, and ordered it. What intrigues me about
infrasound is why we can't hear it. I can see why we have a top
frequency limit to the hearing mechanism because the mass of the parts
must reach a point where they simply can not respond fast enough to the
frequency. My modest understanding of the way the ears work suggests
they are pressure sensitive so if there is a relatively slow pressure
change(20Hz) what stops our ears passing this to the brain? I am
interested in where the filtering occurs, and wonder if the brain gets
the signals but ignores them unless they are over a certain amplitude.
If this is the case, the filtering is cognitive which would be
interesting. We could start asking why it is so consistent, can it
change if we were in fight or flight mode for example.
I like the analogy with the speaker but I wonder if it could be the case
that the speaker is responding to the low frequency but can't transmit
it to us via the air. I have been looking at sub woofers that go down to
low frequencies and the main difference between them and ordinary
speakers is the huge amount of movement they are capable of and the
stiffness of the cone. So they impose a pressure wave on the air rather
than just move it about a bit, if you see what I mean. The stiffness of
the cone helps reduce the harmonic resonance. Headphones get much lower
because they are directly coupled to the ears and the pressure in the
cavity sealed around the ear helps damp the cone, producing better
performance with less substantial hardware.
I really do appreciate you all taking the time to reply to my post and
would welcome any further thoughts you have.
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