What is the key argument to size it?

Let's try with an oscillator f=1kHz. Very rough estimate.

Emitted power is: We =m.v.dv/dt (Watt) with v=Rω and dv/dt=Rω^2

m= ρ.R^3; Total: We = ρ.ω^3.R^5

If put directly in ear: We = W= 1pW/m^2.(eardrum surface).

This determines R = 1 micrometer. Then most certainly much larger than 1 micrometer.

Can we hear an AFM lever driven at very large amplitude of vibration at 1kHz ?

Never tried. Did you ?

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