At5kHz every transverse mode is below cutoff, so \b[no propagating guided mode exists]. A driven boundary can produce an evanescent near field. For n=1 the decay coefficient is $\sqrt{(20\pi)^2-(10\pi)^2}=10\pi\sqrt3=54.4\,\mathrm{m^{-1}}$, an e-folding distance of18.4mm. This is decay along the guide, not attenuation of an otherwise propagating lossless wave.


Solution

Past exam of the physics A course of the University of Cambridge / 2010 / Paper 1 / Section B / B7 / a / Solution

 Solution (source code)