Figure 6.

(a) Contour plot of $\log |det(\mathbf{\text{P}})|$ in the complex frequency plane for a unit ring of N=20 Neumann holes with radius $0.8\sin (\pi /20)$ for standing wave solutions with azimuthal quantum number m=10. Note the presence of two modes very close to the real Ω axis, corresponding to confined quasi-modes. (b,c) The real part of the field u for these confined WBMs at Ω≈14.12 and Ω≈18.90, respectively, normalized to the colour bar shown.