J_m(j_{mn}·r)·cos(mθ) — Bessel function drum modes with nodal lines
Drumhead modes: ψ_{mn}(r,θ) = J_m(j_{mn}·r)·cos(mθ), where j_{mn} is the n-th zero of J_m.
Frequencies f_{mn} = j_{mn}·c/(2πR). Nodal lines (white) are where ψ=0: m diameters + (n−1) rings.
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