# [GIF] The Band Plays On (Chladni figures for a square drum)

Posted 5 years ago
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| The Band Plays OnFollowing up on Drumbeat, which shows one of the vibration modes of a circular drum, here are the nodal lines of a family of vibration nodes of a square drum. Cribbing from the MathWorld article, the vertical displacement of the $(p,q)$ vibration mode of a $1 \times 1$ square drum is $u_{pq}(x,y) = (A \cos \omega_{pq} t + B \sin \omega_{pq} t) \sin(p \pi x) \sin (q \pi y)$,where $\omega_{pq} = \pi \sqrt{p^2 + q^2}$. This is easy enough to turn into a function: ?[p_, q_] := ? Sqrt[p^2 + q^2]; ?[x_, y_, t_, p_, q_, A_, B_] := (A Cos[?[p, q] t] + B Sin[?[p, q] t]) Sin[p ? x] Sin[q ? y]; In fact, the same holds for arbitrary rectangles, so long as the product of sines becomes $\sin(p\pi x/L_x)\sin(p \pi y/L_y)$ where $L_x$ and $L_y$ are the lengths of the sides of the rectangle. The nice thing about squares is that you get an extra symmetry: the $(p,q)$ mode and the $(q,p)$ mode have the same frequency, so any linear combination will also form a standing wave.In the animation, I'm taking the combination $u = u_{7,9} + c u_{9,7}$ and (by letting $c=\tan \theta$) varying $c$ from $-\infty$ to $\infty$. The curves in the animation are the so-called Chladni figures, or nodal lines of the vibration, meaning the solutions of $u=0$.Anyway, here's the code: With[{p = 7, q = 9, A = 1, B = 0, cols = RGBColor /@ {"#F66095", "#2BCDC1", "#393E46"}}, Manipulate[ ContourPlot[ ?[x, y, 0., p, q, A, B] + Tan[(?/2 - .0001) (Haversine[Mod[2 ?, ?]] + Floor[2 ?/?] - 1)] ?[x, y, 0., q, p, A, B] == 0, {x, .01, .99}, {y, .01, .99}, Axes -> False, Frame -> False, ContourStyle -> Directive[CapForm["Round"], Thickness[.01], Blend[cols[[;; -2]], Haversine[2 ?]]], PlotRangePadding -> -0.01, ImageSize -> 540, Background -> cols[[-1]] ], {?, 0., ?}] ] Answer
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Posted 5 years ago - another post of yours has been selected for the Staff Picks group, congratulations !We are happy to see you at the top of the "Featured Contributor" board. Thank you for your wonderful contributions, and please keep them coming! Answer
Posted 5 years ago
 I'm glad you caught the Chladni bug. They really are pretty pictures... :) Answer