Music & Acoustics
Acoustic Room Mode (2,1)
p = sin(mπx/Lₓ)·sin(nπy/L_y)·cos(ωt)

A standing pressure wave of a rectangular room — the (2,1) axial mode — pulsing between compression and rarefaction; such modes color the bass response of every real listening space.
Computed & rendered in pure C# on the frozen OnlyCSharp 1.8 library — animation baked to GIF with the library's own
Gif encoder (no GPU, no ffmpeg, no external packages).