Standing Waves in Pipes — When sound reflects back and forth in a tube, interference creates standing waves at resonant frequencies. Open end: displacement antinode, pressure node. Closed end: displacement node, pressure antinode. Open-open and closed-closed support all harmonics (f₁, 2f₁, 3f₁ …). Open-closed pipes support only odd harmonics (f₁, 3f₁, 5f₁ …) — that is why a clarinet (open-closed near the reed) sounds an octave lower than a flute of the same length.