The gas filling a resonant cavity shifts its Helmholtz frequency.

Evidence

If the f-holes are taped up, the violin can sound like a recording being played through cheap speakers. Filling the violin with helium significantly raises the frequency of the Helmholtz mode, resulting in a tinny timbre. Oppositely, putting sulfur hexafluoride (SF6) in the violin drastically lowers the frequency of the Helmholtz mode. — Eric J. Heller, Why You Hear What You Hear- An…, 1243

Categorization

• CANDY: Violin filled with helium or SF6 changing its timbre is a delightful bar-trick fact grounded in real acoustics.