Helium voices sound high-pitched because formants shift, not because pitch changes

Evidence

In fact, the dramatic effects of helium and SF6 are not due to any change in the frequency of the voice, contrary to the barrage of statements to the contrary. The frequency (and in fact the pitch) stays the same, unless the speaker gets carried away and tries to exaggerate the effect. What does change are the formant frequencies: they are dramatically repositioned. One can also say that the timbre of the voice changes dramatically (see chapter 24). For a half-open straight tube, the resonance frequencies (the formants) are given by fn = (2n – 1)c/4L. If c triples or halves compared to air, a large expansion or contraction of the formants occurs that it is not possible with shape changes alone. In fact, a change in c is equivalent to a change in L, the length of the tube. For example, the effective length of our vocal tract is twice as long for SF6, to mimic the effect of the heavier gas. The drive is free to work at any frequency—only the formants have been changed. The pitch is determined by the drive frequency, not the formant frequencies. — Eric J. Heller, Why You Hear What You Hear- An…, 1713

Categorization

• XREF: Connects to vocal tract resonances as a tube-resonance model (fn formula) and to timbre/source-filter theory of speech production.
• CANDY: The classic helium-voice trick, now explained correctly — great impromptu party fact.