Helium and sulfur hexafluoride change voice timbre, not pitch

Evidence

If the mouth, tongue, and vocal tract are kept absolutely rigid, the pitch of the whistle changes very little as less or more air is forced through. although it may jump to a new mode of the vocal tract at higher velocity. This is not true of cavity-free jet vortex formation, such as happens when a window is cracked open in a breeze; this increases in pitch as the air is forced more rapidly. — Eric J. Heller, Why You Hear What You Hear- An…, 1333 (#expands, via linked claim)

Some sources (and almost all Internet sources) incorrectly assign the whistle frequency to the Helmholtz resonance of the oral cavity. However, the vocal tract is necessarily open all the way to the larynx while one is whistling, since air is being supplied by the lungs! In this circumstance, there is no resonance that belongs only to the oral cavity. This assertion is easily checked: form a whistle, then without changing your mouth or lip shape, and while breathing in or out slowly, pop your cheek with your index finger and listen for the lowest frequency mode. Its frequency will be far below the whistle frequency. Now close off the oral cavity with the back of the tongue and compare the frequencies. Since the vocal tract is open to allow the air to pass through the lips, the same vocal tract resonances used in speech somehow must control the frequency of the whistle. — Eric J. Heller, Why You Hear What You Hear- An…, 1324 (#expands, via linked claim)

Categorization

• XREF: connects to: Whistle pitch derives from vocal tract resonances, not the oral cavity’s Helmholtz resonance — Eric J. Heller, Why You Hear What You Hear- An…, 1324; Fixed vocal tract whistles hold pitch while cavity-free jets rise in pitch with airflow — Eric J. Heller, Why You Hear What You Hear- An…, 1333; Speech excitation is not coupled to tract resonances, unlike most instruments (+1 more)

Broader Connections

• Fixed vocal tract whistles hold pitch while cavity-free jets rise in pitch with airflow #expands (0.85) — Eric J. Heller, Why You Hear What You Hear- An…, 1333
• Whistle pitch derives from vocal tract resonances, not the oral cavity’s Helmholtz resonance #expands (0.80) — Eric J. Heller, Why You Hear What You Hear- An…, 1324
• Nonlinear distortion generates new frequencies that change timbre with volume #expands (0.72)
• Speech excitation is not coupled to tract resonances, unlike most instruments #complicates (0.70)