Sucking a lungful of helium out of a balloon makes your voice sound hilarious. But contrary to popular belief, the switch from air to helium gas doesn’t actually increase the pitch of your voice (at least not very much). Instead, it affects a much more mysterious property of the sound, called “timbre.”
Firstly, what is Helium?
Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among all the elements and it exists only as a gas except in extremely cold conditions.
Helium is the second lightest element and is the second most abundant element in the observable universe, being present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. Its abundance is similar to this figure in the Sun and in Jupiter. This is due to the very high nuclear binding energy (per nucleon) of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have been formed during the Big Bang. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars.
Helium is named for the Greek god of the Sun, Helios. It was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer. Jannsen observed during the solar eclipse of 1868 while Lockyer observed from Britain. Lockyer was the first to propose that the line was due to a new element, which he named. The formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, which is by far the largest supplier of the gas today.
Liquid helium is used in cryogenics (its largest single use, absorbing about a quarter of production), particularly in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium’s other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers—account for half of the gas produced. A well-known but minor use is as a lifting gas in balloons and airships. As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. In scientific research, the behavior of the two fluid phases of helium-4 (helium I and helium II) is important to researchers studying quantum mechanics (in particular the property of superfluidity) and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero.
On Earth it is relatively rare — 5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as the alpha particles emitted by such decays consist of helium-4 nuclei. This radiogenic helium is trapped with natural gas in concentrations up to 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Helium is a finite resource, and once released into the atmosphere, it readily escapes into space.
Back to How Helium Makes Your Voice Squeaky
First, here’s what’s happening inside your throat when you talk: According to acoustics expert John Smith, a biophysicist at the University of New South Wales (UNSW) in Sydney, Australia, you generate sound by rapidly vibrating two small flaps of mucous membrane, called vocal folds, in your voice box. The back-and-forth motions of these folds interrupt the flow of air from your lungs to create “puffs” of sound.
If your vocal folds wiggle back and forth 100 times each second, they produce puffs with a frequency of 100 beats per second (Hz). Additional motions of the vocal folds, such as collisions with each other, generate additional frequencies that are multiples of that fundamental frequency: “harmonics” at 200 Hz, 300 Hz, 400 Hz and so on.
All these frequencies travel together through the vocal tract — the tubelike cavity leading from the voice box up through the throat and mouth to the outside world. Depending on its shape, this tract resonates with certain harmonics generated by your vocal folds, meaning it vibrates in time with them. In doing so, the vocal tract amplifies those resonant harmonic frequencies, making them louder.
So, the harmonics created by your particular vocal folds paired with the shape of your particular vocal tract produce a unique collection of resonant frequencies that, taken together, give your voice its distinctive sound quality, or timbre.
That’s where helium comes in. Sound travels 1,128 feet per second (344 meters per second) through regular air, but it travels 3,041 feet per second (927 meters per second) through helium gas. This is because nitrogen and oxygen molecules that make up the bulk of air are much heavier than helium atoms, so they don’t oscillate back and forth nearly as quickly. (That oscillating is what pushes the sound wave through the gas.)
In physics, the speed of a wave equals its frequency multiplied by its wavelength. So, if a sound wave travels faster through a vocal tract full of helium than it would through a vocal tract full of air, either its frequency or its wavelength must get a boost in a helium-filled cavity, too.
The wavelengths that resonate with the vocal tract depend only on its shape — i.e., the resonant harmonics are the ones whose consecutive peaks fit snugly in the vocal tract — so their wavelengths stay the same regardless of whether the tract is filled with helium gas or air. (Put differently, the gas molecules inside the tract oscillate back and forth the same distance regardless of what molecules they are.)
That means the frequencies of the resonant harmonics must increase in a helium-filled cavity instead. According to Smith and colleagues in “Physics in Speech,” a reference article on the UNSW website, resonant frequencies are several times higher in a vocal tract filled with helium compared to one filled with air.
And that means certain high-pitch components of your voice become amplified relative to the low-pitch components, drastically changing the overall timbre of your voice. “There is less power at low frequencies so the sound is thin and squeaky”.