Who knows how many of you have tried at least once at parties inhale helium of balloons, playing with the fact that This gas temporarily alters the voice! This effect is due to the fact that the timbre of our voice depends on the speed of sound in the gas we breathe, and the speed of sound in helium is about three times faster than the speed of sound in air (about 1030 m/s versus 340 m/s). It’s a question of resonance: the sound waves produced by the vocal cords resonate in the mouth cavity with particular frequencies that are directly proportional to the speed of sound propagation. The result is that the frequencies of our voice are higher and this alters our vocal timbre making it more acute.
Attention: Inhaling any gas other than air can cause decompensation. This is because by introducing gases of a different nature into our lungs we lower the oxygen levels in the lungs increasing the risk of asphyxiation. Breathing gases other than air can create mild to moderate-severe effects that include fatigue, dizziness, shortness of breath, vomiting, vertigo, indigestion, panic, loss of consciousness and suffocation.
Let’s say right away that helium – or any other gas – does not affect in the slightest the frequencies produced by the vocal cords. These elastic structures they vibrate against each other, opening and closing hundreds of times per second, thus producing sound waves with certain frequencies. However, the activity of the vocal cords is not the only factor that determines the stamp of our voice, that is, the particular mix of frequencies that allow us to distinguish one voice from another. The vocal timbre is in fact modulated by the wavelengths that actually resonate in the cavity of our moutha bit like the timbre of a guitar depends on which wavelengths resonate inside its body. In effect, we can therefore consider the cavity of the mouth as a sounding board: the size of the “box” destroys some waves, weakening the respective frequencies of our vocal timbre, and amplifies others, strengthening them in the mix that constitutes our timbre.
The wavelengths “selected” by the oral cavity are fixed by its size: for example, a wave with a wavelength equal to the size of the cavity will bounce back and forth, reinforcing itself, while a wave with a wavelength that is half the size of the cavity will self-cancel as it bounces back into the cavity.
But be careful: the wavelength and the frequency of a wave are not the same thing. From a physical point of view, the wavelength is the distance between two consecutive crests of a wave, while the frequency is the number of crests that pass through a certain point in the unit of time. The two quantities are linked by this simple formula:
frequency = wave speed / wavelength
So, once the oral cavity fixes the wavelengths that make up the harmonics of our voice, the frequencies associated with these wavelengths depend on the speed of the sound waves. This is where helium comes into play: the speed of sound in helium is about 1030 m/swhile in air at the same temperature it is about a third (343 m/s). Looking at the formula above, we understand that, given the same wavelengths, the relative frequencies are higher in helium: to be precise, about three times higher! This means that, by inhaling helium, the harmonics of our voice – which determine our overall vocal timbre – will be higher and therefore our voice will be higher. We repeat: what changes with helium is not the overall frequency of our voice but its timbre, understood as the quality of the sound.
At this point one might ask: why is the speed of sound in helium higher? A notion that is commonly heard – however wrong – is that sound travels faster in denser media. In reality, it’s the opposite: the speed of sound decreases the denser the medium, all else being equal. In fact, helium has about one tenth the density of air, and this is the main reason why the speed of sound is higher in helium. The speed of sound also depends on another very important factor, namely theelasticity of the medium, which in turn depends on other parameters, but without going into details we can say that taking everything into account the result for helium is a speed of sound of approximately 1030 m/s.
There are gases that have the opposite effect to helium, lowering the timbre of our voice. An example is thesulfur hexafluoride (SF6), which is almost 6 times denser than air and therefore produces a “cavernous” tone of voice. The effect lasts longer than that of helium because sulfur hexafluoride is denser than air, so it tends to stay low and has a harder time exiting the lungs.
