Most of us are familiar with this trick: inhale a lungful of helium and you start talking like Donald Duck. Now it may seem a person’s voice has a higher “pitch”, but if this were true, they’d sound more like Mickey Mouse. To understand what’s going on, we need to know a bit about how sound is created.
Sound is simply the vibration of a substance – air, water, wood etc. When we speak, sound is created by the vibration of two folds of tissue (i.e. vocal folds) in our larynx (or voice box). This in turn causes individual air molecules to vibrate and move through space as sound waves.
Did you know? Birds produce sound using a syrinx, which works like human vocal folds.
When sound waves travel, each air molecule hits another but eventually returns to its original position. The result is similar to bouncing a slinky back and forth – part of the wave is compressed (regions known as condensations) and part of it is stretched (regions known as rarefactions). In effect, sound is a compression wave.
The distance between two successive compressions (or rarefactions) is known as a wavelength. And the number of waves passing a fixed point in a given time period is the frequency (cycles per second).
Characteristics of Speech
The three main qualities of speech are loudness, pitch and timbre.
The loudness of our voice is determined by how much the sound wave is compressed - the more it’s compressed, the louder the sound. For example, when air is forced out of the lungs as a scream, the sound wave is much more compressed than when it’s let out as a whisper.
Did you know? The howl of howler monkeys is the loudest sound of any land animal in the world. They can be heard up to 5 km (3 miles) away.
Pitch refers to the frequency of a sound wave - a high pitch sound corresponds to a high frequency and vice versa for a low pitch sound. In speech, the frequency is determined by the vibration of our vocal chords. Therefore, we can use the muscles of the larynx to change the elasticity and tension of the vocal folds and control the pitch of our voice.
Did you know? A male’s voice breaks during puberty because the vocal folds are lengthening.
The third characteristic of speech, timbre (pronounced TAM-ber), is what gives each of us a unique “voice print”. It’s a property of sound that has nothing to do with loudness or pitch. For instance, we can tell a child is speaking and not a woman because of differences in timbre, not pitch.
The sound created by the vocal chords is not a pure tone. Although our voice may have a fundamental (or dominant) frequency, which is determined by the vibration of vocal chord, it also has a number of other frequencies. These are known as harmonics and they vibrate faster than the fundamental frequency. Which harmonics are strongest is determined by the shape of the vocal tract (throat, mouth and nose).
Let’s now return to the question – how does helium affect our voice?
Putting it all Together
In our discussion so far, we’ve been talking about sound traveling through air - a mixture of nitrogen, oxygen and other gases. As a pure gas, helium is seven times lighter than air. Because it is less dense, sound travels about 3 times faster in helium than in air.
In a previous article I discussed how the speed of sound (and light) can be calculated using its wavelength and frequency, where frequency is in units of hertz (cycles per seconds):
Wave speed (m/s) = wavelength (m) × frequency (Hz)
In this context, there are two things to note when discussing what happens to our voice when inhaling helium.
We don’t necessarily talk louder when inhaling helium, which means we aren’t changing the wavelength of our voice The pitch of our voice does not change – the vocal folds vibrate at the same frequency as when we breathe air.
At first glance, this may not seem to make any sense. How can the speed of sound in helium increase if the wavelength (loudness) and frequency of vibration (pitch) remain unchanged?
The answer – helium affects the “timbre” of our voice.
Although the fundamental frequency (pitch) remains unchanged in the presence of helium, there is a shift in the harmonics of our voice. Specifically, the timbre becomes dominated by vibrations with higher frequencies (because the sound is travelling faster) and this leads to the “Donald Duck” effect.
Inhaling helium from a balloon is not necessarily a risky thing. But do keep in mind that the more helium you inhale, the less oxygen your body is getting. And never, ever inhale helium from a pressurized tank... you could end up with helium bubbles in your blood, leading to a cerebral arterial embolism and possibly death.
Principles of Voice Production (tutorials)
Helium (and other inert gases)
Just for Fun
Watch a video of Mel Blanc’s vocal folds (cords) – the voice of Bugs Bunny, Sylvester the Cat, Tweety Pie, Porky Pig, and other cartoon characters.