Chemistry

Why your Voice Sounds Funny after Inhaling Helium



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Just a few puffs from a helium balloon and you can sing just like “Alvin and the Chipmunks”.  Whether this is a desirable goal or not, it is a fun and familiar trick for children and adults alike.  Nearly everyone is familiar with the effect, but the cause is less well known.

Your vocal cords produce sound waves in the air by vibrating.  You make higher pitches by tightening the cords, causing them to vibrate faster.  Lower pitches are from more relaxed cords, and a slower rate of vibration.  The sound wave is carried through the air, traveling at the speed of sound (about 340 meters per second). 

Here’s the catch.  340 meters per second is the speed of sound in air.  Sound travels at different speeds based on the density of what it is traveling through (the medium).  Helium is only about one eighth the density of air (which is mostly nitrogen and oxygen).  Because it is less dense, sound waves can travel more quickly through helium than they can through normal air.  Your vocal cords still vibrate at the accustomed rates, and if everyone were standing in helium, ears included, everything would sound normal.  Your voice gets higher because of the transition from helium to normal air. 

The sound waves that are your voice travel along at the speed of sound, whatever that speed is for the medium they are in.  The speed of sound in helium is about three times faster than the speed of sound in air.  As the sound wave passes from helium into the air, it drops to the slower speed.  This has the effect of compressing the wave, so that its frequency is increased.  The listener (whose ear is in the air, not in helium) then hears the higher frequency sound wave, a voice that is about three times as high as normal.  The voice also gets a little distorted, because there is not a precise barrier between helium and air.  Instead, there is a region in which they are mixed unevenly, so some vibrations change speeds a little sooner or later.  The net result, of course, is a high, squeaky voice.

You can visualize this by thinking about traffic on the highway.  Along a stretch of highway where the cars are zipping along at 60 miles an hour (or 100 kilometers an hour if you prefer), there is usually a fair amount of distance between cars.  Sane drivers will have left themselves at least three car lengths between their car and the next.  When the cars hit a construction zone, traffic slows down, and the cars pack in close together.  To someone standing off to the side of the road, the number of cars per mile is higher in the construction zone than on the rest of the highway.  Your ear registers sound in much the same way, so if the speaker uses helium, their voice starts out on the highway, but the listener, who is in regular air, hears the tightly spaced traffic jam instead.

If someone wanted to be clever, they could reverse the process.  A person could fashion a helmet filled with helium (giving themselves an air hose so they could breathe still) so that their ears would be immersed in a helium environment.  From inside the helium helmet, “normal” speech from outside (in regular air) would sound deeper.  This is because the sound waves would be stretched out upon reaching the helium environment.

Some people actually were clever, except they didn’t mess around with helium and helmets.  Instead, they found a gas that was significantly denser than air, and breathed that in instead.  The gas is sulfur hexafluoride, and is about five times the density of air.  If a speaker inhales this gas, their voice passes from dense sulfur dioxide into less dense air, and speeds up.  The waves are stretched out, and their voice is heard at a much deeper tone than usual.  This has become a popular demonstration, and can be observed in various YouTube clips. 

One word of caution: helium and sulfur hexafluoride are both non-toxic, but can still be deadly.  Any time you are breathing them in, you are displacing the air in your lungs and depriving your body of oxygen.  For a short time this is not a problem, as your body has oxygen stored up in your blood.  For an extended period, you may pass out or even die from lack of oxygen.  Avoid this by using good sense (stop to breathe between every few sentences) and have someone there to help you if things do go awry.

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