Inhaling helium.

1. Jun 10, 2009

leroyjenkens

I just watched this Mythbusters video

And he explains that the reason your voice gets higher when you inhale helium is because helium is less dense than air, so sound travels faster through it, making your voice higher.

But wouldn't sound travel slower through a less dense gas?

Last edited by a moderator: Sep 25, 2014
2. Jun 10, 2009

Bob S

Sound velocity is nearly exactly proportional to sqrt[pressure/ density] = sqrt[meters2/ sec2]= meters/sec, so a lower density for helium at sea level atomspleric pressure means higher sound velocity. For air, the ratio sqrt[pressure/ density] is independent of elevation (barometric pressure), so it is very important for wind instruments (e.g., flute) in orchestras at high elevations, otherwise they would be way out of tune.

3. Jun 10, 2009

uart

Think of the analogy of a wave travelling along a thin spring (like one of those "slinky" toys for example). The stiffer is the spring and the less its mass-density then the faster wave travels. So, for a given stiffness spring then, the lighter the weight of the spring then the faster the wave will travel. Hopefully that makes some intuitive sense to you.

Perhaps surprisingly the "stiffness" of all idea gases is exactly the same (at the same temperature and pressure) irrespective of their mass. By "stiffness" of a gas here were really talking about the amount of pressure increase required to compress the gas by a given amount - it's kind of the same thing as the stiffness of the spring ok.

So the lighter gas is rather like making a lighter weight spring that is still every bit as stiff as it's heavier counter-part. A wave then will travel faster in this lighter weight spring than it would in the heavier one.

4. Jun 10, 2009

FredGarvin

Be careful. The reason your voice sounds higher is due to the fact that the helium in the surrounding volume near your vocal chords makes the natural frequency of your vocal chords increase in frequency. Your vocal chords can vibrate more freely (think of air and hydrogen as springs that apply a resisting force to prevent your vocal chords from vibrating).

The way many people present the reason, some may take it to mean that the speed of sound simply is higher and thus the sound waves travel faster. That is not the reason. For that response to be applicable, everyone listening would have to be in a helium environment.

5. Jun 10, 2009

uart

I always thought it was related to resonant modes in the vocal tract - similar to how organ pipes produce different frequencies depending their different lengths. For example if the principle mode has the length (L) of the vocal tract equal to one half wavelength then the frequency of the principle mode is v/(2L). In other words, if v increases but wavelength stays the same then the frequency must increase.

6. Jun 10, 2009

FredGarvin

The resonant modes are the natural frequencies which is what I mentioned but for a different part. The helium has a specific bulk modulus that is much higher than that of air and thus should lower the resistive force to vibrations.

According to this source, you are more on track than my teachings are:

So it appears it is a combination of the two affects but it is dominated by the resonance of the vocal tract.

Last edited by a moderator: May 4, 2017
7. Jun 10, 2009

leroyjenkens

So to simplify, if I got this right, the sound doesn't move faster through helium, but the vocal cords do?

8. Jun 10, 2009

Bob S

Sound velocity is nearly exactly proportional to sqrt[pressure/ density] = sqrt[meters2/ sec2]= meters/sec, so a lower density for helium at sea level atomspleric pressure means higher sound velocity. The sound velocity in pure helium is about sqrt(29/4) = 2.7 times faster than in air.

9. Jun 10, 2009

Danger

I feel obliged to insert a warning here. Do not inhale balloon helium on any recurring basis. Once a year or so might be okay, but you still shouldn't do it. The Medical Grade stuff is okay. Balloon grade has oil in it to lubricate the tank valves. (At least, it used to. I'm not sure about now.) Inhaling directly from a balloon can be even worse. My wife sells a lot of balloons at work, and most of them have an anti-leak gel injected to make them last longer. If it prevents helium from leaking through mylar, there's a pretty good chance that it would impede the entry of air into your alveoli.

10. Jun 10, 2009

leroyjenkens

So sound moves faster in less dense gases? Doesn't sound move faster in liquids and solids than it does in gases because they're more dense?

11. Jun 10, 2009

Bob S

I said that the velocity of sound in gases is proportional to sqrt[pressure/density]. As you may recall, the mass of a gas in 22.41 liters at STP is equal to the gram-molecular-mass (Avagadro's number of molecules) of that gas in grams; e.g., hydrogen 2, helium 4, nitrogen 28, air 29, oxygen 32, CO2 44 grams, etc..

Last edited: Jun 10, 2009
12. Jun 10, 2009

Phrak

I occationally wondered about these two answers. This result brings up a new raft of questions.

Such as,

.Are they both proportional to the same power of the gas density, and remain 'in tune'?

.If they were not proportional would this effect vowel sounds where the positions of the 2nd and third frequency peaks determine the vowel?

.The inertial of the vocal cords doesn't change. Maybe the two frequencies don't remain properly in sync. ?

I don't expect you to know. Just interesting questions.

13. Jun 11, 2009

leroyjenkens

So sound moves faster through less dense gases? Or just helium? I'm still not clear on that part.

If it moves faster through less dense gases, why does it move faster through more dense liquids and solids?

You have to speak layman to me. Imagine you're explaining it to an 8 year old.

14. Jun 11, 2009

uart

Did you read my post #3 at all Leroy?

15. Jun 11, 2009

leroyjenkens

Yeah, but I'm still confused.
So in simple terms, the less dense the gas, the faster the speed of sound is in that gas? That's what I think I'm reading.

16. Jun 12, 2009

uart

Ok let me re-cap the main points. The wave travels faster if the material is "stiffer" (as in more force to compress by a given ratio) in relation to it's mass density. So there are two factors at play here - the density of the material and it's "stiffness" (see Youngs Modulus). So yes a liquid or solid will be more dense than a gas however it will also be very much stiffer than the gas. Sound would typically travel faster in the solid or liquid compared with a gas, even though the solid or liquid is more dense, but the underlying reason is because of the vastly greater stiffness.

Say for example we were comparing air with water. Liquid water is about 700 times more dense than air but way more than 700 times "stiffer". So sound travels faster in water than in air.

Last edited: Jun 12, 2009
17. Jun 12, 2009

leroyjenkens

What's confusing me, I think, is that I've never heard of materials referred to by their stiffness. So it's taking me a little bit longer to catch on to what exactly that means.

Ok let me see if I understand this....

Sound waves travel faster through materials with greater stiffness and lesser density. So if you have two materials with equal stiffness and one has lesser density than the other, sound will travel faster through the material with lesser density.

Sound waves don't go through liquids and solids faster than gases because liquids and solids have a greater density, but because they have a greater stiffness. The sound travels faster in liquids and solids IN SPITE of them being denser, because their stiffness is so much greater.

So hypothetically, if a gas and a liquid had equal stiffness, the sound would instead move faster through the gas than the liquid because the gas is lighter.

And I probably messed that all up, so I apologize in advance.

18. Jun 12, 2009

uart

Yes that summary is pretty much correct.

BTW. I used the term "stiffness" to keep the explanation as non-technical as possible. Essentially I'm using the term stiffness as a qualitative indicator of the relative amount of force required to produce a certain amount of elastic deformation in a material. (As in a stiff material will require a lot of force to give a particular amount of elastic deformation whereas a less stiff material will require less force for the same deformation).

Technically it would have been more correct to refer to the "bulk modulus" or Young's Modulus of the material here. See : http://en.wikipedia.org/wiki/Young's_modulus

19. Jun 12, 2009

leroyjenkens

Ok cool, thanks for the explanation. But just to clarify, the reason your voice sounds higher when you inhale helium is for the reason FredGarvin gave, and not because the speed of sound is faster in helium?

Here's what FredGarvin said:
Which makes sense to me. But that means that in effect, the explanation given by the Mythbusters guy was wrong. If I'm understanding it correctly.

20. Jun 12, 2009

Nick89

The effect cannot possibly due to the fact that soundwaves move faster in helium than in air (they do, but it is beside the point!). Sure, the waves would travel faster through your throat, but when they leave your mouth, the end up in normal air and should slow down again. So anyone in your vicinity should NOT hear a higher voice!

So (if I remember the mythbusters explanation correctly), yes, Adam was wrong.