Helium Inhalation: Mythbusters Video Explained

[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?

 

[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.

 

[uart]

Think of the analogy of a wave traveling 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.

 

[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.

 

[uart]

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.

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.

 

[FredGarvin]

I always thought it was related to resonant modes in the vocal tract

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:
http://www.phys.unsw.edu.au/phys_about/PHYSICS!/SPEECH_HELIUM/speech.html

If you keep the muscle tensions the same, that is, the frequencies will not change much. There could be a small change because the less dense Helium loads the vocal folds a bit less than the air, but this effect is slight. The effect on the resonances is large, however. Its size depends on how pure the Helium in your vocal tract is.)

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

 

[leroyjenkens]

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

 

[Bob S]

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

Nottrue. Read my post #2:
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.

 

[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.

 

[leroyjenkens]

Nottrue. Read my post #2:
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.

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?

 

[Bob S]

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?

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..

 

[Phrak]

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

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.

 

[leroyjenkens]

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..

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.

 

[uart]

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.

Did you read my post #3 at all Leroy?

 

[leroyjenkens]

Did you read my post #3 at all Leroy?

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.

 

[uart]

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.

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.

 

[leroyjenkens]

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.

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.

 

[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

 

[leroyjenkens]

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

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:

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.

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.

 

[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.

 

[leroyjenkens]

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.

Ok it all makes sense.

So would inhaling hydrogen make your voice even higher than helium does?

 

[uart]

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.

Ok it all makes sense.

No it's completely wrong. But you seem destined to take on whatever is the most incorrect answer offered and I don't have either the time or the patience to explain it to you any further.

 

[leroyjenkens]

No it's completely wrong. But you seem destined to take on whatever is the most incorrect answer offered and I don't have either the time or the patience to explain it to you any further.

I apologize that my stupidity has offended you (somehow), not everyone can be as smart as you.
But nothing anyone has said has contracted what you said, so I don't know where this hostility is coming from all of a sudden.
And apparently I accepted one incorrect answer (according to you), and that somehow makes me DESTINED to accept the most incorrect answer?
If you're going to be arrogant about it, I'd rather not have your opinion anyway.

 

[Nick89]

No it's completely wrong. But you seem destined to take on whatever is the most incorrect answer offered and I don't have either the time or the patience to explain it to you any further.

If you're so sure you are correct then why not explain it? It shouldn't take you that much time... We might even learn something today. What you're doing now is basically sticking your fingers in your ears and shouting "I'm right, deal with it."...