Why does helium make your voice higher?

[David513]

I thought that less dense media made sound travel more slowly, e.g. sound travels slower through water than through a solid. Why, then, would a less dense medium (helium) produce a higher pitch (i.e. faster) sound wave vs. normal air?

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[aldol16]

Speed of sound = sqrt(bulk modulus/density). So lower density means faster travel. This makes sense because the less dense something is, the farther it can travel before having to transfer energy to another particle.

 

[Mad Jack]

Speed of sound = sqrt(bulk modulus/density). So lower density means faster travel. This makes sense because the less dense something is, the farther it can travel before having to transfer energy to another particle.

Pretty much this. Lighter gasses allow sound to travel faster because the gas molecules are moving faster at a given pressure (if I recall correctly).

 

[aldol16]

Pretty much this. Lighter gasses allow sound to travel faster because the gas molecules are moving faster at a given pressure (if I recall correctly).

Yeah, Maxwell speed distribution.

 

[David513]

Thanks for the responses so far.

I know that sqrt(bulk modulus/density) but the hurdle I can't get over is why a solid (more dense) conducts sound faster than air (less dense), but air (more dense) conducts sound slower than helium gas (less dense).

What am I missing?

 

[sanrose]

If we focus only on density, then we can falsely deduce your line of reasoning. However, solid conducts sound faster than air, because of having an extremely huge elasticity (bulk modulus). As a result of which sqrt (bulk modulus/density) of solid is still bigger than air.

Now, if we look only at air and helium, their B is pretty much the same, but He has smaller density. So, sqrt (B/density) of air will be smaller than helium. So, Helium will have higher velocity, meaning higher frequency (i.e. pitch)

 

[aldol16]

I know that sqrt(bulk modulus/density) but the hurdle I can't get over is why a solid (more dense) conducts sound faster than air (less dense), but air (more dense) conducts sound slower than helium gas (less dense).

Remember that there are two variables in the equation for speed of sound. You have bulk modulus as a variable and also density. So having a high speed in a solid is not actually due to density but mostly rather to bulk modulus. You can tell because when you compare a solid as opposed to air, a solid will conduct better despite having higher density.