David513

5+ Year Member
Dec 25, 2010
105
36
Status (Visible)
  1. Pre-Medical
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?
 

aldol16

5+ Year Member
Nov 1, 2015
5,236
4,014
Status (Visible)
  1. Medical Student
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.
 
  • Like
Reactions: 1 users

Mad Jack

Critically Caring
7+ Year Member
Jul 27, 2013
37,202
71,960
4th Dimension
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).
 
About the Ads

David513

5+ Year Member
Dec 25, 2010
105
36
Status (Visible)
  1. Pre-Medical
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

5+ Year Member
Dec 5, 2014
29
40
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

5+ Year Member
Nov 1, 2015
5,236
4,014
Status (Visible)
  1. Medical Student
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.
 
About the Ads
This thread is more than 5 years old.

Your message may be considered spam for the following reasons:

  1. Your new thread title is very short, and likely is unhelpful.
  2. Your reply is very short and likely does not add anything to the thread.
  3. Your reply is very long and likely does not add anything to the thread.
  4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
  5. Your message is mostly quotes or spoilers.
  6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
  7. This thread is locked.