Medium and Wave velocity

[thais]

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Although I've searched older threads I am still confused about the relationship between medium and wave velocity.

If I understood it correctly, sound waves velocity depend on the medium because it needs a medium to propagate while electromagnetic waves do not, because they don't need a medium to propagate. (is this correct?)

However, I'm a bit confused on info given my EK.
They mention that "velocity of the wave is dictated by the medium". (could someone please explain why is that?)
Then, in question #747 of 1001 questions it mentions that "The wave velocity is independent of the medium" even though they are talking about sound waves. ???

Any help will be appreciated! 🙂 Thank you!!

 

[SaintJude]

Sound waves are mechanical disturbances that travel within deformable material medium. They definetely need a medium. Unlike EM waves, they cannot be propagated through a vacuum. You hear nothing in space.

Velocity of a mechanical wave depends (i.e. dictated) by the physical properties of a medium. This means that ONLY changes in the physical properties of a medium will change the velocity of a sound wave. Now, I'm sure you're thinking "But what about v = fλ?" It is still applicable! You can still measure the velocity of a sound wave if given it's frequency. You just cannot predict changes in velocity of a wave when frequency changes. B/c only the physical properties will determine the velocity.

You may go on & ask "Well why do the physical properties 'dictate' velocity of sound waves?"
The speed of sound is dependent upon the ability of molecules to be disturbed, strike nearby molecules & return to its original position--essentially how quickly these sound molecules can vibrate back & forth. Look at the pic below:

The vibrations are just collisions occurring. If a material medium has particles that are very close together (typical of materials w high bulk modulus), this will facilitate the vibration of sound waves--and so will gases with higher temp! Phew! Sorry, usually don't try to post such length responses...

 

[thais]

Wow, thank you so much. It is clear now 🙂
But then what about EK 1001 #747: "The wave velocity is independent of the medium" even though they are talking about sound waves. ?
Thank you very much!

 

[dmf2682]

Wow, thank you so much. It is clear now 🙂
But then what about EK 1001 #747: "The wave velocity is independent of the medium" even though they are talking about sound waves. ?
Thank you very much!

What's the exact question?

 

[thais]

The explanation doesn't seem relevant to the question since the medium doesn't change.
Here it is:

"The speed of sound in air is 340m/s. The sound from a jet moving through air at 100m/s will move at:"

 

[SaintJude]

The explanation doesn't seem relevant to the question since the medium doesn't change.
Here it is:

"The speed of sound in air is 340m/s. The sound from a jet moving through air at 100m/s will move at:"

You are right, thais, they could have written this more clearly. They are saying that the velocity of the source of the sound wave is independent from the velocity of the wave itself.

It doesn't matter how fast the jet is moving. It could be moving 50 m/s or 100 m/s. The speed of sound itself in air will still be 340 m/s, because the velocity of the sound waves depend on the medium it is traveling in: air, in this case.

 

[thais]

Thank you guys! 🙂