# Thread: Speed of sound....help me...

1. Cs(shear wave)=square roots of G/P

where G are the shear modulus of the elastic materials.
For example, for a typical steel alloy, G = 80 GPa and ρ = 7700 kg/m3, yielding the shear velocity cs is estimated at 3200 m/s using the same numbers...

This paragraph is from wikipedia...i am not understand....
why 80/7700=3200?
Can anybody explain?

2.

3. it's the units. you need to pay closer attention to them. G=80,000,000,000 and p=7,700,000

4. Did you forget to take the square root?

5. but i also cannot find the correct answer...

6. Originally Posted by Arcane_Mathematician
it's the units. you need to pay closer attention to them. G=80,000,000,000 and p=7,700,000
Actually, you will want your units in pascals and kilograms (not grams) keeping everything in the MKS system.

7. Sound is not a shear wave.

8. In solid,sound would be a shear wave.
How to calculate the speed of sound wave(longitudinal wave)in a solid?
the square root of [K(bulk mudulus)+(G*4/3)]/density
is this true?

9. can anybody help me.....

10. Originally Posted by ~James~
can anybody help me.....
apparently not.

You were already told that sound is not a shear wave.

11. Originally Posted by Harold14370
Originally Posted by Arcane_Mathematician
it's the units. you need to pay closer attention to them. G=80,000,000,000 and p=7,700,000
Actually, you will want your units in pascals and kilograms (not grams) keeping everything in the MKS system.
I apparently wasn't paying close attention... my bad. I just say the giga and kilo and wanted to convert to grams.

12. Originally Posted by DrRocket
Originally Posted by ~James~
can anybody help me.....
apparently not.

You were already told that sound is not a shear wave.
In solid,sound would be a shear wave.....In wikipedia,it has explain sound is a shear wave when it transfer through solid....Please search "speed of sound" in wikipedia....the part of "speed in solid" has explain this....but give me the reason(Why sound(longitudinal)wave is related to the sear modulus?)

13. Originally Posted by ~James~
can anybody help me.....
Show us your calculation, then maybe somebody can tell what you did wrong.

14. Originally Posted by ~James~
Originally Posted by DrRocket
Originally Posted by ~James~
can anybody help me.....

You were already told that sound is not a shear wave.
In solid,sound would be a shear wave.....In wikipedia,it has explain sound is a shear wave when it transfer through solid....Please search "speed of sound" in wikipedia....the part of "speed in solid" has explain this....but give me the reason(Why sound(longitudinal)wave is related to the sear modulus?)
It is possible to generate shear waves in a solid, but those are not typical sound waves. Normal transmission of sound is via compressional longitudinal waves. Moreover, if you are quoted the speed of sound in a material, without qualification, that speed will correspond to the longitudinal (compression) mode.

Shear wave speed is related to the shear modulus because the deformation mode (strain) is shear strain, of course.

Did you actually read the article, or just copy down formulas without understanding the meaning ?

http://en.wikipedia.org/wiki/Speed_of_sound

15. In order to understand the relationship between longitudinal waves and shear-force, look at the Golden Gate bridge on a windy day. (The wind being low-frequency sound in a way.)

16. Originally Posted by himnextdoor
In order to understand the relationship between longitudinal waves and shear-force, look at the Golden Gate bridge on a windy day. (The wind being low-frequency sound in a way.)
The waves that you see are transverse waves and torsional waves, not longitudinal waves.

Look at the films of the Tacoma Narrows Bridge disaster for larger amplitude transverse waves.