General Questions about Sound and Density...

[userman]

A few questions:

Usually, which molecules are denser? Heavier ones or lighter ones? Bigger ones or smaller ones?




Also, In the sound wave velocity equation
v=(yxP/density)^0.5

The density is on the denominator. But why Does sound travel faster in More dense objects???

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[CardiacArrest]

For the MCAT, understanding equations is the BEST way to do really well on the PS section.

Density = mass/volume. Mess around with the equation. Greater mass, yet smaller volume gives an object more density. Solid>Liquid>Gas.
Size does not matter. You can have 100g of cotton, 100g of brick. Brick wins due to more mass.

I've never seen that sound wave velocity equation before.
I'm more familiar with v=frequency x wavelength.
Given that, in low pressure and in high density objects, sound travels faster. Wavelength increases through more dense objects. Frequency will not be affected, however.
Hopefully someone else can help out here!

 

[UCB05]

A few questions:

Usually, which molecules are denser? Heavier ones or lighter ones? Bigger ones or smaller ones?




Also, In the sound wave velocity equation
v=(yxP/density)^0.5

The density is on the denominator. But why Does sound travel faster in More dense objects???

Even though density increases, which would indicate reduced speed, the bulk modulus, or stiffness, (that "yxP" term in your equation) of the medium usually increases by a greater amount, giving you an overall positive net change in speed. Given two media with the same bulk modulus, sound will travel more slowly through the denser one.

 

[userman]

"Density = mass/volume. Mess around with the equation. Greater mass, yet smaller volume gives an object more density. Solid>Liquid>Gas.
Size does not matter. You can have 100g of cotton, 100g of brick. Brick wins due to more mass."



However, I believe that HEAVIER liquid makes a DENSER gas??

This is the type of information I was looking for...

 

[Gryhu]

This is how TBR explains the speed of sound in different mediums (paraphrased):

The greater the force between two molecules (solid > liquid > gas) the greater the restoring force (less compressible).

The greater the restoring force, the more rapidly molecules are restored to their original condition, the more rapidly they can participate in another wave => faster propagation of sound.

 

[surag]

Hey. About sound travelling through more dense objects faster... Let me try and explain in the easiest way possible

first I will explain how waves travel since sound is a wave. Then I will explain the longitudinal property of waves. Finally I'll explain why this relates to it's variable speed w density

sound is a wave and I'm sure you learn that sound can travel in two forms as transverse and as longitudinal waves. Transverse means the phsical motion is perpendicular to the motion of the wave. For example a string moved up and down creates a transverse wave because the physical motion of the string going UP AND DOWN is perpendicular to the wave moving to the RIGHT.

However in a longitudinal wave the physical motion and the wave move parallel or in the SAME direction. Sound is longitudinal.

This is because sound travels in the direction of the movement of the particles that cause a compression or decompression in these particles. In essense sound is merely the changes in pressure of the material it is in over a distance. This is analagous(sp?) to a slinky that is held at one and and shot out another end. It will go a certain distance-expand and then decompress.

What's important here is that as a result of this property sound requires PARTICLES. Without them sound cannot propagate. Hence in space where there is a vacuum and no particles no compression or decompression can take place and no sound exists. Whereas light is transverse it is NOT dependent on particle movement for propagation hence light travels through space- this is an oversimplification of light but what I said is true

now why does sound move through denser objects better?
The issue with particles is that distortion exists. If particles are closer to each other the energy from one particle can be transferred to the next one easier much like dominos. Closer dominos are more likely to fall in a similar manner than farther apart ones. If they are spaced out too much they won't even touch and the energy won't be propagated!

That is why sound has trouble travelling through rarified gas than denser gas since particles can transfer the energy from sound vibrations with less distortion to closer particles in the denser gas than the lighter one where they are spaced apart more. The same canNOT be said about solids and liquids. Density slows down speed of sound but the increased compressibility allows sound to travel faster in solids over liquids over gas

 

[surag]

Even though density increases, which would indicate reduced speed, the bulk modulus, or stiffness, (that "yxP" term in your equation) of the medium usually increases by a greater amount, giving you an overall positive net change in speed. Given two media with the same bulk modulus, sound will travel more slowly through the denser one.

actually u are correct.