Sound Velocity with Changing Mediums

[linkin06]

---

Members don't see this ad.

what i know: when light goes from air to medium of higher index, velocity slows down, and wavelength decreases since frequency stays the same

i'm getting myself confused w/ sound waves tho.

what i know: sound travels faster in denser medium
in one medium, velocity is constant, and wavelength and frequency change.

what i'm confused: would sound speed up going from one medium to another denser medium? if yes, then does that mean both frequency and wavelength increase? or only wavelength increases?

 

[kobe200LATE]

Sound travels faster in a solid than a liquid than a gas. However, when you are comparing two solids, two liquids, or two gases, the speed of sound decreases with increasing density.

In addition, whenever you travel from one medium to another, the frequency stays constant while the velocity and wavelength proportionally change.

Therefore, when a soundwave travels from one medium to a denser medium (assuming you are comparing a two solids, two liquids, or two gases), then the speed of sound (v) decreases while the frequency stays the same. Using the v = (wavelength)*(frequency) equation, wavelength would decrease proportionally with velocity.

 

[kwokkit]

Sound travels faster in a solid than a liquid than a gas. However, when you are comparing two solids, two liquids, or two gases, the speed of sound decreases with increasing density.

This seems contradictory to me. If sound wave going from lower to higher density, say from Solid 1 --> Solid 2, c decreases, but if sound wave going from lower to higher density, say from Gas 1 --> Solid 2, c increases? Explain?

 

[kobe200LATE]

Yea it does seem a little contradictory, but from what I know, the speed of sound primarily depends on two things: 1) the resistance to compressibility of the medium and 2) the density.

When comparing the speed of sound in a solid vs a liquid vs a gas, the solid will have the highest speed of sound followed by liquid followed by gas because the solid has the highest resistance to compressibility (followed by the liquid). Therefore, sound will always travel faster in a solid than a liquid than a gas.

However, if you are comparing two media with similar resistances to compressibility (ie two solids), then you have to look at their densities. The speed of sound tends to increase with decreasing density. Therefore, when you compare two media with similar resistances to compressibility, then the speed of sound is higher in the media with lower density.

Other factors play into the speed of sound in a specific medium as well. All other things being equal, a medium with increased pressure and increased temperature will result in a higher speed of sound.

This is how I see it so someone correct me if I'm wrong.

 

[kobe200LATE]

So basically for your question,

going from solid 1 --> solid 2, there is no change in resistance to compressibility since they are both solids. Therefore, you just have to compare densities so the speed of sound will be greater in the solid with less density.

However, going from gas 1 --> solid 2 causes an increase in the speed of sound because there is a change in the resistance to compressibility. The solid has a higher resistance to compressibility so even though it has a higher density than a gas, the speed of sound in the solid will be greater than the speed of sound in a gas

 

[BerkReviewTeach]

So basically for your question,

going from solid 1 --> solid 2, there is no change in resistance to compressibility since they are both solids. Therefore, you just have to compare densities so the speed of sound will be greater in the solid with less density.

However, going from gas 1 --> solid 2 causes an increase in the speed of sound because there is a change in the resistance to compressibility. The solid has a higher resistance to compressibility so even though it has a higher density than a gas, the speed of sound in the solid will be greater than the speed of sound in a gas

You have absolutely the right idea and correct information here, but do mind if I suggest a slight twist on your perspective here?

The speed of sound in a medium ultimately depends on the ability for a molecule in the medium to be disturbed, travel and strike a neighboring molecule, and then return to it's original (equilibrium) position to be disturbed again. If a medium has particles that are (a) close together or (b) traveling very fast, then it can propagate sound faster than other mediums. Resistance to compressibility and density are associated with particles that are close together (and their restoring force) and the speed at which particles can restore to their original position depends on their average temperature and the pressure (for a fluid at least). This explains the correlation between our two perspectives.

Particle speed and closeness is just an easier way for me to wrap my head around this concept.

 

[sv3]

You have absolutely the right idea and correct information here, but do mind if I suggest a slight twist on your perspective here?

The speed of sound in a medium ultimately depends on the ability for a molecule in the medium to be disturbed, travel and strike a neighboring molecule, and then return to it's original (equilibrium) position to be disturbed again. If a medium has particles that are (a) close together or (b) traveling very fast, then it can propagate sound faster than other mediums. Resistance to compressibility and density are associated with particles that are close together (and their restoring force) and the speed at which particles can restore to their original position depends on their average temperature and the pressure (for a fluid at least). This explains the correlation between our two perspectives.

Particle speed and closeness is just an easier way for me to wrap my head around this concept.

Hi, I get what your saying but then why the different effects on speed for each factor? I totally understand how resistance to compressibility can increase speed thanks to your particle closeness theory, but I don't see why higher density decreases it then? Higher density would also mean particles are closer no? Perhaps too close? Just curious now........

thanks

EDIT: I think i get it now. Less dense means lighter molecules that can move faster. Resistance to compressibility means they they will transport the distrubance better as opposed to absorbing it...............works in my head at least!

 

[linkin06]

forgot about my thread...so i don't understand what we concluded? i always thought increased density means atoms are close together = they will transmit the sound faster. so why would it slow down from gas to solid?

 

[Dr Gerrard]

The speed of sound depends on two factors: elasticity and density. The
more elastic a medium, the greater the speed. The more dense the medium
the slower the speed of sound. For example steel is 6000 times more dense
than air but 2,000,000 times more elastic than air, so sound travels 16
times faster in steel than in air.

Source (might not be the best):

http://www.newton.dep.anl.gov/askasci/phy00/phy00999.htm

 

[linkin06]

just gonna forget about this. if i can't get a good answer in like 5 posts, way beyond the scope.

 

[Dr Gerrard]

What do you mean, did you not look at that link I posted? That pretty much answers your question.

Density matters as much as elasticity.

As density increases, speed decreases.
As elasticity increases, speed increases.

The density of a metal is less than the density of a gas, but the elasticity of steel is much much greater than a gas, which makes up for the decrease in speed due to density.

However, with two solids, the elasticity is pretty much equal, and so that is when density plays a bigger role.

This is probably not that necessary to know, but I didn't know it before looking it up, and I feel it has helped me understand this better, or at least just given me more confidence in feeling like I can get it.

 

[kwokkit]

Just noticed a small error Dr Gerrard, but great explanation!

The density of a metal is GREATER than the density of a gas, but the elasticity of steel is much much greater than a gas, which makes up for the decrease in speed due to density.