Quiz - Physics - Sound

[ToneNQ]

Yes Rikk that is right. it is a matter of which ((density or compressibility)) is the predominant driving factor on the end result........

Does predominance actually matter to diving? Its a multivariate formula, the question offers one variate only. The question doesn't imply a uni variate relationship, its just a simplified question for a dilatant understanding of physics, like Obama's economics. Its not re-visited in subsequent dive training I've done, at any level, because it doesn't matter.

To suggest that making something denser will allow sound to travel faster through it is also a fallacy. That's the point.

Is any one factor omnipotent? Maybe when PADI embrace elearning it will be easier to update material for the next print run. Not sure what the SSI and SDI materials reference, but it would be easier for them to publish updates. Refreshing instructors is an equal challenge.

 

[saxman242]

Is any one factor omnipotent?

Well, you could make the argument here that sound is slower when something is denser or you could make the argument that it's faster because of the change of pressure with respect to density, but to say sound is faster due to being denser isn't just overly simplified, it's the exact opposite of the truth.

It's one thing to overly simplify (I wouldn't expect padi to get in to the derivative of pressure with respect to density), but they could at least not go with being 180 degrees off.

I can't speak to SDI, but SSI says the same as PADI here.



A much more appropriate thing to say would be "because water is so much less compressible than air". Only takes one more word too.

 

[tursiops]

To suggest that making something denser will allow sound to travel faster through it is also a fallacy. That's the point.

And this misses the point.
One cannot simply say that sounds speed is the square root of K/rho and then say that sound speed and rho (density) are inversely related....unless you ALSO say that K is constant. You can't even approximately say that sound speed is inversely to density unless you say that the material is approximately the same. Air and water are not at all the same. As most have pointed out, the compressibility of air is 20,000 greater than that of water......and this is partly a function of the density of the material. Look at any table of materials and bulk moduluses (is that the plural?), and you'll find that over a broad range as density increases, so does K. that does not mean that there is a mathematical relationship between K and rho, but it does mean that as the material's density increases greatly, the sound speed also increases. Thus, a simple table:
DENSITY SOUND SPEED
air 1 low
water 1000 high

Whether you happen to know the Newton-Laplace formula or not, and whether or not you've ever heard of bulk modulus, the relationship is very simple. As you go from Air to Water, the sound speed INCREASES and the density INCREASES.

The issue is, does the sound speed increase BECAUSE the density increases? The detailed answer is that the SS increases primarily because the bulk modulus increases....but the point is that the bulk modulus increases because you are moving from a gas to a liquid......from a low density material to a high density material. The density changes is trying to reduce the SS by a factor of the square root of 1000; but the bulk moduls is trying to increase the SS by a factor of the square root of 20,000. Bulk modulus wins, obviously, but the only reason the bulk modulus of water is so much greater than that of air is because the liquid is denser than the gas.

 

[NAM001]

Does predominance actually matter to diving? Its a multivariate formula, the question offers one variate only. The question doesn't imply a uni variate relationship, its just a simplified question for a dilatant understanding of physics, like Obama's economics. Its not re-visited in subsequent dive training I've done, at any level, because it doesn't matter.



Is any one factor omnipotent? Maybe when PADI embrace elearning it will be easier to update material for the next print run. Not sure what the SSI and SDI materials reference, but it would be easier for them to publish updates. Refreshing instructors is an equal challenge.

Yes predominance does matter. The issue of what controls speed of sound in many aspects is moot because many factors that have an effect do not apply to diving or the limited environment we dive in. I dont have a chart at hand and am not sure how to past on into the post it but picture if you will a chart SOS is horizontal and depth is vertical the horizontal scale goes from 0 -100 and depth it doesnt matter much. a normal speed of sound plot looks like this on the surface it is about 80 and at say at 1000 feet it is 40 and from there down it gradually increases over the next several thousand feet to perhaps exceed a horizontal value of 80. temp and pressure both increase speed of sound. At the surface region say above 500 ft temperature will vary from there down temp is mostly constant. for grins because i dont remember the exact numbers for the scientists here to argue with. Temp effects SOS say 5 fps per degree. and pressure effects SOS 1fps per 100 ft. at the surface the temperature drives the SOS and is predominant because it has the greatest effect on SOS, or at least change n SOS. Below say 500 ft and below where temp is relative constant say 40 degrees F, depth or pressure becomes the predominant driver to change SOS. Depth does not change physics but depth in deeper water is the only changing variable because all other factors are no longer in control. and are moot because they are mostly constant. All other factors are still involved in the calculation but are no longer a variable in a dynamic problem. In the same spirit, One can not argue the effects of SOS in say wood but it is not an element in diving let alone a variable that effects the SOS in water Salinity also effects the speed of sound but also plays a minor role if I remember in change so it is not a predomonant factor when compared to temperature when you look at going form 33 to 35 ppt salt. Our diving resides for rec diving in the top 100 ft of water. depth although in play has a very minimal impact. salinity also does not normally change because you are int he same area. Likewise all the factors such as elasticity and other things does not alter the speed of sound. Density of material is the predominant driver for divers that drive the change in SOS going form air to water. all other factors play a minor role with in the constraints of air vs water.

here is a link for some SVP's that show the graph thing I was refering to .

sound velocity profile - Google Search

If you look at eh various traces the main differences ar at the shallow region of the plot where temp changes the most. other profiles that deviate greatly have other factors introduced in to them like gulf stream or volcanoes present to alter the normal display.

 

[saxman242]

One cannot simply say that sounds speed is the square root of K/rho and then say that sound speed and rho (density) are inversely related....unless you ALSO say that K is constant. You can't even approximately say that sound speed is inversely to density unless you say that the material is approximately the same

This is what I was trying to get at in the post above yours.

 

[tursiops]

This is what I was trying to get at in the post above yours.

Actually, I think we are saying quite different things. You are saying density is mostly irrelevant, it is compressibility that matters. I would agree with you so long as we are talking only about a gas or a liquid.....but not when we shift from a gas (air) to a liquid (water). In this case, the greatly decreased compressibility of water goes hand-in-hand with its greatly increased density, so talking about density as causing the difference in sound speeds is simply encoding a much more complicated process into the simplest possible language, using commonly understood words, for non-physics majors. You are not going to find the scuba training agencies, particualrly at the basic level that is th context here, going into terms and language like compressibility or bulk modulus.

The entire underwater sound business, in mostly lay language, is at Home.

 

[saxman242]

simply encoding a much more complicated process into the simplest possible language,

This is the issue though... they're over simplifying it, and I get that. I wouldn't expect the PADI OW manual to go into a discussion about the speed of sound being due actually because of the ratio of the compressibility to the density or being the derivative of pressure with respect to density over density, etc. Simplifying it makes sense.

The issue is that in doing that simplification, the implication is that there is a direct proportional relationship between density and speed of sound. If density increases, speed of sound increases. That's just not true. While things that are denser tend to have a faster speed of sound, the density itself is an inversely proportional term. There are other factors that drive up the speed of sound, and while there is a correlation to increased density, it isn't a causation. Yes, liquid water is much denser than air. Yes, the speed of sound is faster in water. You could probably even go as far as saying that water is denser than air for the same molecular reasons that the speed of sound is faster, but that's still not the same thing as saying the speed of sound is faster because it is denser.

A much less wrong simplification would be to say that the speed of sound is faster because water is less compressible. That, in itself, doesn't tell the whole story either, but at least it's pointing in the right direction.

It goes back to the earlier statement about claiming something larger floats because it is heavier than something smaller.




Now, obviously, we're being pedantic about this far beyond a level that's needed on a typical PADI test, but that's what makes these quiz questions in this format more interesting. There's the test answer and there's the right answer and often they're not the same.

 

[tursiops]

density itself is an inversely proportional term

This leaves out the change in compressibility due to density. If you differentiate square-root K over rho, there are TWO terms, one involves the changes in K with density.....how do you include those?

I really don't care about that weight and buoyancy metaphor....it is irrelevant and misleading as to my point above.

 

[saxman242]

Ok, let's break this down to a more molecular level.

The elasticity/compressibility of a material at the molecular level is essentially due to the bond strength between molecules. A higher bond strength will transmit vibrations from one molecule quicker, resulting in a faster speed of sound.

The density is changed by either moving molecules closer to each other (due to increased bond strength) or increased molecular mass. Increased molecular mass requires more energy to accelerate a particle, so the speed of sound is slowed down.

A material with a higher bond strength may be more dense because of the bond strength, but higher density doesn't cause increased bond strength, nor does it mean a higher speed of sound.



Now, in the case of water vs air, you really have both at play. Molecular weight is lower and hydrogen bonding between water molecules are much higher than what you'll get between essentially inert gas pairs. These factors both result in a higher speed of sound and a higher density than air.

 

[tursiops]

Now, in the case of water vs air, you really have both at play. Molecular weight is lower and hydrogen bonding between water molecules are much higher than what you'll get between essentially inert gas pairs. These factors both result in a higher speed of sound and a higher density than air.

Agreed, and my point is that unless you introduce a bunch of new terms and definitions in your OW class, density is the only proxy you have top distinguish between air and water.