Quiz - Physics - Sound

[saxman242]

I don't get you. Please elaborate. Preferably at high school/freshman physics level if possible.

Here's a pretty good HS level discussion: The Speed of Sound in Other Materials

Elastic Properties

The speed of sound is also different for different types of solids, liquids, and gases. One of the reasons for this is that the elastic properties are different for different materials. Elastic properties relate to the tendency of a material to maintain its shape and not deform when a force is applied to it. A material such as steel will experience a smaller deformation than rubber when a force is applied to the materials. Steel is a rigid material while rubber deforms easily and is a more flexible material.

At the particle level, a rigid material is characterized by atoms and/or molecules with strong forces of attraction for each other. These forces can be thought of as springs that control how quickly the particles return to their original positions. Particles that return to their resting position quickly are ready to move again more quickly, and thus they can vibrate at higher speeds. Therefore, sound can travel faster through mediums with higher elastic properties (like steel) than it can through solids like rubber, which have lower elastic properties.

The phase of matter has a large impact upon the elastic properties of a medium. In general, the bond strength between particles is strongest in solid materials and is weakest in the gaseous state. As a result, sound waves travel faster in solids than in liquids, and faster in liquids than in gasses. While the density of a medium also affects the speed of sound, the elastic properties have a greater influence on the wave speed.

Density

The density of a medium is the second factor that affects the speed of sound. Density describes the mass of a substance per volume. A substance that is more dense per volume has more mass per volume. Usually, larger molecules have more mass. If a material is more dense because its molecules are larger, it will transmit sound slower. Sound waves are made up of kinetic energy. It takes more energy to make large molecules vibrate than it does to make smaller molecules vibrate. Thus, sound will travel at a slower rate in the more dense object if they have the same elastic properties. If sound waves were passed through two materials with approximately the same elastic properties such as aluminum (10 psi) and gold (10.8 psi), sound will travel about twice as fast in the aluminum (0.632cm/microsecond) than in the gold (0.324cm/microsecond). This is because the aluminum has a density of 2.7gram per cubic cm which is less than the density of gold, which is about 19 grams per cubic cm. The elastic properties usually have a larger effect that the density so it is important to both material properties.

 

[NAM001]

Oceanographers know that water is compressible...the deep ocean has a lot of pressure...the average depth of the ocean is 4000m, thus 400 ATM!

Here is a portion of a discussion reparding compressability of water

""""""But, squeeze hard enough and water will compress—shrink in size and become more dense ... but not by very much. Envision the water a mile deep in the ocean. At that depth, the weight of the water above, pushing downwards, is about 150 times normal atmospheric pressure (Source: University of Illinois at Urbana-Champaign Ask the Van). Even with this much pressure, water only compresses less than one percent.""""""""

This is an example of how too fine of detail has no use in scuba and why it is considered as a non compressable. In hydraulics the basic premis of using a liquid instead of air in systems is that liquids are non compressables. Perhaps not literally or in every situation but in the world of the topic it is true. Should they say that liquids are compressable but the amount is so small that it is barely zero when it comes to any function related to the subject matter..

As a master instructor do you tell you new OWs that the body is a compressable but does not become 1/2 the size every time you double presssure in the water like air dies. If so what does a student learn form that? Or is it a moot point and to do so just confuses the student with issues and facts not pertainant to enhancing the understanding of the subject matter. When they ask why they have to exhale as they ascent and you answer that it is the air that compresses and not the body. Would it make any difference if you told them the body compresses and decompresses in size but being so so so shall of an amount it is not an issue to worry about as compared to air in the lungs. Every class I have ever listened to will put it out such that the concern with exhaling is that the air in the lungs will compress and expand and the rest of the body does not because it is 78% water and that water is not a compressable item. Is not one of the most basic premises in teaching not to inject things into a lecture that have no significant importance or useful meaning so not to detract form the main subject matter.

Its part of the KISS principle. Now when a prior student sees things like this why would they now question what they learned because some one said water is compressable with out any further qualification of that statement.

I think in the beginning of this thread there was a note that these were basic level questions or a basic forum or something like that. And do not the overly in depth discussion of such deep and involved theory that has no relevance to the basic purpose of the questions detract from that purpose?

Personally I am ready for the next basic question so I can have a chance to contribute to the survey and not to engage in over critique.

 

[Angelo Farina]

@KWS, I agree entirely on the fact that for a basic OW course there is no need to give too much complex explanations.
What I cannot accept is to give a physically wrong explanation (sound speed in water is larger than in air because water has larger density), when there is actually no need of providing such an explanation.
I prefer to do not explain "why", and if they ask, I say to my students "because water is a liquid", which is a fact they accept usually without need of further explanation, or "because water is almost incompressible, while air is greatly compressible".
It is pointing to density as the cause of larger sound speed which is entirely wrong, misleading and inacceptable...

 

[NAM001]

@KWS, I agree entirely on the fact that for a basic OW course there is no need to give too much complex explanations.
What I cannot accept is to give a physically wrong explanation (sound speed in water is larger than in air because water has larger density), when there is actually no need of providing such an explanation.
I prefer to do not explain "why", and if they ask, I say to my students "because water is a liquid", which is a fact they accept usually without need of further explanation, or "because water is almost incompressible, while air is greatly compressible".
It is pointing to density as the cause of larger sound speed which is entirely wrong, misleading and inacceptable...

Many times it is appropriate to give a technically globally wrong explanation so long as it is basically right for the goals of the training at hand and serves to get a concept across in the time prescribed when the concept is what is of importance and not the how. It also depends on whether the topic of the training is SOS or something else. The emphasis on correctness is normally based on the importance to the curriculum or its support of further training. Now I said once that I did not think the SOS to a diver is important. When I talk about it i say when someone talks in the air you have a sole source and that source is in one place and you are in another place. but in water the entire body of water becomes the source and that direction is not easy to determine because you are in the middle of the source. Most likely not correct but it is an accepted explanation for the purpose of getting the point across. The important point is you cant determine direction in water like you can in air. That aspect is test worthy and not the how. The how is just a tool to allow moving on to the next topic. If the topic was sound propagation I would not use that even as a patronizing example. Much like does a balloon with helium rise in the air because it is lighter or because of the density of the gas. are they the same or different ,,, does it really matter when the meat of it is to get across that a tank of air is heavier than a tank of helium. It may or may not matter in the view of the training at hand unless the topic is why it is lighter. My issue with much of the discussion is that we are looking at questions that were made for the support of diving and not doctorate level studies. Those that look at these questions draw their answer form the basic level training they had which is void of the higher education fine points of things that do not relate to scuba. As your occupation may depend on the fine points most others occupations or hobbies do not. Here is an example of that. The formula of PV. double pressure double volume. It is true with in the pressures scuba uses,,,but not neccessarily true above scuba use. If the question comes up like this. you have a tank with 1000 psi in it (25 cuft) and you double the pressure how much CUFT will you have. The answer should be 50 but there will be some that want to go into the non linear regions of air compressablity that does not apply to the purpose of the question which is comprehension of PV relationship. To tell them they are wrong because of the 50 answer , depending how thick of skin they have may be intimidated for no legit reason and avoid getting involved with further questions or scuba studies such as going on to AOW etc. Questions that at the basic level are non bendable and test recall and processing ability. None of us should be initiating some one questioning their past training as it reflects on the agency, the integrity of the instructor and ones basic understanding. In my view these questions are there to reinforce ones understanding of theory and proceedures to promote follow on training and safe practices.

 

[BlueTrin]

Edit: I don’t want to get involved

 

[Jcp2]

I hear what you’re trying to say very clearly now, but don’t know where you are coming from.

 

[Graeme Fraser]

"OK, so if everyone's clear on that, let's now turn to question 2 in your Knowledge Reviews"