Science Buddies: "Ask an Expert"

Hi - Last year I did science buddies sound waves in space experiment and learned no sound travels in a vacuum. This year, I'm going deeper researching if sound moves best in gas, liquid or solid. I found researching it travels best in solids, but I am not sure how to prove it. I know the noise I make needs to be the constant in the experiment. I used a decibel app last year to measure the sound of a bell in the vacuum. Do you have suggestions how I can test a sound I can measure with my decibel app in air, solid and liquid?
Thanks for your help!

Hi Denise,

This sounds like a great project. I think the first thing to consider is the speed of sound in each material. For example, the speed of sound in air (gas) is about 330 m/s (http://hyperphysics.phy-astr.gsu.edu/hb ... ouspe.html). Gasses and Liquids should be straightforward to test, as their sound speeds are relatively low. I did want to ask if you were using a phone application or if there is any equipment that can be used? If you can use equipment, this would be much more helpful, especially considering the short times that would be dealt with. Solids will likely be the hardest to measure. If you can find a local high school science teacher, they should have access to some equipment that will help to make such measurements. What would be useful is an oscilloscope and a pulse generator that can be hooked up to a speaker and microphone. The idea would be similar to that described here : http://en.wikipedia.org/wiki/Ultrasonic_testing.

Let me know if you need more information!

Best of luck,

Matthew

Hi,
In order to prove you hypothesis in a controlled experiment, you would have to use the same material throughout each phase. This would eliminate another lurking variable. For example you could track sound waves through water. Liquid water is the densest so sound waves should travel the best, ice is the next densest, and gas is the least dense. However, since you are using and app this may not be feasible (i'm assuming this is your phone. One way to overcome this is to use a plastic bag or waterproof case to keep your phone from getting wet. You can measure the amount the bag or case obstructs the sound by running a couple of trials using the app with the case and without the case using the same sound. Subtract the two values and you'll get the amount of sound absorbed by the case/bag. Then, in the real experiment, you would just account for this lost sound absorbed by the case or bag used to waterproof the phone.

Another medium you could use is styrofoam - it is not dense, but you can compress it to make it denser.
You may also want to look at the metal gallium - you can get it for about $20 on amazon and is liquid at slightly above room temperature and becomes solid if put in the refrigerator. This is one of the few non-toxic metals that has a low melting point.

In addition, you would have to measure the densities before, and also keep the distances the same. For example, if the microphone was at an end of styrofoam and the sound playing at the other end of the block was 1 foot, the other trial would have to have a denser, compressed block, but still have the sound player and the microphone 1 ft apart. Furthermore, make sure the microphone is in direct contact with the medium (touching the styrofoam of touching the water(using a waterproof case of plastic bag).

If you have anymore questions, please ask!
-Chris

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Hm...if your goal is "air vs water vs solid", you can of course measure in air, but different liquids and different solids will give different results. You have to pick a liquid and pick a solid, and while I'm assuming you mean "human breathable air" helium from a balloon, for example, will give different results than air (and your altitude vs sea level might matter too, if you have sensitive enough equipment)

My instinct would be to pick a phase, then measure different materials in that phase.
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For this sort of thing, liquids are easiest (it is fairly easy to find liquids with different densities - rubbing alcohol, water,olive oil, for example, and measuring them into the same-size container is also easy) but if you're using a cell-phone that might not work very well.

Measuring "water as gas" is pretty much impossible - to measure water in gaseous state, you're talking about steam, which is both dangerous and impractical to stick a cell-phone into. (water vapor like fog isn't good enough, that's mostly air). Measuring a sound wave in ice and water is possible, most other objects are going to be tough to get two phases at human-safe and cellphone-safe temperatures.

I'm not a fan of using anything with air in it (like styrofoam) as it's actually a mix of air and a solid, and yes, compressing it changes it to more of a solid but it is hard to really understand the impact of what you are doing.

My approach to this problem would be to use a bunch of things solids at room temperature, especially those that are easily cut/worked/shaped, as you'd want the same size of object for each (all geometry the same, only difference is density). Suggestions would be clay, wood (you could do several kinds - balsa will be very different from oak), plexiglass, wax, different kinds of plastics.

Thanks everyone! This is lots to think about