Testing Sound Underwater

[MJay]

I want to do a science project testing how the acidity of salt water affects how far sound travels in it (I've read that the more acidic water is, the more sound waves it will absorb), but I'm not sure exactly how to test it. A science teacher at my school recommended using a hydrophone to measure how far the sound will travel before I can't hear it, but I have two worries with that:
1) I won't be able to build a tank large enough for the sound to to be absorbed completely.
2) I won't get results if the acidity of water only affects sound on a large scale, like in the oceans, and not in things as small as tanks.

So, I was wondering if anyone knew if I could get significant results with this project, and if there are ways of testing how far sound travels without having to build an enormous tank. (Is there some kind of underwater decibel meter I could use to test how loud sounds are at certain distances?)

[ScienceExpert123]

waterproof decibel meters are probably really expensive, so I would suggest just using regular decibel meter or a microphone with audio monitoring software. I have an idea instead of a tank: use a pipe that is as long as possible and place a speaker on one side of the pipe and the decibel meter or microphone on the other side. I'm not really sure how accurate this will be compared to the ocean because there are so many variables affecting the ocean (the ocean is never in equilibrium), but it should give you an insight into how pH and salt concentration affects sound. If all the variables of the ocean were kept constant, besides pH or salt concentration, then you would probably see a difference in how sound moves.

let me know if you have any more questions

good luck,
scienceexpert123

[MJay]

Thanks for the quick reply! There's one more thing that I wanted to clear up before I got started. If Sound A travels farther than Sound B, then will Sound A be louder than Sound B at any given distance?

[MelissaB]

Hi,

I've moved your thread to the physical sciences forum, because I think the experts here will be better able to help you.

Good luck!

[deleted-71588]

I want to do a science project testing how the acidity of salt water affects how far sound travels in it

You've picked an extremely difficult area to experiment with. The question "How far does sound travel?" has some philosophy aspects and some mathmatical limits.

Sound is typically modeled using wave theory. When the transmission media changes, there are boundary conditions that cause partial reflections and partial transfers into the adjacent media. The walls of a container used to hold a liquid are boundary conditions that will cause these reflections and partial transmissions. Their shapes and sizes will significantly affect your experiments. In an ocean or lake, the bottom materials and shoreline and plant materials will be significantly different and cause differences in sound propagation behavior.

Mathmatically, the sound never really goes away on its own. As sound spreads it disperses so the sound energy per cubic centimeter is diluted. The question "If a tree falls in the forest and nobody is around to hear it, does it make a sound?" has an application to your problem: If your sound detection equipment (human ear or microphone) is unable to detect the diluted sound energy, is the sound gone? Sound waves can cancel which is the property utilized by sound cancelation headphones.

Now back to sound propagation in a liquid. The density of the liquid will have a major factor in sound propagation. I personally doubt whether the acidity of salt water would directly affect the sound propagation behavior; however, there is probably a correlation between the acidity of salt water and plant/microbial growth and/or liquid density that can affect sound propagation.

You need to decide if you want to study this or a related topic in
1) A sterile (no living materials) physics only way
2) In a simulated natural environment
or find another topic.

If you want to deal with this as a physics only experiment, you should alter your question to something related that can be more easily tested (measured).
If you want to do this in a simulated natural environment, it will take a lot more time, think multiple years as environments are organic and changing and difficult to comprehend.

Coming up with a great science fair project is figuring out something that you are interested in, stays within the science fair guidelines, can afford to do (time, money, space), and has an aspect that is unkown. Lots of luck.