Thread: Proposed Test

Has anybody tried filling a couple of pipes with gravel? If you put a fairly large rock at the end you could close of the end by running some wire through the pipe and save the cost of blocks and PVC end caps.

A brief physics lesson on how sonar works and why the density of the air/water/PVC is important.

I'd say anybody who fishes is very familiar with the reason sonar can see a ping pong ball. Ever try to grab something that is under water? Chances are you missed on the first shot, because the object isn't where it looks like it is. Heck, your arm looks like it is hideously broken when you stick it in the water. That is because light moves at a different speed in water than air, so the light coming to you eyes is bent at the surface of the water. Refraction is the term for the bending.

Same thing happens with sound. Going back to the object under water, you also know that the surface of the water reflects things. That's because when a wave like light or sound passes through a junction between two things like air and water or air and glass, the transfer isn't perfect. Some of the wave bounces back and is reflected, while most of it travels on and is bent by refraction.

What does this mean? When your sonar signal is traveling through the water, when the density of the material the sonar signal is travelling in changes, you get a bending or refraction of the signal and some reflection. The strength of the refraction and reflection depend on how big the difference in density of the materials is. Water is 784 times more dense than air, so this makes for a pretty significant reflection. PVC is only about 1.4 times as much as water, so it produces a comparatively small reflection.

Going a little deeper, this refraction and reflection also explain why we can see a thermocline. The density of water changes with temperature. If the difference in temperature is large enough and abrupt enough, the transfer of the sonar signal into the layer is partially reflected and it shows up on the screen.

Snell's Law?

Just built this...will put this out with a bait bag, a round table saw used blade, and a small bag of ping pong balls. May tape the balls to the ends of pipes or just put them all in a small bag attached. I will do a scan and see if I can see the balls and blade and make it out what they are. Will post soon.

Originally Posted by fishfried

Snell's Law?

That would be it.

kicking back,you should see nearly all if u have di set right,the blade will depend on what angle it is to ducer