I made sets of them one year for holiday presents. There's a physics formula to figure out the length of pipe needed for the note.
Should you happen to need a spare science fair project, the formula is
Pitch n = (Pitch a) x 2^((n-a)/12)
For example, let's say you want to find the frequency of middle C. "a" and "n" are simply reference numbers, let's say they are the number of keys on a piano, numbered left to right. Middle A, the reference pitch (Pitch a) is the 49th key and we know it has a frequency of 440 Hz. Middle C is the 40th key.
So the frequency of Pitch n (Middle C) = 440 Hz x 2^((40-49)/12) = 262 Hz.
To relate frequency to wavelength, wavelength = (velocity of sound in air)/(frequency). The velocity of sound in air is 343 meters/second, so to get a pipe that rings at middle C, we calculate 343/262 = 1.3 meters. Looking at my windchimes on the back porch that are tuned to middle scale, that looks about right--approx. 4 feet.
It's way cheaper to go to the hardware store, buy some copper pipe and a pipe cutter and some craft wood and nylon string, and just cut the sizes you calculate to get the right chord. They ring better if the strings are not knotted at the top but instead sort of threaded through loosely and knotted above the wooden disk that holds them. Like this:
Knots go here
_______|_______
|______________| wooden disk
\\__/ \\__/ \\__/ strings
| | | | | |
| | | | | | metal tubes
| | | | | |
