To start the program do the following:
1. Copy the following link
2. In APGS go to File Open from Web
3. Paste the link and <enter>

Do not open the link directly but from the APGS program

Then press Game Start and click the mouse button

You will see the a Piano Keyboard and you can test it. Each time you click a key the computer will display the frequency in Hertz of the sound that the key produced.

If you click on the red square you will see the following picture: For example, the number under the A4 white key is 440 (Hertz) and this means that A4 vibrates at a rate of 440 times per second. If you look at the previous A (A3) key you will see that this numbers is cut in half. The same pattern holds for any key. Consider a chord that contain two notes. An interesting connection between music and math is that the simpler the proportion the more pleasant the sound (of course this is an over simplification, but it explains why the 12 tone scale was picked for Western music and why the piano white and black keys were designed that way). For example in C4
C5/B5~523/494~1.06~1+1/6 (that is kind of a stretch.
G/C~392/262~1.5=3/2=1+1/2 (Very close)