Heather,
The standard frequency for
A above middle
C is
440 Hz. This is the current standard by which all modern instruments are tuned. Why is A used? Because the frequency for other notes is just too complicated. For example, middle C has a frequency of about 261.62 Hz. I could go into a very long and detailed explanation of harmonic systems and oscillation, but we`ll leave that alone.
The standard frequency for A above middle C has not always been 440 Hz, either. If you want a brief history lesson on the various frequencies that this note has been, check out this link I have provided.
http://www.uh.edu/engines/epi1305.htm
Cheers,
Thomas
