text: is the a note the only note with a rational frequency? i am working a project, and i stated that a is the only note with a rational frequency. am i correct in saying that? i have seen the other frequences represented as numbers, and assumed they were all rounded, but i want to verify.  a4 (standard tuning frequency) = 440 hz yes.  because the frequency of the notes are a geometric sequence with a factor of 2^(1/12) between each note, and 2^(n/12) is irrational except for when n is divisible by 12.  there are 12 half-steps in an octave.  the frequency of a (just below middle c) is 220 hz.  so the frequency of a note that is one half-step above a is 220*(2^(1/12)).  two half-steps up is 220*(2^(2/12)).