Playing with notes and frequencies...

notes.coffee

# Get the frequency, in Hertz, from an octave number.
# `0` corresponds to A0, `1` to A1, etc.
frequencyOf = (octaves) ->
  27.5 * Math.pow(2, octaves)

# Parse a note string into its corresponding octave number.
# Note: this cannot parse sharp (#) notes yet.
parseNote = (string) ->
  point = string.charCodeAt(0) - 65
  offset = point * 2
  offset-- if point >= 2
  offset-- if point >= 5

  octaves = Number string.substr 1
  return octaves + offset/12

usage.coffee

# Use them together to get the frequency of a note:

frequnecyOf parseNote("A4")  #-> 440
frequencyOf parseNote("G5")  #-> 1568
frequencyOf parseNote("D3")  #-> 293.7