function freq (note, tuning) {
	  var m = midi(note)
	  return m === null ? null : midiToFreq(m, tuning)
	}
Get freq of a note in hertzs (in a well tempered 440Hz A4)

@name freq
@function
@param {String} note - the note name or note midi number
@param {String} tuning - (Optional) the A4 frequency (440 by default)
@return {Float} the freq of the number if hertzs or null if not valid note
@example
var parser = require('note-parser')
parser.freq('A4') // => 440
parser.freq('A') // => null
@example
// can change tuning (440 by default)
parser.freq('A4', 444) // => 444
parser.freq('A3', 444) // => 222
@example
// it accepts midi numbers (as numbers and as strings)
parser.freq(69) // => 440
parser.freq('69', 442) // => 442