function midiToFreq(m) {
  return 440 * Math.pow(2, (m - 69) / 12.0);
}
Returns the frequency value of a MIDI note value.
 General MIDI treats notes as integers where middle C
 is 60, C# is 61, D is 62 etc. Useful for generating
 musical frequencies with oscillators.

 @method  midiToFreq
 @param  {Number} midiNote The number of a MIDI note
 @return {Number} Frequency value of the given MIDI note
 @example
 <div><code>
 let midiNotes = [60, 64, 67, 72];
 let noteIndex = 0;
 let midiVal, freq;

 function setup() {
   let cnv = createCanvas(100, 100);
   cnv.mousePressed(startSound);
   osc = new p5.TriOsc();
   env = new p5.Envelope();
 }

 function draw() {
   background(220);
   text('tap to play', 10, 20);
   if (midiVal) {
     text('MIDI: ' + midiVal, 10, 40);
     text('Freq: ' + freq, 10, 60);
   }
 }

 function startSound() {
   // see also: userStartAudio();
   osc.start();

   midiVal = midiNotes[noteIndex % midiNotes.length];
   freq = midiToFreq(midiVal);
   osc.freq(freq);
   env.ramp(osc, 0, 1.0, 0);

   noteIndex++;
 }
 </code></div>