// requires JQuery
sS hjs_humWithFade(double humVolume default 0.25) {
  ret hjs_playHum()
    + hjs(replaceDollarVars([[
      humSound.volume = 0;
      var humVolume = $humVolume; // level when on
      var humShouldBeOn = false;
      
      $.extend($.easing, {
        easeOutExpo: function(x, t, b, c, d) {
          return (t==d) ? b+c : c * (-Math.pow(2, -10 * t/d) + 1) + b;
        },
        easeInOutQuint: function (x, t, b, c, d) {
          if ((t/=d/2) < 1) return c/2*t*t*t*t*t + b;
          return c/2*((t-=2)*t*t*t*t + 2) + b;
        }
      });
  
      function humOn() {
        if (humShouldBeOn) return;
        humShouldBeOn = true;
        hum(humSound.volume); // start playing
        console.log("hum on (vol=" + humVolume + ")");
        fadeHumTo(humVolume);
      }
  
      // TODO: actually turn off in the end
      function humOff() {
        if (!humShouldBeOn) return;
        humShouldBeOn = false;
        console.log("hum off");
        fadeHumTo(0);
      }
  
      function fadeHumTo(vol) {
        $(humSound).animate({ volume: vol }, {
          duration: vol < humSound.volume ? 250 : 500,
          //easing: vol < humSound.volume ? 'easeOutExpo' : 'easeInOutQuint',
        });
      }
      
      function updateHumVolume(vol) {
        if (isNaN(vol) || humVolume == vol) return;
        humVolume = vol;
        console.log("New hum volume: " + vol);
        
        // fade to new value
        if (humShouldBeOn) {
          humShouldBeOn = false;
          humOn();
        }
      }
    ]], +humVolume);
}

Began life as a copy of #1028691