setInterval in module not working

Hey all, I'm trying to write my first module for the pixl and I'm having difficulties with setInterval in the modules function. I was closely following the writing modules page.

I use the code below to develop in the WebIDE. The beep function works fine, so does the second function that just calls the beep function. But then the third function involves calling beep from a setInterval. And that is not working.

What am I doing wrong?

var exports={};
var PIN;
var VOLUME;

function SOUND(pin,volume){
	this.PIN = pin;
	this.VOLUME = volume;
}

SOUND.prototype.beep = function(freq,time){
  analogWrite(this.PIN,this.VOLUME,{freq : freq});
  setTimeout(function(){analogWrite(this.PIN,0); },time);
  print("beep");
};

SOUND.prototype.multibeep_works_fine = function(freq,time,nr){
  this.beep(freq,time);  
};

SOUND.prototype.multibeep_works_not = function(freq,time,nr){
  setInterval(function(){ this.beep(freq,time); }, 1000);
};

exports.setup = function(pin,volume){
	return (new SOUND(pin,volume));
};

var s = exports.setup(A0,0.005);

//works
s.beep(1000,10);

//works
s.multibeep_works_fine(1000,100,1);

//does NOT work?
s.multibeep_works_not(1000,100,1);