I use fiber fill. All that really does is slow down the sound waves and make the box sound bigger.

 

The stuff works by diffusion. It forces the sound waves to go around the individual fibers. It will turn multiple standing waves from many angles into static noise. It is the same principle as a offset perforated dual tube muffler.

 

Not really. The sound waves don't go around the fibers. Sound isn't like water, it doesn't go around obstacles. Sound hits objects and bounces off. When the sound hits the fibers, it bounces off causing some of the energy to be converted from sound into motion as the fibers vibrate. This energy loss causes a slight reduction in sound level, which is one of the downsides of using fiber fill. However, the sound bouncing off the many fibers causes the sound waves to slow down, making the box sound larger.