Cheap sound-proofing

With a noisy 3-year old in the house, I spend a lot of time thinking about sound proofing. A lot of time. It's pretty expensive to get it done right, but I think I have an idea for doing it on the cheap: bubble-wrap. I gather that effective sound insulation is like insulating for temperature. You want to avoid solid surfaces touching each other and use lots of layers. Bubble wrap seems perfect. All those cells of air and layers of plastic. I figure a few sheets ought to do a pretty good job. I haven't gotten desperate enough to try it yet, or maybe I just haven't hit on the right way to do it.