Re: cheap sound proofing
You need absorbers to block the sound from escaping your home into the neighbors'. Ethan Winer explains how it works, and tells you how to build your own absorbers from fiberglas and wood. Not only will you block noise, you might also lower the cost of heating and cooling your trailer.
http://www.ethanwiner.com/acoustics.html
The Ethan Winer link has lots of good info in it, but is is not really applicable here, as acoustic
treatment and room tuning is used to fix sonic issues within your recording/playback space. Absorbers are
not designed to prevent sound leakage (though sometimes this is a pleasant side effect). Sound needs
air to travel. Most sound leaks from doors and windows, not through walls and floors.
There are 3 things you can do to sound
within a room: diffuse it, reflect it, and absorb it. There is way more involved in this than can possibly be discussed within the confines of this thread.
If you want to stop sound from escaping, you are talking about
sound proofing. To do this, you need to create a barrier that will trap the sound within a small 'air lock' to prevent it from migrating out; this is generally done by floating a room within a room and using "weather seals", double doors, etc.