The basics of how sound behaves:
• When sound strikes a surface, some of it is absorbed, some of it is reflected and some of it is transmitted through the surface. Dense surfaces will isolate sound well, but reflect sound back into the room. Porous surfaces will absorb sound well, but will not isolate it.
• The best way to stop sound transmission through a building structure is to isolate the sound source from the structure before the structure has a chance
to vibrate.
• Walls need to be isolated from ceilings and floors, usually by means of dense,
pliable rubber.
• The main ways to minimize sound transmission from one space to another are adding mass and decoupling.
• Every object, every construction material has a resonant frequency at which it is virtually an open window to sound. Different materials have different
resonant frequencies.
• Trapped air (air spaces and air gaps) is a very good de-coupler.
• Airtight construction is an important idea. Sound will get through any small gap. (Sound can leak through openings as small as 1/32")
• Sound bounces back and forth between hard, parallel surfaces.
One of the biggest concepts to understand and appreciate is that acoustic foam or insulation are not going to "soundproof" your room. Both are an extremely effective absorber of ambient, reflected sound and can help make rooms "sound better." Acoustic foam does contribute some sound isolation properties (mostly high frequencies), but is not sufficient by itself to keep sound in or out of a room. Thicker acoustic foam is better at absorbing low frequency sounds. Controlling reflected sound within a room is extremely important.
Isolation construction is not inexpensive. It is important to realize that empty egg cartons, cork squares and carpet scraps are not going to keep sound from leaving or intruding upon your theater or yield a pleasing, neutral, sound within your theater.
One of the keys to getting good, clean sound is removing the sound of the room from the equation, to one degree or another.