Any additional comments or suggestions from those of you who have successfully soundproofed one or more rooms would be greatly appreciated. We're sure there are other solutions not mentioned here that would benefit our readers. If you are able to successfully interrupt the path of the sound, by absorption, reflection or altering the modulation, it is possible to reduce or eliminate the sound received at the recipient's ears. There are many ways to interrupt sound. Absorbers work best when there is some sort of a reflective surface.
All absorbers react differently as they are moved away from the reflective surface.
There is a significant benefit to having an air gap behind the absorber at the mid and low frequencies. Acoustics manufacturers design and tune their products to take advantage of this reflective surface and air gap. A noise barrier can be constructed from almost any non porous material. Low frequency sound contains more energy, because you are modulating a large volume of air to produce the long frequency waves associated with bass and sub bass frequencies. An excellent barrier is lead sheet, it is a dense yet limp material with a very low resonant frequency meaning it is not easily excited.