Sound absorption, sound insulation, and soundproofing have one thing in common. Controlling the sound.
Sound absorption means annihilating the reverberation or residual sound by absorption, and sound insulation means blocking the sound from passing to the other space by reflecting it. Put these together, you get soundproof.
“I only need sound insulation because I only need to keep the sound from leaving this room.” is not a very good Idea. As said above, sound insulation reflects sound to stay within the room, and this means the sound will echo around the room making every sound into a noise. Think about the echo in a cave, or beating a tin bucket with your head inside.
So, if you want to sound proof a room, you need to install sound insulator on the wall, and then install the sound absorber on the sound insulator. The sound insulator will reflect the sound, and the sound absorber will absorb and annihilate the reflected sound and prevent echoing.
The most important thing here is to NOT COVER THE SOUND ABSORBER with wallpaper or any other finishing material. There are numerous micropores on the surface and within the sound absorber that works as traps that locks the sounds from escaping, and blocking these pores with wallpaper will diminish the efficiency of the sound absorber.
Types and characters of sound absorber
1. Porous Type Absorber
Absorbers with numerous pores or void, such as polyester wool, glass wool, urethane foam, or wood wool. As the sound enters the pore or void, the sound loses energy while bouncing around the fibers or the walls of the pore and fade away. The porous sound absorbers have greater efficiency at high frequency level than the low frequency level.
2. Resonator Type Absorber
Resonator type absorbers, such as perforated wood board, uses the resonance effect of small tubular structures to absorb sound. This particular type is useful for reducing sound of specific frequency band, and is more efficient to be used together with porous absorber than being used alone.
3. Membrane Type Absorber
The sound absorbing means are based on a resonance membrane. The resonance membrane transforms the sound wave’s kinetic energy into thermal energy by friction, when the sound wave impacts and vibrates the membrane. The membrane absorber does not have an efficient absorption rate as it annihilates low frequency vibration rather than absorb the sound itself.
When choosing sound absorbers, high absorption rate does not always mean it is a good absorber. Adequate absorption rate depends on the purpose of the space.
For example, if the room is to be used for playing musical instrument, it would be best to choose absorbers with 20~30% absorption rate, to enrich the sound of the instruments.
Absorbers with absorption rate over 50% are recommended for recording studios, to prevent unnecessary sound from mixing in.
These are just simple examples for standard conditions, and you may find information and suggestions for much more complex conditions in publications related to acoustics and sound engineering.