Sound travels as pressure waves through the air. Sound insulation is the resistance to sound; in this case, insulation material reflects the waves. Sound absorption converts the sound to energy or allows it to pass through.
Sound insulation resists the sound waves through reflection, or sending them back into the air. Sound absorption accepts the acoustic energy of the sound wave and transfers it to heat energy. The two functions work together to dampen sound. You can think of sound insulation as noise reduction and sound absorption as noise absorption.
Effects of Sound Absorption
Sound is an energy source and can be absorbed by being "caught" by the molecules around it. For instance, as sound passes through water, the molecules vibrate as it tries to pass. The effect is both the reduction of the sound as well as the transfer of the sound energy to heat. Higher-frequency sound waves take longer to slow down and create more heat when absorbed.
Misconceptions of Sound
Some sources claim that sound travels better through solids and liquid than through air. If this were true, sound would be absorbed in the air better than in solids or liquids. But this is incorrect. Sound might indeed move faster through solids because it is given a specific path, but that is not the most efficient means for it to travel.
Types of Sound Insulation
Sound insulation material also can act as a sound absorber. Any barrier to sound can function as insulation, but the best materials are those that absorb the sound waves. Something as simple as carpeting can absorb the sound of footsteps, as the sound waves are caught bouncing among the carpet fibres. Soundproofing insulation materials can be added between walls or directly onto drywall. Sound insulation materials include cork board, rock wool insulation, fiberboard insulation and specialised acoustical wall coverings.
Sound absorption actually can cause a glass to break, as so many cartoons show when an opera singer hits a high note. Thin glass will absorb the high-frequency sounds and vibrate or resonate to the point of breaking. This has also been known to happen to glass windows, such as church windows that are close to the clanging of large bells.