From the point of view of an acoustic system that receives an incident acoustic energy (Ei), this energy is distrbuted in the acoustic system by three mechanisms:
Transmitted energy (Et) is the portion of acoustic energy that an element lets pass through. It also gives an idea of the insulating capacity of that element.
Absorbed energy (Ea) is the portion of acoustic energy that the element turns into calorific energy by friction.
Reflected energy (Er) is the portion of acoustic energy that the element rejects towards the average emitter like reflection of the incident energy.
These energies divided by the incident energy give rise to the absorption coefficients, reflectance and transmittance. The most important properties of an acoustic material are the coefficient of absorption and the isolation.