This kind of panel is made from porous materials. This way the particles that transport the sound enter the material through the pores and are literally imprisoned inside the material, thus effectively blocking part of the acoustic wave. Materials generally used are: acoustic foam, glass wool, curtains and carpets. Characteristics that contribute to absorption are the porosity of the material and its dimensions and position within the environment. Say we wanted to attenuate a certain frequency, that has been previously over-emphasized by the resonance modes which have marred the environment's acoustics. We have seen how the particles transporting sound reach their top speed at λ/4 distance from the room's wall. So this will be the exact spot where we will position our absorption panel. The latter will indeed capture the moving particles, absorbing the energy and transforming it into heat. If we were to place the panel next to the wall, the frequency in question wouldn't be attenuated. The following diagram illustrates two modes rising from in between two walls and the positioning of two absorption panels in two different positions:
Bearing in mind the spots where the particles' speed is at its highest, we see how panel 1 captures the particles put into motion by resonance frequency f2, whereas it has no effect whatsoever on those moved by frequency f1. Panel 2 on the other hand is placed in the area of maximum particle speed of both frequencies and can therefore attenuate them both.
The density of the material must take into account two conflicting requirements. On the one hand it has to be low enough to allow the particles transporting the sound to penetrate it. On the other hand however, density must be high enough to not let the sound entering the material get back out again; in other words, once they're in, the particles must remain trapped inside. This kind of acoustic panel is particularly efficient when it comes to high frequencies.