Works like this... the air pressure generated by the driver motion presses against the plastic film and that moves the air inside the bag which squirts through the perlite pushing them around and turns the acoustic energy into heat (caused by frictin) and so disipates the energy (particle friction also plays a disipative role here). The thinner the film of the bag the better. Ted Jordan published a series of articles in Wireless World back in seventies I think all about sealed membrane absorbers which used fiberous batting in a low 5 sided box with a sealed membrane on the 6th side, the batting has to be inside the box in contact with the membrane to work effectively.
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