How properly covered internal sides of enclosures for speakers with a passive radiator?

All the passive radiators I have seen use acoustic stuffing material in the the cabinet and/or transmission line.   Just because you have a passive radiator, it doesn't change the need for damping cabinet resonances.   You can use fiberglass insulation, its cheap!, but the dedicated acoustic stuffing material is better and you don't have the working with fiberglass problems.  I use it in all the speakers I have built.

Speakers with passive radiators are more complicated to design than ported speakers. Polyfill is used in some designs, not fiberglass insulation, the polyfill is used not for dampening but to make the cabinet seem bigger than it really is. Sometimes weights are used on the passive radiator also.

Polyfill stuffing for stuffed toys.

VMPS subs with passive radiators were empty..

Hi, thank you for quick response. Usually I cover inner panels of speakers using 10mm felt, it work good to absorb reflection but because the principle operation of the passive radiator is based on the air pressure produced by the woofer. If I understand correctly, if all the internal sides of the speaker are covered with sound-absorbing material, then this reduces the air pressure on the passive radiator and reduces the efficiency of its operation. Please answer if I am  wrong and what you recommend that should be done to reduce the loss of air pressure.