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.

Do not use sound absorbing materials inside the entire cabinet, use black hole 5 or no rez behind the active woofer, just a small strip as a little goes a long way.

Check my enclosures, shown here, 9th photo, back off, innards shown:

Polyfill, be careful of staples, get them straight, hammer them in if needed.

Prior enclosure, on 8" legs, woofer facing down, I had a buzz once, it was a loose staple on the woofers cone 

Now, shorter base, woofer facing front, slightly tilted back (to aim tweeters at seated ear height and time alignment if you believe in that).

No special bracing in that big box. I put lots of Donna's pricey beloved objects on the slightly slanted top, they don't vibrate or move. Very very slight sensation if holding hand on their side.

Cabinet has a rear port, used in prior room 'no wall behind' location. Port closed for this location with wall behind.

My AR2ax were stuffed with ancient fill, now totally filled with polyfill. Not ported, but rather than simply lining the walls like my other ones, stuffed.

JBL used 1’’acoustic fiberglass insulation in their speakers with passive radiators, (except the front baffle). See below: 😎

Mike

https://www.lansingheritage.org/html/jbl/plans/1960s-manual.htm

https://www.lansingheritage.org/html/jbl/catalogs/1979-home.htm

https://www.lansingheritage.org/html/jbl/plans/jbl-plans.htm

OP:  You should definitely try these kind of questions over at DIYaudio.  Lots of builders there with hands on experience.

Thank you very much for your time to help me. Happy New Year.

When I use passive radiators,  I peel fiberglass in about a 3 to 3 inch sheet and line the box with fiberglass.  Remember, anytime you add glass or any type of stuffing, you change the effective box size and how the woofer reacts to that box.  

Also, a passive radiator, works similarly as a port, as you lengthen a port, you lower its effective tuning frequency, by changing the size and mass of a passive radiator, you are also raising or lowering its effective frequency.