Info about damping materials and box volume.

I would like information on the use of the damping material. I know that in a closed box, filling it up all the apparent volume increases slightly. but in the case of a bass reflex, covering the walls with for example 1 or 2 cm of felt and then polyester (as troelsgravesen does) what happens? the volume of the cash register increases, decreases or is it irrelevant?

since my cabinet must be about 5 liters, making a mistake even a little can be significant ... thanks!
 
with 1-2 cm felt and just some polyester on the walls the apparent volume of the cash register loudspeaker box (italian "cassa" 😊) will just vary ever so slightly.
however, the amount of damping material will strongly influence the level of bassreflex port output and reflections/resonances of higher frequencies.
filling the whole box with damping material will result in no or very little output from the port.
 
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For a bass reflex enclosure, it is often recommended to line the back wall, just one side wall and just one of the top or bottom walls.

Lining a reflex enclosure has negligible effect on enclosure volume as the polyester is transparent to bass frequencies and only absorbs the higher frequencies.
 
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Something like that
IMG_20220717_150746.jpg